Abstract: A rotatable member is positioned coaxial with a joint between a first element and a second element. The joint defines an angle to be measured. The rotatable member has coded information thereon. First and second sensing heads are attached to respective elements. Coded information is arranged on the rotatable member so as to periodically pass in readable proximity to the sensing heads once every revolution of the rotatable member. A decoding system is provided for each sensing head. Each decoding system includes threshold circuitry, a state machine, and a pulse generator for providing a timing strobe. A first timer uses a timing strobe from the first sensing head for continuously determining the average elapsed time for one complete revolution of the rotatable member. A second timer uses timing strobes from the first and second sensing heads to continuously determine the average elapsed time for the coded information on the rotatable member to travel from the first sensing head to the second head.

Abstract: A light source which emits light in strips is provided for illuminating a scale of a photoelectric position measuring system. The light modulated by the scale is converted into a sinusoidal scanning signal by a scanning plate. The scanning plate is a semiconductor substrate structured in strips. The width (A) of the light-emitting areas and the width (B) of the interspersed non-emitting areas of the light source is such that defined harmonics are filtered out of the scanning signal. The scanning plate is of such configuration that further harmonics are suppressed.

Abstract: A digital absolute position encoder is provided for determining absolute position of a first member with respect to a second member. The absolute position is determined within a resolution cell of L/2.sup.n, where n is an integer and L is a predetermined span. One of the members has a plurality of, m, tracks of binary indicia distributed over the predetermined span, L, where m is an integer less than n. The other member has a plurality of indicia detectors disposed on the second member for detecting the binary indicia. With such an arrangement, an encoder is provided having "a unit-distance code", yet uses less tracks than that used with a conventional Gray code encoder. Further, the encoder has a reduced number of transitions than that required on the track used with a conventional Gray code encoder. Both of these features simplify the design and fabrication for a given encoder resolution.

Abstract: A programmable encoder having an addressable display device to display an encoder pattern of fiducial marks. The programmable encoder can be programmed to display an encoder pattern on a liquid crystal display. A sensor senses the marks on the pattern and generates sensor signals indicative of the sensed pattern. A controller connected to the sensor receives the sensor signals and outputs control signals which control the movement of a machine, such as an ink jet printer. The programmable encoder can be programmed to display any number of patterns on the addressable display device, and consequently, is highly adaptable to any number of applications requiring position, speed, or acceleration control.

Abstract: The absolute encoder utilizes two or more sensors per track and the output of each sensor is modulated by the track to generate a multiphase analog signal from each track. An analog to digital converter converts each analog signal to a digital value. The multiphase cyclic digital values are combined into a single linear output per track that increases linearly from zero to a maximum over the full span of the measuring scale. Corrections are made for zero drift in the analog signals, for changes in amplitude (gain) of the analog signals and for deviations in the linear values from a perfect straight line. When measuring rotary displacement the methods disclosed may be used to combine the outputs of the geared scales.

Abstract: A light receiving portion which receives light from a light emitting portion through slits formed in a rotating disc is provided with, first, second and third photodiodes. With an output of the second photodiode as a reference value, an output of the first photodiode is compared with the reference value and pulsed by a first comparator, and an output of the third photodiode is compared with the reference value and pulsed by a second comparator. With such a light receiving apparatus, since the output (reference value) of the second photodiode similarly decreases when the outputs of the first and third photodiodes decrease due to a decrease of the light quantity of the light emitting portion, the deviation of the phase difference never occurs between outputs of the first and second comparators.

Abstract: A data output encoder capable of having its state of retaining internal error information and self-diagnostic information reset without recourse to removing power or furnishing a dedicated signal line. A reset pulse generator monitors the changing status of an externally provided output request signal. When the signal status reaches a predetermined pattern, the reset pulse generator generates reset pulses that cause an RS flip-flop circuit to reset the state in which the internal error information and self-diagnostic information are retained.

Abstract: Two beams of laser light are directed onto the surface of a rotary shaft at points (12a, 12b) spaced longitudinally along the shaft from one another. Light backscattered by the optically rough surface of the shaft is detected by means of detectors (18a, 18b) which produce output signals related to the detected intensity. Signal-processing means (20a, 20b) associated with each beam include memory means for storing a reference waveform. The signal-processing means (20a, 20b) provide a signal indicative of the phase of the detected intensity relative to a reference waveform stored in the memory means. A comparator (22) compares the output signals from the signal-processing means (20a, 20b) to provide an indication of torque transmitted through the shaft (12). Alternatively, a single beam and detector may be used to give an indication of angular position of the shaft.

Abstract: A digital position signal generator suitable for gain control and other purposes. A typical embodiment includes a rotary disk notionally divided into a plurality of sectors of the same angle about the disk axis. Some of the disk sectors have radial extensions which are detectable by photocouplers. The photocouplers are fixedly disposed about the disk axis with angular spacings each greater than the angle of each disk sector. Thus, with the rotation of the disk, the photocouplers produce binary zero and one outputs in various predetermined combinations nonsequentially representing successive predetermined angular positions of the disk. A position data generator circuit such as a microcomputer is provided which has stored thereon meaningful position data sequentially representing the successive predetermined angular positions of the disk. Inputting each output combination of the detectors, the position data generator circuit puts out a corresponding one of the meaningful position data.

Abstract: An absolute encoder is comprised of a movable member formed thereon with a plurality of slit patterns to define a plurality of tracks (T.sub.1, T.sub.2) which are bit-coded according to a digital code system such as a binary coded quaternary system indicative of absolute addresses, and which are arranged in parallel manner from a higher order to a lower order. A light source irradiates an illuminating light onto the movable member. A pair of photodetector arrays 1, 2 receive the illuminating light through the slit patterns to output detection signals (A.sub.0.sup.+ -A.sub.3.sup.-) for the respective tracks. A processing circuit processes the detection signals to produce bit signals (P.sub.0 -P.sub.3) and to decode the same to read an absolute address of the movable member. This processing circuit has an operation circuit (3-20) for logically processing lower order detection signals (A.sub.0.sup.+, A.sub.0.sup.-) obtained from the lower order track T.sub.1, with higher order detection signals (A.sub.2.sup.

Abstract: An absolute encoder has a code plate formed with an absolute pattern and an incremental pattern. A first detector detects the absolute pattern, and outputs an absolute signal. A second detector detects the incremental pattern, and outputs an incremental signal. The absolute encoder stores the absolute signal according to an external input signal, and forms a corrected absolute signal on the basis of the incremental signal input after the absolute signal is stored.

Abstract: A novel position sensing device and novel method for determining position use an electronic memory to retain a look up table for calibrated position indicia on a scale. The position indicia are preferably arranged along the scale so that at least two indicia are positioned at unique distances apart in each length segment of the scale. A reader electrically reads the at least two indicia of a segment and the electronic memory look up table allows the at least two indicia to determine the segment, position within segment, and the position relationship between the position within the segment and the reader. Preferably, the indicia are slits in a scale bar, and the reader is a light source and charge-coupled device. The position of the slits is preferably determined using more than one pixel of the charge-coupled device.

Abstract: An encoder comprises a rotatable or longitudinally displaceable body having a plurality of sawtooth-shaped regions which lie one behind the other in the direction of rotation or displacement. Each sawtooth-shaped region is separated by a steep edge that extends transversely to the direction of rotation or displacement. Upon movement of the body due to its sawtooth-shaped configuration, the distance of the surface of the sawtooth-shaped regions changes relative to a distance sensor arrangement. An evaluation circuit receives the output signals of the sensor arrangement and determines the rotation angle or the displacement of the body. The sensor arrangement comprises two distance sensors being offset relative to one another in the direction of movement by less than one sawtooth-shaped region forming distance signals, with the distance signals input to the evaluation circuit. The evaluation circuit evaluates only one of the distance signals in order to determine the angle or displacement of the body.

Abstract: The invention provides an optical encoder for use with various density code wheels. The encoder provides a geometric offset for a photodetector that is fully illuminated. A circuit offset is provided for a photodetector that is not illuminated. The encoder comprises a light source; a principal photodetector; a pair of secondary photodetectors on opposite edges, respectively, of the principal photodetector; and a code wheel having plural alternating windows and spokes which respectively transmit or block light from the light source. The code wheel is mounted between the light source and the photodetector. The principal photodetector has a unit width and a first length. The secondary photodetectors each have a half-unit width and a second length; the second length is less than the first length.

Abstract: A precision temporal pulse 10 indicating accurate rotational velocity and precise angular position of a object rotating about an axis is generated. A "perfect word" 12 binary code sequence is applied to a binary code track 16 of an encoding disk 13. Binary code bits are synchronously detected as they pass through a code reader 22, that synchronously generates a bit by bit replica of the sequence of bits detected by the reader 22. The bits are stored in a shift register 28, and are synchronously compared to a stored reference value by logic circuitry. A temporal pulse is generated when the logic circuitry has identified that the stored value is equal to the reference value.

Abstract: A position-encoded screw, and a method and system for calibrating a position-encoded screw. In one embodiment, the screw includes a threaded portion and a set of position-encoding marks arranged along a spiral path around the threaded screw portion. To calibrate the screw, a set of sensors detects each mark as the screw is incrementally advanced past the sensors, and the absolute position of the screw is independently measured using an auxiliary device such as a precision electro-optic scale. The output of the sensor set is processed to generate encoded position signals representing encoded displacements of the screw relative to a known position. A set of the independently generated absolute position signals, one corresponding to each encoded position signal, is stored in the form of a look-up table, so that each absolute position signal may be read from the table in response to a corresponding encoded position address signal.

Abstract: A position detector for use in, for example, a robot hand can detect any of a plurality of positions on a movable element. The position detector includes a coding member having a first row of absolute codes and a second row of incremental codes both formed therein in a direction of movement thereof. Positional data in binary codes are serially recorded in the first row whereas regularly formed incremental codes corresponding to respective binary codes are recorded in the second row. The position detector further includes a plurality of light emitting and receiving elements for detecting the codes of the first and second rows and a serial-parallel converter for converting serially detected binary codes of the first row to parallel rows of binary codes based upon detection signals of the second row. The binary codes in each parallel row are equal in number to recording bits of the positional data.

Abstract: A combined optical and capacitive absolute positioning encoding system uses a plurality of metallized strips deposited onto a glass scale substrate. A movable transceiver is positioned adjacent to the scale. The transceiver generates interrogating electrical an optical signals which interact with the scale. The movable transceiver also carries with it receiver mechanisms for receiving the signals after interaction with the scale. Signal processing electronics are provided to determine the relative positions of the scale and transceiver by comparing the transmitted, interrogating signals with the received signals. The system is able to determine the relative positions of the scale and transceiver even if power to the system is interrupted, or if the scale and transceiver are moved relative to one another very rapidly.

Abstract: The present invention relates to a rotary encoder for detecting a rotational angle. A rotary encoder according to the present invention comprises a rotating body provided on its surface with a plurality of digital code patterns representative of absolute angle ranges and a plurality of relative angle patterns representative of relative angles, a high-order angle data producing means (1,2,5,7) for reading the absolute angle patterns to produce high-order angle data, and a low-order angle data producing means (1,2,5,7) for reading the relative angle patterns to produce low-order angle data. The rotary encoder according to the present invention ensures medium resolution by means of the digital code patterns and attains finer resolution by means of the relative angle patterns, whereby the rotational angle can be detected with superior resolution.

Abstract: An apparatus is provided for generating a pulse train, the apparatus comprising a member having a plurality of spaced marks disposed thereon with at least one of the spaced marks being a reference mark. The member is in communication with means for generating the pulse train as the plurality of spaced marks move. Additionally, the pulse train has a periodicity defined by the same edge of two successive pulses and the plurality of spaced marks are configured so that the periodicity of the pulse train remains constant.

Abstract: Computer controlled numbering of consecutive business forms is provided for both computer controlled presses and in the form of a retrofit unit for conventional rotary presses. The retrofit unit includes a disk provided with radially oriented metallic strips of predetermined locations on the disk, and mounted on a suitable shaft of the press. The disk cooperates with a stationary transducer mounted, for example, on a stationary press wall to send signals to the control unit to fire the printer.

Abstract: A position detecting device such as a needle position detecting device which detects a needle position with the aid of a rotor. In the position detecting device, the rotor and a position detecting annular member are abutted against each other, and thus the use of screws can be eliminated, and the annular member can be readily adjusted in angular position with a tool which is inserted through the holes of the rotor and engaged with the adjusting means of the annular member.

Abstract: A relative position transducer for determining the angular position of a first member angularly movable relative to a second member includes a coded track on the first member, a read head mounted on the second member, drive means for oscillating the read head to scan over and read a limited region of the coded track, and a microcomputer for interpreting the code read by the read head as the position. A first sensor detects when the read head is in a predetermined reference position and a second sensor detects the end points and direction of the read head scan. The sensor outputs are connected to the computer for correct interpretation of the read head output. An interplation is used to improve the accuracy of the position determination.

Abstract: An electronic postage meter is disclosed which includes a plurality of print devices each individually settable to a desired position so that the print devices may print a desired postage value; a plurality of d.c. motors, one for each of the plurality of the print devices, a different one of the d.c. motors being coupled to a different one of the print devices for setting the respective print device to a respective desired position; actual position sensing means for providing signals related to the actual positions of the print devices; and a microcomputer coupled to the d.c. motors and to the actual position sensing means. The microcomputer in response to signals from the actual position sensing means, a desired postage value and programming of the microcomputer sequentially controls the d.c. motors to set the print devices to respective desired positions for printing the desired postage value.

Abstract: A position detection encoder includes a code plate having a plurality of slots formed therein, a first light emitting element and a first light receiving element disposed on one side of the code plate, and a second light emitting element and a second light receiving element disposed on another side of the code plate. A first light beam emitted from the first light emitting element and received by the second light receiving element passes simultaneously through a same one of the plurality of slits of the code plate as does a second light beam emitted from the second light emitting element and received by the first light emitting element. Detection signals having differing phases are respectively obtained from the first and second light receiving elements.

Abstract: An encoder having two encoder units. An encoder having two encoders each having means for storing offset between said two encoder units and means for reading and correcting the offset. An encoder having an encoder with means for storing amplitude difference and phase difference between the encodes and means for reading and correcting the amplitude difference and phase difference, and further having two encoder units and means for generating an offset erasing signal between the two units. An encoder having an encoder that has means for storing offset between each unit and the offset erasing signal generating means, and means for reading and correcting the offset, and further having two encoder units and means for generating an offset erasing signal between the two encoder units. An encoder has means for storing amplitude difference and phase difference between the units, and means for reading and correcting the amplitude difference and phase difference.

Abstract: A sequence of random numbers of a desired length is produced using an algorithm for shifting a binary code M bits and adding 0 or 1 so that all produced codes are different from each other. An absolute encoder of a desired resolution is provided using such a sequence of random numbers. Where a slit pattern of M-series random number codes is employed, a circuit for generating a pattern comprising a combination of the M-series random number codes and 0s is employed to provide an absolute encoder having a resolution of 2.sup.k.

Abstract: A low cost, high resolution optical encoder for a brushless DC motor comprises reflective sensors, and a movable disk cooperating with a stationary pattern to provide a pattern of reflective and non-reflective areas for the sensors that can be used to generate motor positional information as well as commutation drive signals.

Abstract: A method and apparatus for simultaneously detecting and comparing a plurality of upper patterns indicative of Gray code patterns and having greater pattern pitches and a plurality of lower patterns indicative of pure binary code patterns and having smaller pattern pitches to correct the upper pattern detection data and thereby detect an absolute position.

Abstract: The objective of this invention is to provide a compact rotary encoder with no axial play, and in order to achieve this objective, the cylindrical operating shaft is supported by the cylindrical support shaft extending up from the bottom surface of the rotary encoder base member so that the attachment portion of the control knob and the fitting portion of the cylindrical operating shaft can be made longer, whereby inclination in the control knob and play in the fitting portion of the operating shaft can be reduced while keeping the overall height of the rotary encoder small.

Abstract: A position encoder includes a plurality of encoded elements or disks, each of which has indicia representative of a bit of a five bit digital word of a Gray code. Each disk is mechanically coupled to a lever or movable member. A sensor is positioned adjacent to each disk so that the sensors produce digital signals in response to the indicia. The indicia are arranged so that if only one bit of any word is changed, then the modified word will either be a) identical to a word representing an adjacent position of the lever, or b) different from all the words which represent lever positions.

Abstract: A method of interpolating between two signals comprising the conversion of each signal into digital form and use of the converted signals to address a memory device at each address of which is pre-stored the result of interpolation for a particular combination of values of the signals.One embodiment of the method resides in a shaft angle encoder having: sensor means regulated by an encoder disc to produce two analogue output signals, analogue to digital conversion means which convert the said analogue signals, a memory device in which the said results are pre-stored and memory addressing means which use the converted signals to cause corresponding pre-stored results to be output from the memory device.

Abstract: The system unit of an IBM compatible personal computer generates a distinctive audible "Beep" signal from the system unit's own sound transducer in response to the successful decode of a barcode label in a keyboard emulation data collection device such as a barcode reader. To initiate the generation of such a Beep by the system unit, the keyboard emulation system simulates a benign error condition which does not affect the operation of any application programs resident in the system unit and which does not require any modifications to the operating system or any special device drivers for its proper operation.

Abstract: A pen-type computer input device is used to move a cursor on the display of a computer system or enter data such as of figures, letters, numerals, or the like into a computer system. The pen-type computer input device includes a shank, a ball rotatably supported on the tip end of the shank, the ball having a plurality of identifiable regions on an outer peripheral surface thereof, and a circuit including a sensor unit for detecting the identifiable regions in response to rotation of the ball, and for detecting the directions in which the ball rotates and the distances by which the ball moves two perpendicular directions in a plane, and producing an output signal representative of the directions and distances. Since the sensor unit can detect the distance by which the pen-type computer input device moves without any mechanical motion transmitting mechanism incorporated in the shank, the pen-type computer input device may be small in size and can be handled with ease.

Abstract: An incremental position encoder and method are disclosed herein for a close loop position control system, for example, for positioning a printing device in an electronic postage meter. The incremental position encoder and method includes three channels, first and second of which provide signals for coarse position determination and first or second and third of which provide signals for fine position determination.

Abstract: A shaft angle encoder is provided with a symmetrical code wheel and pattern of photodetectors for providing an index pulse indicative of shaft angular position. The code wheel has two concentric index tracks each of which is symmetrical in a circumferential direction and are symmetrical with respect to each other and also with respect to a data track for indicating increments of angular position. In an exemplary embodiment there is a pair of windows of unit width in the one index track with a spoke therebetween with a width of at least two units. A pair of lateral windows extend away from each other more than a unit width along the second track. An opaque area at least two units wide is provided between the lateral windows in the second track.

Abstract: A reflective optical rotary encoder device detects an angular speed or position of a rotatable member with a light beam applied to and reflected from a reflective optical encoder disc. The reflective optical rotary encoder disc has an annular encoder track for reflecting the light beam, the encoder track being composed of an annular pattern of angularly spaced pit units each including a plurality of radially spaced arcuate pits, and a pit-free annular focus servo track for focusing the light beam onto the encoder track, the focus servo track extending along and being disposed radially substantially centrally in the annular encoder track.

Abstract: Electrodes (6) provided on the inner periphery of a rotary plate (3) are different in number from those provided on the outer periphery. A slide knob (12) moves a contact part (5) along the plane of the rotary plate (3). Thus, the number of electric pulses generated every rotation of the rotary plate (3) when the contact part (5) is in the inner periphery is different from that when the contact part (5) is in the outer periphery.

Abstract: A method of decoding spectral data that has been encoding by modulation of a spectrum of light to produce data bits from a source using on/off and off/on transitions to represent each data bit. A start data bit with a constant width precedes each segment of light representing the data bits. The intersection of the output of spectral detectors and a hysteresis curve determines a transition through which information is obtained from the encoded data.

Abstract: A position measuring apparatus of the type that includes a measuring scale and a scanning unit is provided. The measuring scale includes a measuring graduation track and a reference mark track associated with the measuring graduation track. The reference mark track includes a plurality of reference marks, a predetermined number of which are further designated as marked or unmarked. The scanning unit provides for scanning the measuring scale and evaluating the signal produced by such scanning. The apparatus also includes an operating switch associated with the scanning unit. The operating switch provides for selection of at least two modes of operation of the scanning unit: a first mode in which all the reference marks on the reference mark track are evaluated, and a second mode in which marked or unmarked reference marks on the reference track are evaluated.

Abstract: An absolute position encoder comprising a rotary code plate for obtaining sine wave outputs of a plurality of channels each having a different number of cycles per revolution, A/D converters for respectively A/D-converting the sine wave outputs of the plurality of channels for constituting rotational angular data of the code plate, and a ROM for storing at corresponding addresses interpolated data obtained by interpolation of angular data of one wavelength of the channels into a predetermined number of divisions sequentially using outputs of the A/D converters corresponding to the channels as address data, and for storing at corresponding addresses corrected data obtained by correction of an interpolation error included in interpolated data of upper channels having small numbers of cycles based on correction address data using interpolated data of lower channels having large numbers of cycles as the correction address data, wherein an absolute position of a moving body is detected by synthesis of the interpol

Abstract: A tape twine detecting device is disclosed, in which the rotation direction of a reel base is detected on the basis of signals having waveforms of at least three different widths generated by the rotation of the reel base and a tape twine occurrence indicating signal is sent, when it is judged that the rotation is reverse.

Abstract: A measuring device is provided in which a graduated plate is scanned in at least two scanning locations. The scanning signals from the different scanning locations are transmitted to a testing circuit which determines whether the phase displacement between the scanning signals exceeds a limit value. If the limit value is exceeded, one of the scanning locations is weighted higher than the other. The weighting is implemented by increasing the components of the signals from one scanning location while decreasing the components from the signals from the other scanning location.

Abstract: An absolute position type of displacement detector employs multiple grating tracks. The pitches of the gratings in the respective tracks are set to be just slightly different so as to detect an absolute position from a small number of analog signals by a vernier. The number of tracks is set to be three or more so as to multiply the vernier, thereby enabling absolute position measurement in a wide measurement range.

Abstract: A multi-turn absolute encoder comprising a shaft angle encoder for measuring the turning angle of a rotating shaft, and a turn detector for detecting the number of turns of the rotating shaft, wherein the turn detector uses a magnetic bubble element for sequentially transferring magnetic bubbles in a pattern in accordance with a magnetic field generated by a permanent magnet turned by the shaft, and a detector to detect the number of turns of the shaft from the positions of the magnetic bubbles which are transferred in the pattern.

Abstract: A rotation angle detecting device, that outputs a position data and a control data, each having a prescribed number of bits, as a serial data. A position-data generating circuit, that includes first detecting circuits, that are turned on or off according to the angle of rotation of a rotary shaft, outputs a position data, having a prescribed number of bits, according to the angle of rotation. A state-data generating circuit, that includes second detecting circuits, that are turned on or off according to the signals from the first detecting circuits, outputs a state data, having a prescribed number of bits, that indicate the states of operation of the first detecting circuits. The state data is transformed to a control data after passing through a data selector and a control code generating circuit. The last bit of the serial data is a parity bit.

Abstract: A position sensor for supplying digital signals corresponding to the position of a movable part in non-contacting, reliable fashion. A number of individual electrodes are applied on a substrate with a layer of a-Si:H applied thereon. A common, large-area collector electrode is applied over the a-Si:H layer. A light beam is moved over the collector electrode to activate a predetermined combination of photosensitive pixels which may be photodiodes or photoresistors. These pixels are situated between the individual electrodes and the common collector electrode. The output signals of the photodiodes are in the form of a digital code that characterizes the location of the light beam.

Abstract: A non-contact type potentiometer employing a triangular wave encoder includes a triangular wave encoder for outputting a triangular wave signal consisting of a LEAD signal and a LAG signal having the phases different from each other. A pair of comparators are provided for waveforming the triangular wave signals into a pair of rectangular wave signals. An up/down signal converter is provided for converting each of the rectangular wave signals into up/down directive pulses, and an up/down counter is provided for receiving the directive pulses and outputting an absolute digital signal which is supplied to a D/A converter for converting the absolute digital signal into analog position step signals.

Abstract: A pulse generator and a disk sector boundary signal generator are implemented using an optical pattern of alternating light and dark radial lines or segments which are adhered to and rotate in unison with the rotor of a motor and are sensed by an optical sensor. The output of the optical sensor includes error conditions including sensor error, pattern error component, the wow component of the motor and the flutter component of the motor. A first pulse generating means eliminates sensor and pattern error components and the flutter component leaving only the wow component of the sensor output. A second pulse generating means that eliminates the higher frequency error components leaving pattern error components, wow components and flutter components, where the system is designed to generate flutter components with a frequency at least ten times the frequency of the wow component. The first pulses serve as disk sector boundary reference pulses.

Abstract: An optical rotary encoder includes four photosensor elements aligned in a square fashion. A first pair of diagonally aligned photosensor elements develop output signals of inverse phase with respect to each other as a slit plate rotates. The thus obtained output signals are applied to a comparator to obtain a first control signal. A second pair of diagonally aligned photosensor elements develop output signals of inverse phase with respect to each other as the slit plate rotates. The output signals from the second pair of photosensor elements are applied to another comparator to obtain a second control signal.