Electricity: measuring and testing – Magnetic – Displacement
Patent
1988-06-15
1990-03-06
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Displacement
32420724, 324233, 336 45, 34087032, G01B 714, G01B 730, G08C 1906, H01F 2100
Patent
active
049069241
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to displacement transducers.
DESCRIPTION OF PRIOR ART
Linear variable displacement transducers (LVDT's) are well known and have numerous applications, including use in position-sensing probes for measuring work pieces on co-ordinate measuring machines and machine tools. A typical LVDT comprises three coils wound on a bobbin, and a movable slug within the bobbin. An oscialltor provides an excitation signal to one of the coils (the primary coil). The other two coils (the secondary coils) are connected in mutual opposition. The degree of coupling from the primary to the respective secondary coils depends on the position of the slug within the bobbin. The combined output of the secondary coils has an amplitude which is linearly variable with the displacement of the slug from a null position.
Whilst such arrangements are widely used, they do have disadvantages. Since the output from the secondary coils is amplitude modulated, it is necessary to process it linearly with relatively high gain in order to ensure a useful signal under all operating conditions. For the same reason, the output signal is relatively subject to noise.
An article "LVDT interface chip's functional blocks offer versatility" by Zahid Rahim, EDN, May 29th 1986, pages 159 to 168, both discloses a typical arrangement of the above type, and also describes a displacement transducer while uses a multiple coil arrangement in a variable phase circuit rather than a variable amplitude circuit. Such a circuit would minimise the above-noted disadvantages. In this arrangement, two primary coils are driven in quadrature, that is by sine and cosine signals. The sine and cosine signals are coupled to a secondary coil by the movable slug. Thus, the output of the secondary coil is the vector sum of the two iput signals, and the phase of this vector sum varies with the position of the slug. Being a phase modulated signal of constant amplitude, the output has relatively low susceptibility to the problem of noise and does not require such high gain amplification. Variable phase transducers using similar principles are described in U.S. Pat. No. 4,556,886 and in an article "Linear Position Transducer Changes Phase Instead of Amplitude" by Frank Yeaple, Design News, Nov. 5th, 1984, pages 180-181.
For such devices to work well, however, the two quadrature inputs need to be closely balanced. For example, if the amplitude of one of the inputs were to drift relative to the other, this would cause a phase change at the output, falsely indicating a movement of the slug. It would also obviously be desirable, if possible, to use a less complicated coil arrangement.
U.S. Pat. No. 4,112,365 and European Patent Application No. 169,633 show arrangements in which an oscillator drives a filter network containing a single coil and a capacitor. A movable coupling member varies the impedance of the coil in dependence on displacement, and thus causes a phase shift in the input signal from the oscillator. The phase shift can be detected to indicate displacement. However, the phase shift is non-linear with displacement, unless a specially shaped coupling member is used. Also, the filter network is just a simple resonant circuit, which means that as the coupling member is moved, the amplitude response of the network to the frequency of the oscillator varies appreciably.
SUMMARY OF THE INVENTION
The present invention provides a displacement transducer comprising an oscillator; a coil; an element movable in relation to the coil to vary the inductance of the coil; a network configured around the coil and having an input terminal connected to receive the output of the oscillator, the network being arranged to pass the frequency of the output of the oscillator but to give it a phase shift dependent on the position of the element;and means connected to an output terminal of the network for detecting the phase shift,
characterised in that the network includes two coils or two sections of a coil, and movement of said element varies their mu
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"Linear Position Transducer Changes Phase Instead of Amplitude" by Frank Yeaple, Design News, Nov. 5, 1984, pp. 180-181.
"LVDT Interface Chip's Functional Blocks Offer Versatility" by Zahid Rahim, EDN, May 29, 1986, pp. 159-168.
Renishaw plc
Strecker Gerard R.
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