DE3817098A1 - Method for the electrical representation of a physical measured variable in the form of an impedance change - Google Patents

Abstract

The invention relates to a method in which a first physical effect variable has a consequent impedance change of a measuring impedance as measured variable, simultaneous compensation being carried out of undesired, superimposed impedance changes as a result of further physical effect variables, that is to say disturbances. The method is intended to deliver all the information required for measured value acquisition with a low expenditure and a high measuring accuracy. According to the invention, it is proposed that, apart from the actual active measuring impedance which is exposed to all physical effect variables, a second, passive measuring impedance is provided. The second, passive measuring impedance is advantageously connected in series with the first and is exposed only to the disturbances. The currents flowing across the two impedances generate a voltage drop, which corresponds to an external desired value, across the passive impedance. The difference between the two voltages across the active and the passive impedance corresponds to the measured value.

Description

State of the art

For the acquisition of measured values in the form of impedance changes As a rule, the Wheatstone bridge is used. These consists of four branches or two half-bridges each two branches. In at least one half-bridge is located an active branch that is affected by the influence of physical Modulations of the impedance nominal value experiences. The non-active or passive branches provide reference quantities for the active ones Branches in the form that they are in a differential circuit for compensation of the DC component (impedance nominal value) in serve modulated signal. The exit of the bridge leads in the differential deflection of the active branches in the first Approximation proportional measuring signal.

However, the accuracy of the Wheatstone bridge is ver limited to several factors:

• 1. The Wheatstone bridge can only be supplied with an impressed voltage. The effective measuring current is thus dependent on several parameters, such. B .:
  • 1.1 of the impedance of the leads, which grows with the distance between the voltage source and measuring bridge,
  • 1.2 of the current deflection of the measuring bridge, which changes the effective impedance of the bridge,
• 2. between the degree of differential deflection and the output signal of the bridge is only in the first Approximation linear relationship, the resolution of the measurement Due to the bridge function, the signal is subject to a Damping, which manifests itself in a linearity error,
• 3. parasitic influences on the bridge such. B. under different, the degree of deflection of the individual Branches dependent effects of operating temperature. These leave over the linear comparison of the two Half bridge signals only a partial compensation of due to the disturbances caused by error.

Other disadvantages of the Wheatstone bridge, which in particular in practical use are:

• 1. the difficulty of measuring current by simple means to calibrate
• 2. the balance of the bridge offset, which is a very high Additional effort required,
• 3. the normalization of the output signal on the basis of a Gain calibration, on the differential amplifier also caused additional expenses.

Overall, they are considering all errors sources with the Wheatstone bridge achievable Meßgenauig typically at 0.3%.

The inventive method

The method according to the invention relates to the use two measuring elements in the form of impedances, either in Single circuit or operated in a different arrangement can.  

As measuring elements z. B. be used: Dehnungs strip, temperature sensor, inductive, capacitive or potentiometric displacement sensors, photosensitive elements, etc.

The inventive method is the example of the series circuit of strain gauges to a half bridge be which all are required for the measurement Information at low cost and high accuracy supplies.

In the half-bridge of pure ohmic resistors are both Branches the physical disturbances advantageous in the same Manner (eg same operating temperature), d. H. both are active receptors in terms of this effect sizes.

The optimal conversion of the measurement contained in the active branch Signals in a proportional electrical signal is there achieved by that conducted over both branches measuring currents which generate voltage drops above the passive branch which correspond to the respective external setpoints and that the difference between the voltages of the active branch and the passive branch maps the measured value and, if necessary is reinforced by an appropriate value.

The regulation of the voltage drop in the passive branch takes place circuitry in such a way that the actual value with a externally selectable setpoint is compared and that the output voltage of the control amplifier, the advantageous reinforced with virtually infinite gain factor Control deviation corresponds to the measuring branch a the reference branch imparts the same or similar measuring current.

The drawing shows a representation of the circuit in the Prin zipschaltbild.

The half-bridge is electrically connected to the transmitter via five lines. The lines 7 and 8 carry the measuring current through the half-bridge, the lines 9 to 11 transmit the potential relevant for the implementation of correspondingly high-impedance inputs of the evaluation circuit. This arrangement ensures high reliability even with long Meßlei lines.

The described method is free from system errors that Effective measurement accuracy ten times higher than that Full bridge method, is only limited by the Manufacturing tolerances and long-term drift of the measured value recording involved components.

No. function 1 external setpoint input 2 control amplifier 3 differential amplifier 4 differential amplifier 5 Transducer = active measuring branch 6 Reference sensor = passive reference branch 7, 8 Forward and return line of the measuring current loop 9, 10, 11 Measuring signals (voltage paths) 12 Measured value proportional voltage output 13 Feedback signal to the controller proportional to the voltage drop at the reference branch

Claims (5)

1. A method for electrical representation of the impedance change of a measuring impedance as a result of a first physical We kungsgröße, the measured variable and the simultaneous compensation of unwanted, superimposed impedance changes due wei terer physical effects variables, the disturbances, characterized in that in addition to the actual, active Meßimpe danz, all is exposed to physical effects variables, nor a second, passive Meßimpedanz is provided, which is advantageously connected in series with the first and exposed only to the disturbances that flows over both impedances same or similar currents that produce a voltage drop above the passive impedance, the one corresponds to the external reference value and that the difference between the two voltages above the active and the passive impedance corresponds to the measured value. 2. The method according to claim 1, characterized in that the external setpoint is adjustable within wide limits. 3. Process according to claims 1 and 2, characterized gekennzeich net, that on the change of the external setpoint the Calibration of the measuring system can be done. 4. A circuit according to claims 1 to 3, characterized gekennzeich net, that a variable gain amplifier the voltage drop at a Reference sensor with a setpoint compares the rule deviation by a preferably infinite factor ver strengthens and that the amplified signal in the series circuit from the transducer and reference transducer is fed.   5. A circuit according to claims 1 to 4, characterized gekennzeich net, that the voltage drops across the transducer and subtracted from the reference transducer via differential amplifier be and that the result of the subtraction ent the measured value speaks.