Position Transducer for Digital Systems

1. Field of the Invention

This invention relates to an improved position transducer and more particularly to method and apparatus for indicating the position of a core member moved relative to a variable differential transformer.

2. Description of the Prior Art

One type of position transducer used in the prior art is a linear voltage differential transformer (LVDT). Typically, such a sensor generates an electrical signal proportional to the displacement of the transformer core with respect to a reference position in response to an external physical motion. The transformer core is generally of high permeability material which couples AC signals induced in the transformer primary into secondary windings of the transformer. The secondary voltage is attenuated by the core position and this induced voltage is either in phase or 180.degree. out of phase with the excitation. In this manner the relative movement of the core controls the output of a modulator/demodulator network.

Normally in position transducers employing an LVDT the demodulators use precision full wave rectifiers and filters as described, for example, in U.S. Pat. No. 4,140,998. Recently the need for an LVDT in a digital system has become apparent. Since all analog to digital converters utilized in current digital systems are relatively fast, i.e., conversion of <30 .mu.S, as compared to typical LVDT excitation frequencies, the lack of adequate demodulator output filtering could lead to system instability due to positive feedback. This positive feedback results from analog to digital conversion of the demodulated and filtered LVDT secondary waveform during the peak and subsequently the valley of the ripple. Thus, the system time constant is dictated by the tolerable ripple and is usually a minimum of twice the period of the excitation frequency.

In the position transducer employing an LVDT according to the present invention a pulse signal from a clock source is converted to a sinusoidal waveform of a predetermined frequency which is required for excitation of the LVDT. LVDT secondary waveform is modulated by the relative movement of the core member of the LVDT. The LVDT primary waveform is applied to a phase shift network. The output of the phase shift network is applied to a high gain squaring circuit to generate a square wave. This square wave is utilized to trigger a one shot which provides a gating signal to the primary and secondary sample and hold circuits. The waveform from the LVDT secondary is buffered by a differential amplifier in order to obtain a waveform referenced to the system ground. This waveform is applied to the secondary sample and hold circuit which is gated by the one shot. In this manner each of the primary and secondary sample and hold circuits produces DC voltages proportional to the peak of the LVDT primary and secondary waveforms. The response of the primary and secondary sample and hold outputs to changes in the LVDT is only one period of the excitation frequency. Furthermore the system has minimal ripple caused by the droop of the sample and hold circuit. This droop can easily be limited to 1 millivolt. By utilizing a gating signal pulse of a preselected width relative to the period of the excitation frequency, the sample and hold circuits acquire the primary and secondary sinusoidal waves within 0.5% of their peak value. With this approach any error due to phase shift between LVDT primary and secondary waveforms which typically is 3% is minimized.

It therefore is the primary object of the present invention to improve position transducers.

It is another object of the invention to enhance the performance of a modulator/demodulator network to indicate the position of a movable core member relative to a variable differential transformer.

It is still another object of the present invention to reduce the response time of a variable differential transformer indicating movement of the core member relative thereto.

It is still another object of the present invention to improve system performance of a variable differential transformer with modulator/demodulator network having zero ripple.

It is still a further object of the invention to provide method and apparatus for a variable differential transformer system compatible for use with a digital system.

FIG. 1 is a block diagram of the position transducer system according to the present invention, and

FIG. 2 is a timing diagram showing some of the waveforms that occur in the system shown in FIG. 1.

Referring now to FIG. 1 there is shown a block diagram of the position transducer having a primary 14 and a secondary 16. Movable member 10 varies the secondary 16 of the transducer with respect to the primary 14. It should be understood that transformer 12 may be a linear variable differential transformer (LVDT) or a rotary variable differential transformer (RVDT). The transducer uses a modulator/demodulator circuit which requires a stable clock signal 20, for example from a CPU clock (not shown). The signals 20 are divided in a digital counter 22 by a number N to obtain twice the appropriate LVDT excitation frequency. For purposes of this application a frequency of about 7.5 KHz has been used. The output 24 from divider 22 is supplied to a flip-flop circuit 26 to produce a precision bipolar square wave 28 at the required excitation frequency, 3.5 KHz. This square wave 28 is applied to a low pass filter 30 to produce a sinusoidal waveform 32.

Signal 32 is applied to primary 14 of LVDT 12, a primary sample and hold circuit 36, and a phase shift network 38. The phase shift network 38 shifts the phase of the primary waveform by a predetermined angle. In this particular application a phase shift of 84.degree. is desirable. The resulting phase shifted signal 40 is supplied to a squaring circuit 42 to produce a square wave 44. The falling edge of this square wave is utilized to trigger a one shot 46 to produce a pulse train 48. The width of these pulses 48 is selected so that the falling edge of these pulses occurs at the peak of the LVDT primary and secondary sinusoidal waveforms. The pulse train 48 is applied to a gate driver 50 which translates this pulse train typically TTL to a level compatible with a sample and hold MOS type switch. The resultant gating signals 52 and 54 are utilized by sample and hold circuits 36 and 46, respectively. The period of the one shot is selected to produce a pulse width which provides a gating signal for the primary sample and hold circuit 36 and a secondary sample and hold circuit 46 which will be described more fully hereinafter. For this application a pulse width of 1/14 of the period of the excitation frequency is preferred. In this manner the peak values of the primary and secondary waveforms are acquired in the primary and secondary sample and hold circuits. The output of the LVDT secondary 16 is buffered by a differential amplifier 60 to produce an output signal 62 which is acquired by the secondary sample and hold circuit 46.

The sample and hold circuits 36 and 46 thus acquire the peak values of the primary and secondary sinusoidal waveforms. In this manner DC level signals 70 and 72 are produced which are proportional to the changes occurring in the LVDT primary and secondary, respectively. The waveforms of FIG. 2 illustrate the timing relationship between the signals discussed earlier. The sample and hold outputs 70 and 72 are then digitized for use in a digital system. This may be accomplished, for example, by A to D converters whose output is read by a CPU. The CPU can read both sample and hold outputs 70 and 72 and take the ratio and determine the position accurately.

It has been shown that in this modulator/demodulator system the sample and hold circuits acquire and retain primary and secondary sinusoidal waveforms, respectively, within 0.5% of their maxima. Furthermore, any error due to phase shift between primary and secondary waveforms which is typically about 3.degree. is minimized. It has been found that by using a one shot with 2% accuracy over temperature, an overall position accuracy of 0.5% is easily attainable over the ambient temperature range -55.degree. C. to +100.degree. C. despite typical phase shift from primary to secondary of 3.degree..

The above described invention provides an improved position transducer using an LVDT with the described modulator/demodulator network to obtain optimal response to position changes. In contrast to the existing LVDT systems using a precision rectifier and filter, the disclosed mechanization provides a minimum ripple system whose response time is one period of the excitation frequency. Thus this invention provides significant improvement over the LVDT mechanizations which utilize precision full wave rectifiers and filters.

It should be apparent from the foregoing description that the present invention provides many advantages over the conventional position transducers. Although only a preferred embodiment is specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teaching to those skilled in the art.