Abstract: A circuit for controlling a rotating member is provided with an input capture register which reads out the value of a free running counter at a timing of an FG (frequency generated) pulse generated in accordance with a rotation of a rotating member. A central processing unit performs an interrupt operation based on a PG (phase generated) pulse generated in accordance with a rotation of the rotating member and an interrupt operation based on the FG pulse. A pulse width modulation circuit rotates the rotating member by use of a speed error signal and a phase error signal which are obtained by the interrupt operations of the central processing unit. A rotating member driver is also provided. The central processing unit calculates the phase error signal from an output of the input capture register based on an FG pulse generated after the generation of the PG pulse.

Abstract: A motor controller of the present invention has a frequency generator which outputs, in accordance with a rotation of the motor, a predetermined number of rotational signals every time the motor makes one rotation. A period signal representative of the period of the rotational signals is calculated, and the period signal and a predetermined value are subtracted to obtain a speed error signal. The error signal is stored at a predetermined address of a memory. A filter which performs filtering by use of a plurality of error signals in the memory and a subtracter which subtracts the error signals and the contents of the memory are provided. A selecting circuit outputs the error signal when the error signal is outside a predetermined range and outputs an output of the filter when the error signal is within the predetermined range. Thereafter, the selecting circuit outputs an output signal of the subtracter. A control signal is produced from an output of the selecting circuit.

Abstract: A hydraulic pump (16) is driven by an induction motor (14) from a source of multiphase alternating current (10) of either fixed or variable frequency. The pressure is applied to a hydraulic load, which may vary, and change the pressure. A pressure sensor coupled to the pump produces pressure signals. The motor includes plural windings, typically three, associated with a number of poles. Each motor winding is driven by a different phase of controlled alternating current. A rectifier (46) coupled to the multiphase AC source generates direct current, which is filtered to minimize DC ripple voltage. A controllable multiphase inverter (54) is coupled to the rectifier and to the motor windings, for converting the direct voltage into multiphase alternating motor drive voltages of controllable amplitude and frequency.

Abstract: In a linear feedback servo control system, the motion profile for each axis is pre-filtered before it is introduced to the servo loop for that axis. The motion profile is pre-filtered by a filter having an inverse amplitude response to the amplitude response of the servo loop. Therefore, the composite amplitude response of the filter/servo loop combination is approximately ideal for all relevant frequencies. The pre-filtering is done using constant delay filters, whether high pass, low pass or hybrid, that exhibit a phase lag corresponding to a time delay that is essentially constant at all relevant frequencies. Each axis of synchronized motion is pre-filtered with constant delay filters having the same time delay constant, but selected so that the motion of each axis provides sufficient accuracy within the intended bandwidth of that particular axis. Consequently, while phase lag is not zero, there is essentially constant delay for all axes at all relevant frequencies.

Abstract: A robot control system generates an offset signal which is added to the control signal for a robot when the velocity error is small. The offset signal is, in each case, based on a previous offset signal added to the then current robot control signal which is updated regularly on discrete increments of time when the velocity error is small. Thus, with the control system when the velocity error is low adds the control signal to the then current offset signal to provide a corrected signal to control the robot. When the velocity error signal is high, the generation and addition of the offset signal is disabled.

Abstract: A servo motor control circuit for a video signal playback apparatus including a wave-shaping circuit, a speed error detector, a first filtering circuit for filtering torque ripple components of servo motor speed error signals characterized in that the gain in much higher in a low frequency band than in any other frequency band, a second filtering circuit for attenuating the low frequency gain of signals output from the first filtering circuit, a phase error detector, an adder, and a motor controller, resulting in a decrease in low frequency band phase distortion and a decrease in the time required for servo motor stabilization.

Abstract: A control unit of a direct-current motor used in a magnetic recording and reproducing device or the like, which generates a digital error signal by processing a rotational frequency signal (a rotational frequency signal and a rotational phase signal when necessary) detected from a motor to be controlled, and furthermore, obtains a digital correction signal by processing the value of the power source voltage and corrects the digital error signal using the correction signal to compensate power source voltage fluctuation.

Abstract: In a motor control system, a first servo loop for a phase system and a second servo loop for a speed system are provided the pull-in operation of the motor during the transition time period is made stable, and also the operation efficiency of the low-speed CPU is not deteriorated. In the motor control system, the first servo loop is arranged by a speed detector for outputting a voltage corresponding to a frequency of a first reference signal used to set a rotational speed of the motor, a phase comparator for comparing a phase of the first reference signal with a phase of a control signal; and an adder for adding an output of the speed detector to an output of the phase comparator.

Abstract: Method and apparatus for robust integral-pulse control of servodrives of unknown dynamics. According to the current value of the controlled object position, a microprocessor controller presets the current values of pulse duration and pulse amplitude as functions of the position control or velocity control error. The current value of the pulse period is a function of the sum of (difference between) the current values of orthogonality and robustness parameters when the object moves with acceleration (when the object moves with deceleration). The orthogonality parameter is calculated in proportion to the current value of pulse duration under the condition of orthogonality of the adjacent pulses and the Indeterminacy Principle. The robustness parameter is a function of the current object acceleration/deceleration.

Abstract: A media transport system for controlling the position and velocity of media in a document production apparatus marking engine having a media positioning system driven by a motor, includes a motion control loop to control the motor; a sensor adapted to detect the position of the media and to create a trigonometric signal characteristic of the position of the media, the trigonometric signal comprising frequency and phase components; a resolver electronics subsystem adapted to trigonometrically process the trigonometric signal to create a resolved signal; and means for comparing the resolved signal to a reference clock signal of predetermined frequency and phase to provide an error signal used to control the motor that drives the media positioning system. The trigonometric signal may include a sine portion and a cosine portion. The sine portion of the trigonometric signal is sin(.omega..sub.e *t+.phi..sub.e) and the cosine portion of the trigonometric signal is cos(.omega..sub.e *t+.phi..sub.e); where .omega..

Abstract: A carriage motor control device that uses speed determination means (3) for determining a set speed value (SN12) to be set on the basis of an aimed angle value (SN13A) indicative of an aimed rotation angle, an aimed speed value (SN13B) indicative of an aimed rotation speed and the rotation angle value, speed control means (4) for generating a speed control value (SN7) for making the rotation speed value closer to the set speed value on the basis of a difference between the rotation speed value and the set speed value, angle control means (5) for generating an angle control value (SN8) for making the rotation angle value closer to the aimed angle value on the basis of a difference between the aimed angle value and the rotation angle value, selection means (8) for selecting the speed control value as a motor drive control value when the rotation speed value is larger than a first speed value and selecting the angle control value as the motor drive control value when the rotation speed value is smaller than the

Abstract: An electric contactor for the motor of a reciprocating pump such as an oil well walking beam pump, and other cyclic loads, uses the power drawn by the load to determine the phase angle of the pump without the need for a separate phase signalling sensor. The contactor couples the electric pump motor to a power line, and has a power sensor and peak detector. The power sensor can be responsive to RMS AC power or simply to current amplitude and polarity. The motor passes through a minimum power at the extreme top and bottom of a pump stroke, and the phase position at the top of the stroke is preferably detected by sensing for the power reversal that occurs as the momentum of the pump causes regeneration by the motor. The contactor can have overcurrent and/or undervoltage protection features, can include contacts for rearranging the motor windings to vary pump operation, and can report the phase to a pump control and monitoring system, whereby process variables are assessed as a function of phase.

Abstract: A sensor such as a quartz angular rate sensor is operated in a closed-loop mode including feedback of a control signal. The sensor produces a modulated carrier frequency signal. For stable operation of the feedback loop, the control signal is obtained from the modulated carrier frequency signal by complex demodulation of the modulated signal to produce an in-phase demodulated signal and a quadrature-phase demodulated signal, and the in-phase demodulated signal and the quadrature-phase demodulated signal are filtered by a complex equalizing filter to produce an equalized in-phase signal and an equalized quadrature-phase signal. The equalized in-phase signal represents information sensed by the sensor, and phase of a demodulation reference signal is adjusted to minimize appearance of the information in the equalized quadrature-phase signal. The equalized in-phase and quadrature-phase signals are complex modulated at the carrier frequency to produce the control signal.

Abstract: A motor servo system for controlling a motor includes a first synthesizing portion which synthesizes a speed detection signal and a phase detection signal with each other to produce a synthesized signal which is in turn filtered by a low-pass filter portion and then becomes a first motor control signal. The first motor control signal is applied to an equivalent motor portion (software motor) which is included in an observer and has a transfer function of as the same as that of the motor. A detection signal which is detected by the equivalent motor portion and a synthesized signal from the synthesizing portion are compared with each other by a first comparing portion. An output signal of the first comparing portion is applied to a second comparing portion or a second synthesizing portion.

Abstract: A digital current regulator for controlling the operation of an inductance device such as an AC motor. A plurality of independently controllable insulated gate bipolar transistors are selectively enabled to vary the instantaneous phase and magnitude of current applied to an AC motor. To minimize induced back EMFs during current switching operations, a number of safeguards are utilized. Switching between adjacent states only is provided to minimize the magnitude of instantaneous current changes. A "ready blocking" function limits the maximum switching frequency to avoid significantly derating the switching device capabilities, while a "time out" feature assures system operation above at least a minimum switching frequency to further minimize the magnitude of instantaneous current changes. A time hysteresis function is provided to avoid switching device response to false feedback control signals.

Abstract: A control device for a moving body such as a focusing lens or the like is arranged to vary time intervals at which a speed control action is performed on a motor according to a driving amount required for moving the moving body to a target position by means of the motor. The arrangement enables the device to prevent an adverse effect of vibrations, etc. and to accurately accomplish position control.

Abstract: A low cost control system for precise positioning employs an a.c. motor and a variable frequency power source for the motor. When the desired position is approached, the frequency of the source is decreased to a low constant value, and the motor is stopped in timed relation to the leading edge of a feedback signal from a position responsive feedback sensor.

Abstract: A positioning control for linearly displacing an object (31) has a direct current motor (1) and a gear (2,21) for converting rotary into linear motion, a current supply (4) of current pulses having given frequency, amplitude and duration; a pole reverser; a rotatable operating element (51) coupled to an angular step generator (5) and a sensor of the sense of rotation. The pulse frequency of the current supply (4) is determined by the angular step generator (5) and the polarity of the pole changer is determined by the sensor of the sense of rotation. The characteristic curve of the speed of displacement of the object (31) as a function of the pulse frequency is kinked, in that for low pulse frequencies the speed increases proportionally slowly, and up from the level of a pulse frequency threshold, the speed increases markedly quicker. The invention is useful for microscope focussing drives.

Abstract: Where T.sub.1 is a period of a dynamic operation range of a phase comparative waveform of a motor servo apparatus, and a period T.sub.0 of a reference phase signal is measured, the reference phase signal is delayed by T.sub.3 =T.sub.0 -T.sub.1 /2 to obtain a gate signal. The dynamic operation range is started by the gate signal. Thus, the operational center of the phase system is brought in line with the dynamic operation range. The operation is made symmetrical in the dynamic operation range to achieve a rapid servo lock and a stable phase control over the motor servo apparatus, even at the start of the motor or when an external perturbation occurs.

Abstract: A waveform processing circuit for a pulse generator provided between the pulse generator and a motor control circuit, comprises a frequency dividing circuit and a control signal generation circuit. The frequency dividing circuit outputs, on the basis of an A-phase pulse signal and a B-phase pulse signal outputted from the pulse generator, a pulse signal having a duty ratio of 1:1 and being inverted in response to rise of the A-phase pulse signal and a pulse signal having a duty ratio of 1:1 and being inverted in response to fall of the A-phase pulse. The control signal generation circuit generates, on the basis of these two pulse signals, a traveling amount signal and a direction discrimination signal. Thus, the motor control circuit carries out, on the basis of the traveling amount signal and the direction discrimination signal, motor control without undergoing the influence of a duty ratio of the pulse generated from the pulse generator.

Abstract: A motor speed controller implements a speed control to phase control switch over. When the switch over occurs on initial offset between the reference speed signal phase and the current motor position phase is determined. The reference speed signal is then offset by this phase difference thereby achieving a quicker servo lock.

Abstract: A motor driving control apparatus having a function for performing an individual test operation of a servomotor driving control apparatus or an inverter apparatus by generating test operation command in the control apparatus in place of a command from an external controller, whereby both a function check and a performance check can be performed to attain an improvement in reliability and case of operation.

Abstract: A controller is disclosed which uses pulse width ("PWM") and pulse frequency ("PFM") modulated signals to control a variable to a desired value, the PWM and PFM signals generated through calculation of the difference between the variable and the desired value and then by routing either the PWM or the PFM to the final control element or elements, thereby performing the functions of a proportional and integral controller. Derivative action is also disclosed by differentiating the difference, after filtering, and using the relationship between the difference and the differential to determine the routing of the output signal to the final control element or elements.

Abstract: The present invention relates to a vibration wave motor of the type in which an electro-mechanical energy conversion element is disposed on a vibration member and a frequency voltage is applied to the element to form a travelling vibration wave on the vibration member and a moving member is driven by the vibration wave. In the present invention, when the motor has assumed a resonance or the vicinity thereof when speed adjustment is being effected with the frequency of the periodic voltage changed, the change of the frequency to any frequency exceeding the then frequency is inhibited so that the drive frequency is not changed to a value exceeding the resonance frequency.

Abstract: A device for controlling the rotational speed of a motor carries out feedback control by a speed difference control signal and a phase difference control signal. A first time period from a reference time point to a time when a newest detection pulse is outputted is calculated every time a predetermined number of detection pulses are counted by detection pulse counters. A second time period from the reference time point to the time when a command pulse, which is assigned an ordinal number equal to the ordinal number of the newest detection pulse and which thereby corresponds to the newest detection pulse, is calculated on the basis of the total number of pulses counted by the detection pulse counters in the time period from the reference time point to the time when the newest detection pulse is outputted, and the period of the command pulse.

Abstract: A speed control circuit for a servo motor for counting frequencies generated in response to a revolution of the servo motor and for converting the frequency to a control voltage of the servo motor, including a controller for receiving a basic clock signal after outputting a latch control signal every period of a frequency generator signal and for outputting a load signal following a period of the basic clock signal, a counter for counting the basic clock signal after loading an initial value once the load signal is output from the controller, a latch for latching the counted value of the counter when the latch control signal is output from the controller, a digital/analog converter for converting the counted value latched to an analog signal a, and a speed error control ouput section for performing buffer amplification on the analog signal and for outputting a as a speed error control signal the buffer amplified analog signal.

Abstract: A method of synthesizing load invariant dc drive system comprising positive armature current feedback of exactly specified nature and value of its transfer function. The system transfer function independent of load is realized while stability and dynamics of the system are controlled by additional voltage loop.

Abstract: A direct drive motor system for controlling driving of the joints of a multi-jointed robot, comprising a motor portion, a rotation detecting portion, a position control portion, a speed control portion, a driving circuit and a tuning portion, wherein the motor portion is of an inductor type, the rotation detection portion uses an optical rotary encoder or magnetic resolver, and the position control portion is arranged to control in a feedback manner the rotational position of the motor with a tertiary servo system with software. The system's driving circuit has a feedback control loop for the current of the motor coil, and a detection circuit for the current includes a small signal isolator. The tuning portion tunes the servo systems of the speed control portion and the position control portion. As a result, the system simultaneously satisfies various requirements of such a direct drive motor.

Abstract: A motor control system and related method are provided for phase and frequency locked operation of multiple spindle motors in a computer disk drive environment or the like, for coupling two or more disk drive units together to provide expanded data storage capability. The control system comprises a hybrid digital and analog network for producing a pulse width modulated control signal to control a disk drive spindle motor in a manner achieving frequency and phase locked relation with a reference signal. Multiple spindle motors are similarly driven in relation with the same reference signal, whereby the spindle motors are frequency and phase locked with respect to each other to permit, for example, parallel data transfer among the multiple disk drive units.

Abstract: A digital servo system comprising source means of a clock signal, counter means, latch means which latches a predetermined lower bits of the output of the counter means, means for obtaining a servo control output in response to the latch means, and sequencer means which is applied with a digital data of which the pulse interval or the phase is processed, and controls passing or nonpassing of the clock signal to the counter means in response to the digital data and obtains a latch pulse for the latch device.

Abstract: A servomotor drive control system in which current control of a servomotor is performed by a microprocessor on the basis of a current feedback signal of the servomotor an interrupt generating circuit interrupts the microprocessor a circuit for storing an interrupt period and a current command applied to the servomotor are also included in the system. The arrangement is such that an interrupt period is set in dependence upon the rating of the servomotor being driven and controlled.

Abstract: In a method for producing a pulsed setting voltage (v). said voltage is pulse-duration modulated with an electric control variable. The setting voltage serves to act on a setting drive having an inductance, particularly an electric motor setting drive (9) of a throttle valve (11) of an internal combustion engine. In order to obtain at the same time good periodic excitation of the setting drive as well as a low electric loss power in the setting drive, the setting voltage which is pulsed with a high clock frequency (f.sub.0) is pulse-duration modulated with a low fundamental-modulation frequency (f.sub.m) as well as a control variable.

Abstract: A tapping machining method for moving a tapping tool to a commanded position in synchronism with rotation of a spindle, to subject a workpiece to threading, and thereafter rotating the spindle in the reverse direction and withdrawing the tapping tool from the workpiece in synchronism with rotation of the spindle. The steps of the tapping machining method include moving and positioning a spindle, namely the tapping tool (4), from an initial point (Pi) to an approach point (Pr) in the depth direction (along the Z axis) while being rotated at a constant velocity. Then a pulse distribution calculation is started by using the pitch of a screw and pulses of a frequency proportional to the rotational velocity of the spindle, and threading is performed while the tapping tool is moved toward a target position (Pe) by the distributed pulses.

Abstract: A synchronism discriminating circuit comprises a phase comparator supplied with a reference signal having a reference frequency and a reference phase and with an input signal of which a synchronism with respect to the reference signal is to be discriminated, for comparing the phase of the reference signal with the phase of the input signal and for producing an output signal responsive to a phase difference between the two signals, where the reference signal is made up of a series of pulses, and a discriminating circuit supplied with the reference signal and with the output signal of the phase comparator, for counting the pulses in the reference signal during a period in which the phase difference is within a predetermined range and for producing a synchronism discrimation signal when a predetermined number or more pulses in the reference signal are continuously counted.

Abstract: A digitally controlled rectifying system used for driving a d.c. motor through antiparallel connected forward and reverse converters. The firing phase angle of the converters is shifted by 180.degree. at a time of gate switching operation for the converters so that the load current is formed to become zero, and after the gate switching operation the firing phase angle is shifted to a position at which the load current is substantially zero, whereby the time lag of control occurring at the switching operation of the converters can be minimized irrespective of the load condition.

Abstract: A method and apparatus for controlling the acceleration of the frequency at the beginning of a pulse train and the frequency at the end of the pulse train, so that a movable element with little rigidity, such as a robot arm, can be started and stopped smoothly without vibration. The method includes storing frequency function in a memory, which frequency function represents an acceleration characteristic or a deceleration characteristic, reading the frequency function out of the memory at a predetermined interval, computing a frequency at the present time, and generating a number of pulses, corresponding to the product of the computed frequency and the predetermined time interval, during the predetermined time interval.

Abstract: A control circuit is used during slow or stop-motion playback operation of a video tape device to drive video tape intermittently. In order to avoid errors in the stopping position of the tape, a pulse width modulated (PWM) signal is provided to a motor controller circuit associated with the capstan motor of the video tape device. A frequency signal generator coupled to this motor generates a frequency signal that varies with the capstan motor speed, and this frequency signal is provided to a retrigger monostable multivibrator which generates a PWM signal whose duty ratio changes in proportion to the capstan motor speed. A drive pulse generator and a brake pulse generator provide drive start and brake start signals, respectively, when it is desired to commence and stop movement of the tape.

Abstract: A digital control device includes a first controller which generates a control reference signal of a first controlled variable of a controlled system, a first frequency converter which converts said control reference signal into pulses of a frequency proportional to a magnitude thereof, a second frequency converter which converts a measured value of a second controlled variable of said controlled system into pulses of a frequency proportional to a magnitude thereof, and a second controller which obtains a difference between the output pulses of said first frequency converter and the output pulses of said second frequency converter and which controls said second controlled variable by using said difference as a control reference value.

Abstract: A catenary-like section of traveling web material hanging between a support point and a stepping motor-driven reel rotating to collect or dispense the material is maintained by regulating the stepping motor rate of rotation in accordance with a control signal output of an ultrasonic transducer operating in the pulse/echo mode to continuously monitor the distance between the low point of the catenary-like section and the transducer. A microcomputer connected between the transducer and the motor accelerates and decelerates the rate of stepping motor rotation in accordance with a predetermined program to maintain the catenary section during changes in the distance between the section low point and the transducer.

Abstract: In a control device of an AC motor wherein a torque equivalent to that of a DC motor is created by controlling the instantaneous values of its stator current, or in a vector control device, signals from a position detector such as a resolver coupled to the AC motor are utilized to form stator current instruction signals and to provide rotation position feed-back signals and rotation speed feed-back signals for the AC motor, thereby to perform precise servo-control.

Abstract: A device for controlling the phase of an output pulse sequence relative to an input pulse train of the same frequency comprises a multiplier receiving the pulse train from a frequency generator and emitting to a divider a rectangular waveform having a frequency m times that of the pulse train. The divider is controlled by a comparator to divide the frequency of the rectangular waveform by a factor of m.div.1, m-1, or m, depending on whether the actual phase difference between the pulse train and the sequence is less than, greater than, or equal to a predetermined phase angle, the actual phase difference being communicated to the comparator by a counter registering during each cycle of the pulse train a number of pulses proportional to the phase difference.

Abstract: An apparatus forming an incremental motion system is described for controllably moving a load from a first position to a second position in a forward mode to track a moving object field in response to an external reference signal. The apparatus moves the load from the second position back to the first position in a reverse mode. A motor having a rotatable shaft supplies the moving force to the lens shuttle. An optical tachometer having a rotating apertured element fixedly secured to the shaft to rotate therewith generates a signal, upon serving each aperture, in each of the forward and return modes. The signals corresponding to served apertures define feedback signals having corresponding frequencies. A forward control circuit is responsive to the frequency of the reference signal and the frequency of the feedback signal in the forward mode to provide a forward control signal to an amplifier which energizes or drives the motor.

Abstract: An apparatus is provided for controllably moving a load in a return mode back to a first or home position by a motor driven shaft, which shaft moves the load toward a second position in a forward mode to track a moving object field. An apertured fan-shaped timing disk or element is secured to the rotating shaft to move therewith. A position feedback sensor senses the angular displacement of the shaft by generating a feedback signal upon optoelectronically sensing an aperture in both the forward and return modes. An optoelectronic sensor detects the passage of the moving object field to provide a return signal after the object field has passed a predetermined position. A motor and its amplifier controllably move the load back to the first position in the return mode in response to the return signal. The inertia of the system causes the shaft to rotate to move the load to the second position in the return mode after responding to the return signal.

Abstract: A phase-sensitive transducer apparatus comprises first and second relatively movable members wherein the first relatively movable member has a plurality of windings and the second relatively movable member has a winding. Means are included for applying a first input signal to one of the windings of the first relatively movable member and a second input signal to another of the windings of the first relative movable member. An output signal is induced on the winding of the second relatively movable member. The first and second input signals are sinusoidal in nature of substantially identical frequency and peak amplitude and are phase-displaced by a predetermined amount, and the output signal is substantially constant in peak amplitude and variable in phase during relative movement of the first and second relatively movable members.

Abstract: Method and apparatus for accurately positioning a blank angular position transducer element for boring at its center. A relatively high frequency carrier signal is applied to the blank transducer element to be bored and is thereby induced in a master rotor element which is rotated at a constant speed in an operative relationship with the blank. The rotor is made deliberately eccentric with respect to its axis of rotation, and the induced signal contains a carrier signal, a component indicative of relative angular position, and a once-per-revolution eccentricity error component, if the blank element is also eccentric with respect to the axis of rotation. Error detection circuitry isolates the once-per-revolution eccentricity error component, and resolution circuitry resolves it into x and y directions for application to error display devices, or to actuators connected to move the blank transducer element automatically until the x and y components are both zero.

Abstract: A level detection system and a servo system employing the level detection system are disclosed. The level detection and servo system employs the use of a reasonably broad band-pass characteristic to avoid criticality in narrow band-pass systems. The level detection system includes an input level comparison circuit responsive to an input signal having a recurrent frequency and a reference signal. A filter having a band-pass characteristic which will pass the recurrent frequency of the input signal and its side-bands is responsive to the input level comparison circuit. The output of the filter is coupled to a second comparison circuit. The second comparison circuit is employed as a control means to drive the servo motor in the servo system.

Abstract: In a position transducer arrangement one or more AC operable position transducers are energised by the output of a variable amplitude oscillator. The output signals from the oscillator and the or each transducer are demodulated, or detected in turn by the same demodulator and the demodulated oscillator signal is compared with a reference signal to produce a difference signal. The difference signal is used to adjust the amplitude of the oscillator signal in the sense to reduce the difference signal to zero.