DE19845871A1 - Tightening screws in bone structures - Google Patents

Abstract

The method involves using a controllable screw driving system to detect the starting torque and rotation angle. The maximum starting torque is estimated according to an algorithm, and when this is reached, a gradient process is started with the cutoff condition gradient DELTA M/ DELTA eta is less than or equal to 0, where the gradient is formed from the differential quotients of the starting torque/angle characteristic curve. An Independent claim is included for a device for tightening screws.

Description

The invention relates to a method for tightening screws ben, in particular in bone structures, with a controllable to driven screw system, in which a tightening torque M and an angle of rotation ϑ are determined during tightening and a gradient ΔM / Δϑ from the difference quotient in a gradient method the tightening torque / angle of rotation characteristic curve is formed, the lowering of which is used to switch off the tightening system after reaching a threshold torque M _S.

The invention further relates to a device for tightening screws, in particular in bone structures, with a controllably driven screw system, which has a measuring device for measuring a tightening torque M and an angle measuring device for measuring an angle of rotation ϑ, which are connected to a motor control unit which in Depending on a gradient ΔM / Δϑ, which is formed from the difference quotient of the measured tightening torque / angle of rotation characteristic, gives a shutdown signal after reaching a threshold torque M _S.

From a publication (Bauer G .: Bolting technology - Bolting system in Bartz, W. J .; Wipler, E. (ed.): The automated Assembly with screws. Procedures, requirements, profitability compare. Ehningen: expert-Verlag 1988, pages 116-149) a process for the yield point controlled tightening of screws known with a controllably driven tool. With this The procedure is constantly during the tightening of the screw Rise in torque or tightening torque over a fi xen angle increment calculated. As long as the screw is stretched within the so-called Hook's straight line  the torque increase or the gradient ΔM / Δϑ, which from the Dif reference quotient of the tightening torque / angle of rotation characteristic is constant. After proportionality has been exceeded the gradient drops sharply at the limit of the screw material. With a certain drop in the gradient ΔM / Δϑ the An drawing process ended.

In order to avoid premature switching off before reaching the yield point in the event of an irregular torque curve, the tightening system controller is not given an absolute switching gradient, but a percentage drop - for example 50% - of the maximum gradient determined in each case. In order to avoid premature switching off in the lower area of the tightening torque / angle of rotation characteristic curve, a so-called threshold torque M _{S was} introduced.

In contrast to the screws that are common in technology Materials is a very inhomogeneous material and its bones Strength varies considerably depending on the bone type and condition Lich. The ultimate is therefore subject to bone screw fixation tive torque fluctuations. In addition, the Mo characteristics / angle of rotation characteristics, especially in the case of osteoporosis noticeable drops in torque in the tightening area are characterized.

There is therefore no prior information for bone screwing bone strength and the maximum tightening torque ment and the maximum screw-in angle. The problem with the Application of the known yield point controlled tightening process is particularly strong in bone surgery Nonlinearities in the tightening range of the tightening torque / Angle of rotation characteristic.

A disadvantage of the tightening process controlled by the yield point is that that it is unable to switch off early due to Mo prevent slumps.  

The object of the present invention is therefore the known to improve the yield point controlled tightening process so that Early shutdowns prevented due to torque drops be, and that the procedure also in bone surgery is reversible.

The object is inventively achieved in that the maxi male tightening torque M _max according to a predetermined Einschraubalgo algorithm _Max is estimated as the estimated maximum tightening torque M *, and that upon reaching the _Max formed threshold value M _S extended from the estimated maximum torque M * the Gradientenver with the switch-off condition gradient ΔM / Δϑ ≦ O is started.

By estimating the maximum tightening torque M _Max according to a predefined screw-in algorithm, the risk of early shutdowns due to torque drops is considerably reduced.

According to a preferred embodiment of the invention, the specified screwing-in algorithm is carried out by a linear regression analysis in screw-in tests from the context of the maximum tightening torque M _Max and the screwing-in energy E that results from the area below the tightening torque / angle of rotation characteristic curve during the threading process Cut the thread beforehand or determine the screw when cutting with a self-tapping screw.

In principle, it is also possible to use screw-in tests obtained screwing algorithm by including the to show data obtained in vivo when screwing in tern, so that a "learning system" arises, in which the Zu reliability of the estimation of the maximum tightening torque with increasing application, becomes more precise.

The linear regression analysis of the relationship between the maximum tightening torque M _Max and the screwing energy E provides preliminary information that enables a reliable estimate of the maximum tightening torque M _Max or the estimated maximum tightening torque M * _Max from the integration of the tightening torque / angle of rotation The characteristic curve enables the determined screw-in energy.

According to a further preferred embodiment of the invention, the estimated maximum tightening torque M * _Max with the insertion energy E | _{8UMDR estimated} after 8 turns.

The estimated maximum tightening torque M * _Max for a screwdriving application without thread pre-cutting is based on the equation

estimated.

By estimating the maximum tightening torque M * _Max with the insertion energy E | _8UMDR after 8 turns, the anatomical relationships of the human cervical vertebrae and the surgical instructions for the insertion of cancellous bone screws (1.75 mm thread pitch) are taken into account with a maximum of 8 permissible turns.

According to a further preferred embodiment of the invention, the maximum tightening torque M _Max is the estimated maximum tightening torque M * _Max for a screwdriving case with a threaded section according to the equation

estimated.  

In the case of screwing with a thread cut, the estimated maximum tightening torque M * _Max is already available at the start of the tightening process.

According to a further preferred embodiment of the invention, the predetermined screw-in algorithm is determined by a linear regression analysis in screw-in tests from the relationship between the maximum tightening torque M _Max and the gradient ΔM / Δϑ of the torque / angle characteristic curve. For human cervical vertebrae, the estimated maximum tightening torque M * _{Max is determined} at a given absolute threshold torque M _Sabs .

The estimated maximum tightening torque M * _Max becomes at an absolute threshold torque M _Sabs of 300 mNm according to the equation

estimated.

M * _Max is thus determined without determining the insertion energy E | _8UMDR estimated.

This makes it possible to use the screw system independently of Use thread cutting process.

According to a further preferred embodiment of the invention, the switch-off condition must be achieved as a function of a tightening range M _A / M * _Max for a predetermined number of angular steps with a negative secant slope or gradient.

This enables the switch-off sensitivity to be staggered Lich. The screwing or tightening process is therefore only then canceled when the switch-off condition ΔM / Δϑ ≦ 0 applies and a predetermined number of angular steps with negative ones Secant slope, so-called descent steps, without interruption  is reached. This will increase the number of possible early shutdowns decreased.

The individual embodiments and screw-in algorithms can can also be combined with one another.

According to a further preferred embodiment of the invention, a first estimated maximum tightening torque M * _Max1 in dependence on the insertion energy E during thread cutting and a second estimated maximum tightening torque M * _Max2 by means of the secant pitch ΔM / Δϑ in the tightening range with a threshold torque M _S = 0 , 3 M * _Max1 . estimated. The first estimated maximum tightening torque M * _Max1 and the second estimated maximum tightening torque M * _Max2 become a total estimated _value M * _Max according to the equation M * _Max = K ₁ .M * _Max1 + K2.M * _Max2 , with K ₁ and K ₂ as weighting factors.

The reliability of the estimation of the maximum tightening torque M _Max is further increased by the double estimation.

From DE 27 58 674 C2 a device for tightening screws with a controllably driven screw system be known. The screw system has a measuring device for measuring the tightening torque M and an angle device for measuring an angle of rotation ϑ. The measuring devices are connected to a motor control unit which has a calculation circuit for calculating the gradient of the tightening torque / angle of rotation characteristic or a gradient ΔM / Δϑ. For comparison with a threshold torque M _S , the engine control unit contains a comparator which activates a switching device after reaching the threshold torque M _S and emits a switch-off signal after passing through a range of negative or less gradient.

This device is under construction for self-tapping screws partial connections made from a variety of different materials suitable. She sees a separate control for the thread cutting process and for the final tightening process.  

The disadvantage of this device is that it is only for yourself cutting screws is suitable, the tightening mo ment / angle of rotation characteristic curve that with medium tightening moments through a range of low or negative slope running.

Another disadvantage is that strong non-linearities occur or torque drops in the tightening range of torque / torque angle characteristic curve, particularly for bone screw connections occur, result in early shutdowns.

Another object of the present invention is therefore that to improve known device so that early shutdowns as a result of sudden drops in torque, and that before The inventive method described here can be carried out can.

This object is achieved in that the motor control unit has at least one microcontroller in which at least one screw-in algorithm can be stored, with the aid of which the maximum tightening torque M _{Max can} be estimated by the microcontroller from the values of the measuring devices as an estimated maximum tightening torque M * _Max is.

Controller by storing a Einschraubalgorithmus in the microcomputer, with the aid of Einschraubalgorithmus the maxima le torque M _Max estimates, is advantageously prevented that dips before reaching the maximum tightening torque M _max or the ge estimated maximum tightening torque M * _Max as a result of moments of Tightening process is undesirably canceled.

According to a further preferred embodiment of the invention the screw system can be placed on a training session, via which the screw system can be calibrated. Here are about the training unit specified tightening torques on the screw system imprintable.  

By using the training session it is easy Way possible to calibrate the tightening system.

According to a further preferred embodiment of the invention is the tightening of screws simu with the training session can be used. For this purpose, the training unit has a simulator, on which the handpiece of the screw system can be attached. In order to it is possible to handle the tightening system in vitro work out.

But it is also possible that screwing in or tightening Train screws with a tool by hand.

Further details of the invention emerge from the following detailed description and the attached drawing in which preferred embodiments of the invention are illustrated, for example.

The drawings show:

Fig. 1 is an illustration of a screw system with standardized basic input, hand piece and foot switch,

Fig. 2 is a side view of a handpiece in section,

Fig. 3 is a screw system as a block diagram,

Fig. 4 is a block diagram of the engine control unit of Fig. 3,

Figure 5 is a block diagram of the meter electronics of a current measuring device motor.,

Fig of a second Einschraubalgorith mus * _Max sensitivity. 6 is a flow diagram of a simple estimate of M when using a gradient staggered shut,

Fig. 7 is a flow diagram of a third Einschraubalgorith mechanism with a two-fold estimate of M * _Max weighting factors with Ge K ₁ and K ₂ for the individual estimates using a gradient method with ge Staffelter cut-off point,

Fig. 8 is a flow diagram of a fourth Einschraubalgorith mechanism with a two-fold estimate of M * _Max and egg ner weighting of the individual estimates factors with Gewichtsfak K ₁ and K _2, and an estimate of a residue at zugswinkeis θ _RA using a Gradientenver driving staggered cut-off point,

Fig. 9 is a flowchart of a fifth Einschaubalgorithmus by simple estimation of M * _Max from a tapping process using a Gradientenver driving by a simple shut-off condition .DELTA.M / Δθ ≦ 0,

Fig. 10 is a flowchart of a sixth Einschraubalgorith mus dual estimation of M * _Max factors with weight K ₁ and K ₂ of the individual estimates using a gradient USAGE dung staggered switch-off sensitivity,

Fig. 11 is a flowchart of a seventh Einschraubalgo algorithm dual estimation of M * _Max with the weight factors K ₁ and K ₂ of the individual estimates and an estimate of the residual tightening angle θ _RA under Ver use of a gradient method with staggered cut-off point,

FIG. 12 is a flowchart of an eighth Einschraubalgorithmus simple estimate of M * _Max and in assess Zung residual tightening angle θ _RA using a gradient staggered Abschaltemp sensitivity,

Fig. 13 is a flowchart of a ninth Einschraubalgorith mechanism with a simple estimate of M * _Max with a threshold torque from solutes M _S by means of a supply Sekantenstei .DELTA.M / Δθ using a Gradientenverfah Rens staggered cut-off point,

Fig. 14 is an illustration of the principle of estimating a remaining to tightening angle θ _RA by means of the secant .DELTA.M / Δθ below a Schwellmomen tes M _S = 0.5 M * _Max,

Fig. 15 is a flowchart for the Einschraubalgorithmus of Fig. 6,

Fig. 16 is an illustration of a training session with up setztem handpiece,

Fig. 17 is a front view of a Kalibrieraufbaues with a handpiece of a screw system simulator and tripod

Fig. 18 is a front view of a unit of a simulator exercise,

Fig. 19 is a plan view of the simulator of FIG. 18 and

Fig. 20 is a block diagram of a control unit for a simulator.

A device for tightening screws consists essentially of a controllably driven screw system ( 1 ) which is connected to a personal computer ( 2 ). The screw system ( 1 ) consists of a handpiece ( 3 ), a base unit ( 4 ) and a foot switch ( 5 ).

The handpiece ( 3 ) consists essentially of a motor-gear unit and an optical angle encoder ( 7 ) an angle measuring device ( 55 ), both of which are surrounded by a V2A steel housing ( 8 ). On the output shaft ( 9 ) of the motor-gear unit ( 6 ) an adapter ( 10 ) made of V2A steel with a small three-jaw chuck ( 11 ) for receiving a tool ( 12 ) is attached. The tool ( 12 ) can be designed as a drill, a tap or as a hexagon for screwing osteosynthesis screws. Other tools can also be included.

The motor-gear unit ( 6 ) consists of a DC motor, which is designed as a bell-armature motor, and a planetary gear. With the motor-gearbox unit ( 6 ), a load torque of 0.8 Nm in continuous operation is possible for the screwdriving event, which can also rise briefly to up to 2.0 Nm.

The optical rotary encoder ( 7 ) is connected to the motor directly via a shaft. It has the incremental, optical rotation angle sensor ( 7 ), which consists of a rotating metal plate benblende, which is scanned with optoelectronic miniature light barriers. The rotary encoder ( 7 ) shows a total of 500 pulses per revolution. Appropriate electronics quadruple this resolution. This enables a rotation angle resolution of Δϑ = 360 ° / (4,500) = 0.18 °.

The base unit ( 4 ) essentially has a measuring device ( 13 ) for measuring a tightening torque M and an engine control unit ( 14 ). The measuring device ( 13 ) is designed as a motor current measuring device ( 15 ) for indirect measurement of the tightening torque M via the motor current I _A. The motor current measuring device ( 15 ) has measuring electronics ( 16 ). The motor current or armature current I _A is measured via a measuring resistor ( 17 ) of 100 mΩ, which is designed in series between the motor control unit ( 14 ) and the motor. The voltage signal U is then preamplified by a measuring amplifier ( 18 ) with a gain factor V = 5. Since the current measurement is at a relatively high potential of 28 volts (operating voltage of the motor), but the operating potentials of the amplifier stages are only ± 15 volts, an isolation amplifier ( 19 ) with the amplification is required to overcome the potential differences in the next stage V = 1 switched. The subsequent active Bessel low-pass filter ( 20 ) of the 3rd order with a cut-off frequency of 100 Hz serves to smooth the measuring signal which has been pulsed up to that point. The additional amplification of the measurement signal results in the total gain of the circuit being V = 10. In order to obtain a positive output signal from the measurement electronics ( 16 ) regardless of the direction of rotation of the motor, the filter stage is followed by a precision rectifier ( 21 ). This is a full wave rectifier with a grounded output. After successful zero adjustment, the processed measurement signal U _M is fed to an 8-bit analog-to-digital converter ( 27 ) of the engine control unit ( 14 ).

The motor control unit ( 14 ) consists essentially of a central microcontroller ( 22 ) or microprocessor of the type 68 HC 11, a motor controller of the type HCTL-1100 and a power amplifier ( 24 ). The motor control unit ( 14 ) uses the motor control unit ( 14 ) to control the speed. The Mikrocon troller ( 22 ) passes the setpoint values for speed and angular acceleration to the motor controller ( 23 ). With the help of the current angular position and an integrated programmable digital filter, the motor controller calculates the necessary control data for the motor. The result is available as a pulse width modulated output signal with the corresponding polarity on the signal lines PWM and "direction of rotation" for the following power amplifier ( 24 ).

The power amplifier ( 24 ) mainly consists of an H-bridge output stage with MOSFETs, which are blocked or switched on in accordance with the control signals PWM and "direction of rotation". The pulse width modulation frequency for the engine control unit is 25 kHz.

Since the central microcontroller ( 22 ) is relieved of the complex calculation of the engine control data by the engine controller ( 23 ), it can take on further tasks. The Mikrocon troller ( 22 ) receives via a serial interface ( 25 ) from the personal computer ( 2 ) the commands for its individual types of operation of the tightening system ( 1 ), which it converts into the corresponding control commands for the motor controller ( 23 ) and transmits there. At the same time, the interface ( 25 ) also serves to transmit torque / angle of rotation characteristics recorded with the screw system ( 1 ). Furthermore, the microcontroller ( 22 ) controls a status display ( 26 ) with luminescent diodes in accordance with the selected operating mode. The speed of the screwing system ( 1 ) can be continuously adjusted using the foot switch ( 5 ). The foot switch ( 5 ), which has a Hall sensor as a signal sensor, delivers an output voltage proportional to the switch position, which is limited to 0 to 5 volts by a subsequent voltage divider. This analog speed signal is then digitized with an internal 8-bit analog-to-digital converter ( 28 ) of the microcontroller ( 22 ), converted by the microcontroller ( 22 ) into the corresponding motor control unit and sent to the motor controller ( 23 ). In the operating mode screws with screw-in algorithm, the speed is constant at 30 min ^-1 . The foot switch ( 5 ) only takes on the function of an emergency stop button.

The actual main task of the microcontroller ( 22 ) is to regulate the screwing-in process. For this purpose, the motor controller ( 23 ) supplies the current position of the rotary encoder ( 7 ) and the motor current measuring device ( 15 ) a voltage between 0 and 5 volts proportional to the motor current I _A. An 8-bit analog-to-digital converter ( 27 ) integrated in the micro controller ( 22 ) converts the signal voltage U of the motor current measuring device ( 15 ) into a digital signal and calculates the associated tightening torque M using a calibration characteristic.

With the help of the measured angle of rotation ϑ and the tightening torque M as well as an implemented screw-in algorithm, the switch-off time for the screwing system ( 1 ) is determined and the motor is stopped.

The motor controller ( 23 ) completely takes over the regulation of the motor speeds. For this purpose, the motor controller ( 23 ) performs a position control internally. After each scan, the next desired position from the motor controller ( 23 ) is determined using the predetermined linear acceleration and speed. From the current position of the angle encoder ( 7 ), the position error is then calculated, which in turn is used by the digital filter of the motor controller to calculate and output the new motor control data.

After restarting the tightening system ( 1 ), the microcontroller ( 22 ) initializes the digital filter of the motor controller ( 23 ). As soon as the microcontroller ( 22 ) has received a control command from the personal computer ( 2 ), the command is interpreted and the routine of the selected operating mode is started. Depending on the operating mode, the motor control commands, such as start, stop, specification of the desired speed or angular acceleration and others, are transmitted to the motor controller ( 23 ) and the status display ( 26 ) is triggered.

For the operating modes drilling, tapping and screwing without screw-in algorithm, stepless speed control in left and right-hand rotation is possible, in which the microcontroller ( 22 ) continuously converts the speed setting of the foot switch ( 5 ) into the set speed of the motor controller ( 23 ) and sends it. While the drilling speed is limited to 400 min ^-1 , screwing and tapping can only be done with max. 60 min ^{-1 are} driven. In the operating mode screwdriving with a screwing algorithm, the microcontroller program calculates the switch-off time for the screwdriving process at constant speed of 30 min ^-1 in clockwise rotation from the data from the measuring device ( 13 ) or motor current measuring device ( 15 ) and the current angle of rotation ϑ of the motor controller ( 23 ).

When recording the tightening torque / angle of rotation characteristics, the current angle of rotation ϑ and the tightening torque M are measured at a constant speed of 30 min ⁻¹ in clockwise rotation for a given number of revolutions and temporarily stored by the microcontroller ( 22 ). Following the recording of the characteristic curve, the value pairs of tightening torque / angle of rotation are transmitted to the personal computer ( 2 ) via the serial interface ( 25 ).

To protect the motor-gear unit ( 6 ), an overcurrent protection device in the microcontroller program is also available. As soon as the permissible peak or permanent motor current is exceeded, this leads to the motor being switched off. After the selected operating mode has been ended by the user or by the operating procedure itself (screwing with screwing algorithm, characteristic curve recording, etc.) and the microcontroller ( 23 ) has transmitted the status of the operating routine to the personal computer ( 2 ), the microcontroller ( 22 ) stands for the next command from the personal computer ( 2 ) is available.

Various screwing-in algorithms for regulating the screwing-in process of bone screws are stored in a memory of the microcontroller ( 22 ).

The specified screw-in algorithms are based on the relationship between the maximum tightening torque M _Max and the insertion energy E

or between the maximum tightening torque M _Max and the gradient ΔM / Δϑ of the tightening torque / angle of rotation characteristic curve, which was determined by a linear regression analysis in screw tests during the thread cutting process during thread pre-cutting or thread cutting with a self-tapping screw and during the tightening process. On the basis of this relationship, an estimated maximum tightening torque M * _Max ab is estimated during the screwing-in process and a threshold torque M _{S is} determined therefrom, upon reaching which the gradient method is started with the switch-off condition ΔM / Δϑ ≦ 0 and possibly other conditions.

The linear regression analysis follows the form:

Y = bx + a

with Y as a dependent random variable, X as an independent Zu if variables, a as the intercept of the regression line and b as the regression coefficient of the regression line.

For bone screw connections, especially for screwing in Screwing into a cervical vertebra, the following were one screw algorithms developed.

A first screw-in algorithm for a screwdriving case without a thread pre-cut is essentially divided into two sections:

• 1. Estimation of the maximum expected tightening torque M _Max with the screwing energy E, according to the formula
  is calculated after 8 screw turns during the thread cutting process. The estimated value M * _Max for the maximum tightening torque results from a determined regression line of the insertion energy E | _8UMDR after 8 screw turns and the maximum tightening torque M _Max according to the equation:
  
• 2. Start of the gradient process at a threshold torque M _S = 0.5 M * _Max with the switch-off condition ΔM / Δϑ ≦ 0.

In a second screw-in algorithm for a screwdriving application without thread pre-cutting, a graduation of the switch-off sensitivity is also introduced. The screwing or tightening process is therefore only interrupted when Δϑ ≦ 0 applies, and since a predetermined number of angular steps with negative secant pitch, so-called descent steps, has been reached without interruption. As the torque difference ΔM = M * _Max -M becomes smaller, the number of permissible descent steps is gradually reduced, thereby increasing the switch-off sensitivity. The staggering is given in the following table:

A flow diagram of the second screw-in algorithm is shown in simplified form in FIG. 6. A flow diagram for the second screw-in algorithm is shown in FIG. 15.

In a third screw-in algorithm for a screwing case without thread pre-cutting, a second estimate of an estimated maximum tightening torque M * _{Max2 is carried out} using the insertion energy E in the tightening range of the tightening torque / angle of rotation characteristic curve at a threshold torque M _S = 0.5 M * _Max1 to increase the security of the estimation of the maximum tightening torque M _Max . M * _Max1 is the estimated value for the maximum tightening torque from the first estimate of the tapping process. The estimates are weighted according to the correlation coefficients from the regression analysis with the factors K ₁ and K _{2 in} order to take into account the quality of the individual estimates. The total _{estimated value} M * _Max thus results from the sum of the weighted individual _estimates M * _Max1 and M * _Max2 .

The third screw-in algorithm can therefore be described in the following Steps can be divided:  

• 1. Calculation of the first estimated value of the maximum tightening torque M * _Max1 according to the equation:
  
• 2. Estimation or calculation of the maximum tightening torque M * _Max from the insertion energy E at the threshold torque M _S = 0.5 M * _Max1 . The second estimate M * _Max2 results from the regression line according to the equation:
  
• 3. Determination of the total estimated value M * _Max after the equation:
  M * _Max = K ₁ .M * _Max1 + K ₂ .M * _Max2 .
  The weighting factors K ₁ and K ₂ are 0.4 and 0.6 according to the correlation coefficients of the first and second estimates.
• 4. Start of the gradient process at the threshold torque M _S = 0.5 M * _Max with the staggering of the switch-off sensitivity of the second screw-in algorithm.

A flow diagram of the third screw-in algorithm with two-fold estimation of the maximum tightening torque is shown in FIG. 7.

According to a fourth screw-in algorithm for a screwdriving application without thread pre-cutting, in addition to the two-fold estimation of the maximum tightening torque M _Max, a residual tightening angle ϑ _{RA is} estimated using the secant pitch ΔM / Δϑ at a threshold torque M _S = 0.5 M * _Max . In a further switch-off condition, the estimated residual tightening angle ϑ * _{RA is} taken into account in a modified gradient method.

The fourth screw-in algorithm is divided into the following five steps:

• 1. Calculation of the first estimate of the maximum tightening torque M * _Max1 according to the equation from the insertion _energy E during the thread cutting process after 8 screw rotations, according to the third screwing algorithm.
• 2. Calculation of the second estimated value of the maximum tightening torque M * _Max2 according to the equation from the insertion energy E during the tightening process at the threshold torque M _S = 0.5M * _Max1 according to the third screw-in _algorithm .
• 3. Determination of the total estimated value M * _Max according to the third screw-in algorithm.
• 4. Estimation of the estimated residual tightening angle ϑ * _RA using the secant slope ΔM / Δϑ of the tightening torque / angle of rotation characteristic for the threshold torque M _S = 0.5 M * _Max .
• 5. Start of the gradient process at the threshold torque M _S = 0.5 M * _Max with a gradation of the switch-off sensitivity according to the following table:
  

The principle of estimating the remaining residual angle ϑ _RA is shown in FIG. 14.

A flow diagram of the fourth screw-in algorithm is shown in FIG. 8.

A fifth screw-in algorithm for a screwed case with a thread pre-cut is essentially divided into the following work steps:

• 1. Determination of the estimated value of the maximum expected tightening torque M _Max with the calculated insertion energy E from the thread cutting process after 8 turns of the thread cutter. The estimated value M * _Max for the maximum tightening torque results from the determined regression line according to the equation:
  
• 2. Start of the gradient process at the threshold torque M _S = 0.5 M * _Max with the simple switch-off condition ΔM / Δϑ ≦ 0 or with the staggered switch-off sensitivity according to the table in the second screw-in algorithm.

For the screwing process with thread pre-cut, the estimated value M * _{Max is} therefore already available before the screwing or tightening process.

A flowchart for the fifth screw-in algorithm with the simple switch-off condition is shown in FIG. 9.

In a sixth screwing-in algorithm for a screwing case with a thread pre-cut, a second estimate is made using the secant pitch or the gradient ΔM / Δϑ in the tightening range at a threshold torque M _S = 0.3 M * _Max1 . M * _Max1 results from the first estimate of the tapping process with a tap. According to the quality of the individual estimates, the weighting factor of the first estimate is K ₁ = 0.4 and the weighting factor of the second estimate is K ₂ = 0.6.

The sixth screw-in algorithm is essentially divided into the following sections:

• 1. Calculation of the first estimated value of the maximum tightening torque M * _Max from the screw-in energy E in accordance with the equation of the fifth screw-in algorithm.
• 2. Second estimate of the maximum tightening torque M _Max by means of the secant pitch ΔM / Δϑ in the tightening range at the threshold torque M _S = 0.3 M * _Max1 . The second estimate M * _{Max2 is} calculated from the regression line after the equation:
  
• 3. Determination of the total estimated value M * _Max according to the equation of the third screw-in algorithm with the weighting factors K ₁ = 0.4 and K ₂ = 0.6.
• 4. Start of the gradient process at the threshold torque M _S = 0.5 M * _Max with the graduation of the switch-off sensitivity according to the second screw-in algorithm.

The flowchart of the sixth screwing-in algorithm is shown in FIG. 10.

In a seventh screw-in algorithm for a screwing case with a thread pre-cut, the residual tightening angle ϑ _{RA is also} estimated and its estimated value ϑ * _{RA is} taken into account in the switch-off conditions of the gradient method.

The seventh screw-in algorithm is divided into the following five sections:

• 1. Calculation of the first estimate M * _Max according to the sixth screw-in algorithm.
• 2. Calculation of the second estimate of the maximum tightening torque M * _Max according to the sixth screw-in algorithm.
• 3. Determination of the total estimated value M * _Max according to the sixth screw-in algorithm with the weighting factors K ₁ = 0.4 and K ₂ = 0.6.
• 4.Determine the estimated residual tightening angle ϑ * _RA using the secant slope ΔM / Δϑ of the tightening torque / angle of rotation characteristic for the threshold torque M _S = 0.5 M * _Max .
• 5. Start of the gradient process at the threshold torque M _S = 0.5 M * _Max with the staggering of the switch-off sensitivity according to the table of the fourth screw-in algorithm.

The flow diagram of the seventh screw-in algorithm is shown in FIG. 11.

An eighth screw-in algorithm for a screwdriving case without thread pre-cutting is essentially divided into the following sections:

• 1. Estimation or calculation of the estimated value of the maximum tightening torque M * _Max from the insertion energy E during the thread cutting process after eight screw turns.
• 2.Determining the estimated residual tightening angle ϑ * _RA using the secant slope ΔM / Δϑ the tightening torque / turning angle characteristic curve for the threshold torque M _S = 0.5 M * _Max .
• 3. Start of the gradient process at the threshold torque M _S = 0.5 M * _Max with the graduation of the switch-off sensitivity according to the following table.
  

The flow chart of the eighth screwing-in algorithm is shown in FIG. 12.

In order to be able to use the screw system independently of the threading process, a ninth screw-in algorithm is divided into the following steps:

• 1. Calculation of the estimated value of the maximum tightening torque M * _Max using the secant slope ΔM / Δϑ at an absolute threshold torque of M _SABS = 300 mNm according to the following equation of a regression line:
  
• 2. Start of the gradient process at the threshold torque M _S = 0.5 M * _Max with the staggering of the switch-off sensitivity according to the staggering of the second screw-in algorithm.

A flow chart of the ninth screw-in algorithm is shown in FIG. 13.

The differentiators, integrators and estimators presented in individual flowcharts are implemented in the microcontroller ( 22 ) by program. In principle, however, it is also possible to use corresponding electronic components.

The screw system ( 1 ) can be placed on a training unit ( 29 ). The training unit ( 29 ) essentially consists of a simulator ( 30 ) and a control device ( 31 ) with a control unit ( 32 ). The control device ( 31 ) is connected to the personal computer ( 2 '). The simulator ( 30 ) consists essentially of a magnetic hysteresis brake ( 33 ) with a braking torque MB, which depends only on an excitation current I _{B of} the coil of a fixed brake magnet ( 34 ). To monitor the angle of rotation, a rotary encoder ( 35 ) is connected via a shaft coupling ( 36 ) to a rotor shaft ( 37 ) of the hysteresis brake ( 33 ). The simulator ( 30 ) also has a parking brake ( 38 ). The parking brake ( 38 ) prevents the rotary movement of a rotor ( 39 ) connected to the rotor shaft ( 37 ) during a demagnetization process of the hysteresis brake ( 33 ). The parking brake ( 38 ) is designed as a disc brake, which has a metal plate ( 40 ) with a friction lining, the rods of two lifting magnets ( 41 ) via two train ( 42 ) and two guide rods ( 43 ) with return springs ( 44 ) against a rotor flange ( 45 ) can be pressed with a friction lining. In order to prevent a possible torque ripple due to residual magnetism during operation of the hysteresis brake ( 33 ), the hysteresis brake ( 33 ) is demagnetized before each simulation with the aid of the parking brake ( 38 ) and a demagnetization control electronics ( 46 ).

The rotary encoder ( 35 ) is built as an optical, incremental rotary encoder corresponding to the optical rotary encoder ( 7 ).

During the demagnetization process, the magnetic field of the hysteresis brake ( 33 ) is continuously reversed by the demagnetization control electronics ( 46 ) and is thereby slowly reduced from a nominal value to zero. As a result, the magnetic dipoles of the rotor ( 39 ) pass through the entire hysteresis loop several times with a continuously decreasing maximum field strength until they are finally oriented with the same probability in all directions. The prerequisite for this is that the rotor ( 39 ) does not move during the process. This is prevented by the parking brake ( 38 ). After the demagnetization process, the solenoids ( 41 ) are switched off again and the metal plate ( 40 ) is moved back into its initial position by the restoring force of the restoring springs ( 44 ). The solenoids ( 41 ) are switched at about 24 volts with a current consumption of about 300 mA and can generate a maximum pulling force of about 10 N depending on the stroke. The stroke that can be covered with the parking brake ( 38 ) is approximately 2 mm.

The control unit ( 32 ) consists mainly of a central microcontroller ( 47 ) of the SAB 80535 type with an external program and data memory ( 48 ), a hysteresis control electronics ( 49 ) and the demagnetization control electronics ( 46 ).

Before the simulation operation, the personal computer ( 2 ') uses a calibration characteristic of the hysteresis brake ( 33 ) to calculate the control data of the tightening torque / angle of rotation characteristic to be simulated and stored in the personal computer ( 2 ') for the hysteresis control electronics ( 49 ). The calibration characteristic is located in the external program memory ( 48 ) of the control unit ( 32 ) and is previously transmitted from the central microcontroller ( 47 ) via a serial interface ( 50 ) to the personal computer ( 2 '). The modified tightening torque / angle of rotation characteristic curve is then transferred from the personal computer ( 2 ') to the microcontroller ( 47 ) of the control unit ( 32 ) and stored in the data memory ( 48 ). During the simulation of the screwing-in process, the microcontroller ( 47 ) continuously determines the current rotational angle position from the rotational angle data. From the modified tightening torque / angle of rotation characteristic in the data memory ( 48 ), the microcontroller ( 47 ) looks for the associated tightening torque or control signal for the hysteresis control electronics ( 49 ) and thus controls it via an external digital-to-analog converter ( 51 ) and an amplifier ( 52 ) the hysteresis control electronics ( 49 ). The hysteresis control electronics ( 49 ) in turn generates the necessary excitation current I _{B of} the brake magnet ( 34 ) of the hysteresis brake ( 33 ) from the control signal for the desired braking torque M _B.

For calibration, the screwing system ( 1 ) is placed on the simulator ( 30 ) of the training unit ( 29 ) with the aid of a tripod ( 53 ) and clamped in the three-jaw chuck ( 11 ) of the handpiece ( 3 ) with a receptacle ( 12 ) 54 ) of the rotor shaft ( 37 ) of the simulator ( 30 ). The screw system ( 1 ) can then be calibrated by comparing it with impressed moments.

When training the screwing-in process of, for example, osteosynthesis screws, the personal computer ( 2 ') coordinates the entire training course over the training unit ( 29 ). The central microcontroller ( 47 ) of the control unit ( 32 ) takes over the control of the simulator ( 30 ) during the simulation operation including the demagnetization process and thus relieves the personal computer ( 2 '). Communication between the personal computer ( 2 ') and the central microcontroller ( 47 ) takes place via the serial interface ( 50 ) using a defined protocol and acknowledgment procedure.

The training process essentially follows the following steps:

• - creation of a training plan,
• - simulation of the M / ϑ characteristics,
• - evaluation of training results,
• - repeated training of individuals or of all M / ϑ characteristics.

Before the training, a training plan is first created in the form of a text file. Here the user gives his name, the date, the desired training modalities, the number of screw-in attempts and the type of screw and bone to be trained. The personal computer ( 2 ') then randomly selects the desired number of tightening torque / angle of rotation characteristics of the selected screw and bone type and also enters them in the training plan. For the simulation of the individual torque / angle of rotation characteristics, the personal computer ( 2 ') calculates the control data for the control unit ( 32 ) by means of interpolation from a calibration characteristic of the simulator ( 30 ) and sends them to the central microcontroller ( 47 ). The central microcontroller ( 47 ) then carries out the simulation of the screwing-in process until the user terminates the tightening process. The personal computer ( 2 ') evaluates the individual simulation, saves the result in the training plan and starts the next simulation. After all screwing-in processes of the training plan have been simulated, an overall assessment is carried out and also stored in the training plan. The training plan can be saved and printed out for documentation. The repeated simulation of the entire and individual tightening torque / angle of rotation characteristics of the training plan is possible, so that the success of the training can be recorded over the long term.

Claims (51)

1. Method for tightening screws, in particular in bone structures, with a controllably driven screw system, in which a tightening torque M and a rotation angle ϑ are determined during tightening and a gradient ΔM / Δϑ from the difference quotient of the tightening torque / in a gradient method Rotation angle characteristic curve is formed, the drop of which is used to switch off the screwing system after reaching a threshold torque M _S , characterized in that a maximum tightening torque M _{Max is} estimated according to a predetermined screwing-in algorithm as an estimated maximum tightening torque M * _Max , and that when the from the estimated maximum tightening torque M _Max formed threshold value M _S the gradient method with the switch-off condition gradient .DELTA.M / Δθ ≦ O is started. 2. The method according to claim 1, characterized in that the predetermined screwing-in algorithm by a linear regression analysis in screw-in experiments from the context of the maximum tightening torque M _Max and the screwing energy E, which results from the area below the tightening torque / angle of rotation characteristic curve, is determined during the thread cutting process. 3. The method according to claim 1 or 2, characterized in that in human cervical vertebrae, the estimated maximum tightening torque M * _Max with the insertion energy E | _{8UMDR is estimated} after eight turns. 4. The method according to claim 3, characterized in that the estimated maximum tightening torque M * _Max according to the equation
is appreciated. 5. The method according to claim 3, characterized in that the estimated maximum tightening torque M * _Max for a screwing case with a thread cut according to the equation
is appreciated. 6. The method according to any one of claims 1 to 5, characterized in that the predetermined screw-in algorithm is determined by a linear regression analysis in screw-in tests from the context of maximum tightening torque M _Max and the gradient ΔM / Δϑ of the tightening torque / angle of rotation characteristic. 7. The method according to claim 6, characterized in that the threshold torque M _{S is formed} according to the equation M _S = 0.3 M * _Max . 8. The method according to claim 6, characterized in that the estimated maximum tightening torque M * _{Max is determined} at a predetermined absolute threshold torque M _SABS in human cervical vertebrae. 9. The method according to claim 8, characterized in that the maximum tightening torque M * _Max at an absolute threshold torque M _SABS of 300 mNm according to the equation
is appreciated. 10. The method according to claim 9, characterized in that an estimated residual tightening angle ϑ * _{RA is} estimated with the aid of the gradient ΔM / Δϑ of the tightening torque / angle of rotation characteristic at a threshold torque M _S = 0.5 M * _Max . 11. The method according to any one of claims 1 to 10, characterized in that the threshold torque M _{S is formed} according to the equation M _S = 0.5 M * _Max . 12. The method according to any one of claims 1 to 11, characterized in that the shutdown condition depending on the position in a tightening range M / M * _Max for a predetermined number of angular steps with a negative secant slope he must be sufficient. 13. The method according to any one of claims 1 to 12, characterized in that a first estimated maximum tightening torque M * _{Max1 cut} depending on the insertion _energy E at the thread and a second estimated maximum tightening torque M * _Max2 depending on the insertion _energy E in the tightening range the torque / angle of rotation characteristic is estimated at a threshold torque M _S = 0.5 M * _Max1 . 14. The method according to any one of claims 1 to 12, characterized in that a first estimated maximum tightening torque M * _Max1 depending on the insertion _energy E cut the thread and a second estimated maximum tightening torque M * _Max2 by means of the secant pitch ΔM / Δϑ in the tightening range at a threshold torque M _S = 0.3 M * _{Max1 is} estimated. 15. The method according to claim 13 or 14, characterized in that a weighting of the individual estimates is made before multiple estimates of the estimated maximum tightening torque M * _Max . 16. The method according to claim 15, characterized in that a total estimated value M * _Max according to the equation M * _Max = K ₁ .M * _Max1 + K2 from the first estimated maximum tightening torque M * _Max1 and the second estimated maximum tightening torque M * _Max2 .M * _Max2 , with K ₁ and K ₂ as weighting factors. 17. The method according to any one of claims 1 to 16, characterized ge indicates that the screws with a constant speed get dressed by.   18. The method according to claim 17, characterized in that the screws are tightened at a speed of about 30 min ^-1 . 19. Device for tightening screws, in particular in bone structures with a controllably driven screw system, which has a measuring device for measuring a tightening element M and an angle measuring device for measuring a rotation angle ϑ, which are connected to a motor control unit which is a function of one Gradient ΔM / Δϑ, which is formed from the differential quotient of the measured tightening torque / angle of rotation characteristic, gives a switch-off signal after reaching a threshold torque M _S , characterized in that the motor control unit ( 14 ) has at least one microcontroller ( 22 ) in which a Screw-in algorithm can be stored, with the aid of which the maximum tightening torque M _{Max can} be calculated as an estimated maximum tightening torque M * _Max by the microcontroller ( 22 ) from the values of the measuring devices ( 13 , 55 ). 20. The apparatus according to claim 19, characterized in that the screw system ( 1 ) has a handpiece ( 3 ) with a motor-gear unit ( 6 ) which can be controlled by a base unit ( 4 ). 21. The apparatus according to claim 20, characterized in that the drive motor is designed as a DC motor. 22. The apparatus according to claim 21, characterized in that the motor is designed as a bell armature motor. 23. Device according to one of claims 20 to 22, characterized in that the handpiece ( 3 ) has a rotation angle encoder ( 7 ) for continuous measurement of the rotation angle ϑ, which is connected to the motor via a shaft. 24. The device according to claim 23, characterized in that the rotary encoder ( 7 ) is designed as an optical, incremental rotary encoder. 25. The apparatus of claim 23 or 24, characterized in that the angle encoder ( 7 ) in conjunction with an electronic evaluation unit has a rotation angle resolution of Δϑ ≦ 3.93 °. 26. Device according to one of claims 19 to 25, characterized in that the screw system ( 1 ) has a motor current measuring device ( 15 ) for indirect torque measurement. 27. The apparatus according to claim 26, characterized in that the motor current measuring device ( 15 ) has measuring electronics ( 16 ), in which the motor current I _{A is} measured via a measuring resistor ( 17 ) and the measuring signal via a measuring amplifier ( 18 ) is strengthened ver. 28. The apparatus according to claim 27, characterized in that the measurement signal is filtered and amplified via a Bessel low-pass filter ( 20 ). 29. The device according to claim 27 or 28, characterized in that the measurement signal in a rectifier ( 21 ) rectifies and is supplied as the measurement signal U _{M to} the engine control unit ( 14 ). 30. Device according to one of claims 19 to 29, characterized in that the motor control unit ( 14 ) in addition to the Mi krocontroller ( 22 ) has a related Tor controller ( 23 ) which controls the motor via a power amplifier ( 24 ) . 31. The device according to any one of claims 19 to 30, characterized in that the microcontroller ( 22 ) is connected to the motor current measuring device ( 15 ) via a first analog-digital converter ( 27 ). 32. Device according to one of claims 19 to 31, characterized in that the microcontroller ( 22 ) via a second analog-digital converter ( 28 ) with a foot switch ( 5 ) is connectable bar. 33. Device according to one of claims 19 to 32, characterized in that the microcontroller ( 22 ) via an interface ( 25 ) with a personal computer ( 2 ) can be connected. 34. Device according to one of claims 19 to 33, characterized in that the screw system ( 1 ) on a trai ningseinheit ( 29 ) can be placed. 35. Apparatus according to claim 34, characterized in that the screw system ( 1 ) via the training unit ( 29 ) can be calibrated. 36. Apparatus according to claim 35, characterized in that predetermined tightening torques M can be impressed on the screw system ( 1 ) via the training unit ( 29 ). 37. Device according to one of claims 34 to 36, characterized in that the tightening of screws can be simulated with the training unit ( 29 ). 38. Device according to one of claims 34 to 37, characterized in that the training unit ( 29 ) has a simulator ( 30 ) on which the handpiece ( 3 ) of the screw system ( 1 ) can be placed. 39. Apparatus according to claim 38, characterized in that the simulator ( 33 ) has a brake for generating a braking torque M _B. 40. Apparatus according to claim 39, characterized in that the brake is designed as a magnetic hysteresis brake ( 33 ) whose braking torque M _B depends on an excitation current I _{B of} a fixed brake magnet ( 34 ). 41. Apparatus according to claim 39 or 40, characterized in that a rotary encoder ( 35 ) via a shaft coupling ( 36 ) with a rotor shaft ( 37 ) of a rotor ( 39 ) of the brake ( 33 ) is connected. 42. Apparatus according to claim 41, characterized in that for the simulation by the braking torque M _B a corresponding screw tightening torque M can be generated as a function of the screwing angle ϑ corresponding to the specified tightening torque / angle of rotation characteristic. 43. Device according to one of claims 40 to 42, characterized in that the simulator ( 30 ) has a parking brake ( 38 ) through which a rotary movement of the rotor ( 39 ) can be prevented during a demagnetization process of the hysteresis brake ( 33 ). 44. Device according to one of claims 41 to 43, characterized in that the rotary encoder ( 35 ) is designed as an optical, in incremental rotary encoder. 45. Device according to one of claims 38 to 44, characterized in that the simulator ( 30 ) with a control device ( 31 ) is connected. 46. Apparatus according to claim 45, characterized in that the control device ( 31 ) has a central microcontroller ( 47 ) for controlling the simulator ( 30 ). 47. Apparatus according to claim 46, characterized in that the central microcontroller ( 47 ) is connected to an external program and data memory ( 48 ). 48. Device according to claim 46 or 47, characterized in that the central microcontroller ( 47 ) via a hysteresis control electronics ( 49 ) with the hysteresis brake ( 33 ) is connected. 49. Device according to one of claims 46 to 48, characterized in that the central microcontroller ( 47 ) via a demagnetization control electronics ( 46 ) with the hysteresis brake ( 33 ) is connected. 50. Device according to one of claims 46 to 49, characterized in that the central microcontroller ( 47 ) is connected to the angle encoder ( 35 ). 51. Device according to one of claims 46 to 50, characterized in that the central microcontroller ( 47 ) with a personal computer ( 2 ') can be connected.