GB2340242A - A position detector - Google Patents

Description

2340242 IMPROVEMENTS RELATING TO POSITION DETECTORS This invention relates

to improvements in position detectors for measuring relative movement and/or displacement, of the type described in GB 1513567.

Accordingly, the present invention provides a position detector comprising a first member; a plurality of substantially contiguous, substantially spherical identical balls of magnetic material carried by the first member, the balls being disposed side by side in a row in point contact with one another and being constrained against movement relative to one another; a second member, the first and second members being relatively movable in a direction parallel to the line of point contact between the balls in said row, and a transducer which is carried by said second member and which comprises transmitting means for producing a periodically varying magnetic field through the row of balls, and means for sensing phase displaced variations produced in the magnetic field as a result of the relative movement between the first and second members and for producing signals denoting the relative positions of the first and second members, said sensing means comprising at least two pick-up coils which are disposed adjacent to and spaced along the row of balls and the line of point contact therebetween; wherein the ratio of the distance between the pick- up coils and the line of point contact between the balls to the diameter of the balls is in the range of 1.25 to 1.37.

When the ratio of pick-up coil height to the ball diameter falls within this range the accuracy of measurements obtained by the position detector is maximised, More preferably, the ratio of the height of the pickup coils from the line of point contact to the ball diameter is 1.28: 1.34.

most preferably, the ratio of the height of the pickup coils from the line of point contact to the ball diameter is 1.31.

Conveniently, the first member comprises a tube for containing the balls which is made from a fibre reinforced plastic material, most preferably carbon fibre reinforced plastic. Use of such materials assists in achieving the desired ratio mentioned above, particularly in small size position detectors in which the ball diameter is in the region of 5mm.

Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIGURE 1 illustrates diagrammatically one embodiment of the invention, showing only one pair of transmitter and pick-up coils for clarity; FIGURE 2 illustrates diagrammatically the arrangement of four pairs of coils.in the measuring device shown in Figure 1; FIGURE 3 illustrates diagrammatically a second embodiment of the invention, showing only one pair of transmitter and pick-up for clarity; and FIGURE 4 illustrates diagrammatically a third embodiments of the invention, which includes four pick-up coils.

In Figure 1, a ball container 1, of non-magnetic material, which is preferably be tubular, is fixedly mounted to a part 2 of a machine by a link 3. A transducer comprising several pole pieces 4 (of which only one is shown for clarity), each pole piece 4 carrying a pair of coils comprising a transmitter coil 5 and a receiver or pick-up coil 6, is fixedly connected to a second part 7 of the machine by a link 8. The parts 2 and 7 are relatively movable in the direction shown by the double-headed arrow and it is the relative displacement resulting from this relative movement which is to be measured. The container 1 houses a plurality of substantially spherical, identical, steel balls la, arranged side by side in contact with each other in a straight line which is parallel to the direction of relative movement of parts 2 and 7. The coils 5 and 6 are placed on opposite sides of the line of balls la, and the pole pieces 4 are aligned so as to execute motion relative to the line of balls la in a direction parallel to that line when the parts 2 and 7 move relative to each other. It is immaterial whether part 2 is stationary and part 7 moves, or vice versa, or whether both parts move in opposite directions and/or at different speeds, so long as relative displacement therebetween occurs parallel to the line of balls la.

The transducer employs a number of pole pieces which are periodically spaced along the line of balls la, the relative positions of these pole pieces being determined by the number of poles used and by the diameter of the balls. Each transmitter coil is supplied via lines 50 with a signal of periodic waveform, e.g sinusoidal, rectangular etc., the relationship between the phase of the signal supplied thereto with that of the signal supplied to any other transmitter coil being directly related to the relative spacing of these two transmitter coils along the row of balls. The signal supplied to each transmitter coil produces a magnetic field between that transmitter coil 5 and the associated receiver coil 6, which magnetic field is affected by the relative movement of the balls la between these two coils 5,6. The resulting variation in the magnetic field produces corresponding variations in the signal induced in the receiver coil 6. Thus, when the outputs in lines 60 from all the receiver coils are summed, the resultant signal, after filtering where necessary, is a constant amplitude sinusoidal wave signal whose phase is directly proportional to the relative displacement between parts 2 and 7 due to relative movement therebetween. The resultant signal, after suitable processing, is displayed on a readout panel in digital form which can be connected to a printer for a permanent record to be made.

An example of a four pole piece arrangement is shown in Figure 2, where the adjacent pole pieces are spaced by (2d + d/4) where d is the diameter or pitch of each ball la, and the phases of the signals fed to transmitter coils 5b, 5c and 5d are displayed relative to that of the signal fed to transmitter coil 5a by 90", 1800 and 2700 respectively and the resultant signals in receiver coils 6a, 6b, 6c and 6d are summed as described above. It should be noted that it is possible for the same signal to be fed to all the transmitter coils 5a, 5b, 5c and 5d, shown in Figure 2, any phase shift required due to the relative positions of the pole pieces along the path being applied to the output signals from the respective receiver coils before they are summed.

Figure 3 shows an alternative arrangement of the transducer and container 1 which facilitates the accurate aligning of one to the other. The container 1, as in the embodiment of Figure 1, is mounted on the part 2 by a link 3, and the arrangement and operation of the transducer with respect to the container 1 is likewise similar to that described for the embodiment of Figure 1, the difference being that in this embodiment, the pole pieces 4 of the transducer are fixed to a carriage 9 which is mounted on container 1 for sliding motion therealong. The connection between the pole piece 4 and part 7 of the machine is provided by a flexible link 10, replacing the rigid link 8 shown in Figure 1.

Referring to Figure 4, a plurality of substantially spherical, identical, steel balls la are arranged in a suitable housing 1, side by side in contact with each other in a straight line. A transducer is located around the balls la, the transducer comprising transmitter coils 5 and pick-up coils 6 which are co-axial with the transmitter coils and also co-axial with a line 10 joining the centres of the balls. The balls and the transducer are relatively movable in directions parallel to the line 10, being mounted on respective parts (not shown) of a machine which are also relatively movable in directions parallel to the line 10, and it is the relative displacement of these two machine parts which is to be measured. The manner in which the balls and the transducer are mounted on the machine parts may be similar to the mountings described above with reference to Figures 1 and 3.

The transmitter coil 5 comprises a number of transmitter coil portions 51, connected in series, the centre of each coil portion being spaced a distance d/n where d is the diameter or pitch of the balls la and n is the number of pick-up or receiver coils 6. In this embodiment, four pick-up coils 6a, 6b, Gc and 6d are provided (i.e n=4). Each pick-up coil, for example 6a, comprises a number of pick-up coil portions, for example 6a 1, 6a 2, 6a 3 etc. which are spaced apart at intervals of length d. In this embodiment, each pick- up coil portion is surrounded by a respective transmitter coil portion 5', so that the centres of adjacent pick-up coils are interleaved, the centre of pick-up coil portion 6a 1, for example, is spaced by d/4 from the centre of pick-up coil portion 6b 1.

The transmitter coil is supplied with a signal of periodic waveform, e.g. sinusoidal, rectangular, etc. which produces a magnetic field parallel to the line 10. The variation in the magnetic field resulting from relative axial movement between the balls la and the transducer produces corresponding variations in the signals induced in each pick-up coil 6a, 6b, 6c or 6d. The phase of the output signals from the pick-up coils is then adjusted in correspondence with the relative pitch displacement of the pick-up coils. Thus when the output signals are summed, after filtering where necessary, the resultant signal is a constant amplitude, sinusoidal waveform of which the phase is directly proportional to the relative displacement due to the relative movement of the 7 machine parts.

Alternative, the transmitter coil portions which surround the coil portions of each respective pick-up coil may be connected in series, and a separate signal supplied to each group of transmitter coil portions, the relative phases of the signals supplied corresponding to the pitch displacement of the respective associated pick-up coil output signals is required.

It should be noted that is possible for the coil portions of the pick-up and transmitter coils of the embodiment of Figure 4 to be produced on flexible printed circuits which are wrapped around the ball housing.

When producing position detector devices as described above for different applications it has been noted that in some cases the accuracy of measurements obtained by the device is considerably lower than in others. It was thought that a number of different factors might be responsible for this but until now it has not been known which factors are the most critical. It was initially thought that the varying magnetic properties of the individual balls la themselves could be to blame. It was also considered that the ball container 1 may have an effect depending upon its material and the uniformity of its cross- sectional wall thickness. However, extensive research has now shown that in all the above described embodiments, it is dimensions of certain components which are important in optimising the accuracy of the device.

In particular, it has been found that the ratio of the distance of the pick-up coils 6 from the line of point contact of the balls la (the pickup coil height) to the diameter of the balls la is critical in ensuring optimum accuracy. The closer that the pickup coils 6 are to the balls la, the better will be the strength of the signal induced in the coils. However, distortion of the induced signal will also increase. Therefore, it is known that a compromise between these two opposing factors must be found. However it is not yet fully understood why the actual ball diameter is also important in addition to the distance of the pick-up coils from the balls.

It has been determined that the preferred ratio of the pick-up coil height to the ball diameter is in the range of about 1.25 to about 1.37. A preferred ratio is in the range of about 1.28 to about 1.34. The most preferred ratio is 1.31.

As described above, the signal induced in the pickup coils 6 is varied by relative movement of the line of balls la which alters the magnetic field produced by the signal which is supplied to the transmitter coils 5. The maximum voltage induced in a pick-up coil 6 will occur when the coil is over the centre of a ball and the minimum voltage induced will incur when the pick-up coil is over the point at which two balls contact one another. It is the difference between the maximum and minimum induced voltages which is processed to obtain the overall induced sinusoidal wave signal. Therefore, if the difference between the minimum and maximum voltages is too small, there is a greater chance of errors occurring, leading to inaccuracy in measurements obtained by the device.

When the ratio of the pick-up coil height to the ball diameter is at the most preferred value of 1.31, the minimum signal induced has been found to be 94% of the maximum signal, leaving a useful signal of 6% for processing. However, when the ratio of pick-up coil height to ball diameter rises to 1.56 for example, the minimum voltage induced rises to 98.4% of the maximum signal leaving only 1.6% for processing, thus leading to an increased risk of errors.

Typical practical embodiments of the position detecting devices described above use a stainless steel tube for the ball container 1. As well as being non-magnetic, the ball container must be sufficiently strong and rigid to support a line of balls, ball bearings with a diameter of 12.7mm (0.511), without bending in the middle when the container itself is only supported at either end. It must also be resistant to fluids including water and industrial coolants since such devices are typically used on machine tools. Hence, stainless steel is an particularly attractive choice. However, in some applications, especially where a small device is required, for example with balls of diameter in the region of 5mm, it is difficult to obtain the desired ratio of pick-up coil height to ball diameter using a stainless steel tube. This is because, due to the presence of a former around which the pick-up coil is wound, the wall of the tube must made very thin, eg approximately 0.3mm. This reduces its strength and rigidity and it is difficult to ensure uniform wall thickness, leading to bowing or buckling of the ball container tube along its length in use.

Accordingly, an alternative is to form the ball container from a fibre reinforced plastic material, in particular carbon fibre reinforced plastic. Thin walled tubes of such material can be produced by winding the material round a mandrel which defines the internal diameter of the tube. The exterior surface is then ground to reduce the wall thickness to a desired value. Using such material is advantageous because it can easily be produced to the required dimensions but retains sufficient rigidity, strength and resistance to fluids.

Thus, the present invention provides an improved position detector device, the configuration of which is designed to ensure optimise the accuracy of measurements obtained by the device. It will be apparent to those skilled in the art that modifications to the precise arrangements described above can be made without departing from the scope of the invention.

Claims (6)

CLAIMS:

1. A position detector comprising a first member; a plurality of substantially contiguous, substantially spherical identical balls of magnetic material carried by the first member, the balls being disposed side by side in a row in point contact with one another and being constrained against movement relative to one another; a second member, the first and second members being relatively movable in a direction parallel to the line of point contact between the balls in said row, and a transducer which is carried by said second member and which comprises transmitting means for producing a periodically varying magnetic field through the row of balls, and means for sensing phase displaced variations produced in the magnetic field as a result of the relative movement between the first and second members and for producing signals denoting the relative positions of the first and second members, said sensing means comprising at least two pickup coils which are disposed adjacent to and spaced along the row of balls and the line of point contact therebetween; wherein the ratio of the distance between the pickup coils and the line of point contact between the balls to the diameter of the balls is in the range of 1.25 to 1.37.

2. A position detector as claimed in claim 1, wherein the ratio of the distance of the pickup coils from the line of point contact to the ball diameter is in the range of 1. 28 to 1.34.

3. A position detector as claimed in claim 1, wherein the ratio of the distance of the pick-up coils from the line of point contact to the ball diameter is 1.31.

4. A position detector as claimed in any of claims 1 to 3, wherein the first member comprises a tube containing the balls, the tube being made from fibre reinforced plastic.

5. A position detector as claimed in claim 4, wherein the tube is carbon fibre reinforced plastic. 10

6. A position detector substantially as hereinbefore described with reference to the accompanying drawings.