EP0363382A1 - Ameliorations apportees a des codeurs rotatifs - Google Patents

Ameliorations apportees a des codeurs rotatifs

Info

Publication number
EP0363382A1
EP0363382A1 EP19880904253 EP88904253A EP0363382A1 EP 0363382 A1 EP0363382 A1 EP 0363382A1 EP 19880904253 EP19880904253 EP 19880904253 EP 88904253 A EP88904253 A EP 88904253A EP 0363382 A1 EP0363382 A1 EP 0363382A1
Authority
EP
European Patent Office
Prior art keywords
signal
signals
pulse train
data
rotation
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19880904253
Other languages
German (de)
English (en)
Inventor
Daniel T. Slattery
David Ernest Biddlecombe
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
SSD Ltd
Original Assignee
SSD Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by SSD Ltd filed Critical SSD Ltd
Publication of EP0363382A1 publication Critical patent/EP0363382A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
    • G01P13/00—Indicating or recording presence, absence, or direction, of movement
    • G01P13/02—Indicating direction only, e.g. by weather vane
    • G01P13/04—Indicating positive or negative direction of a linear movement or clockwise or anti-clockwise direction of a rotational movement
    • G01P13/045—Indicating positive or negative direction of a linear movement or clockwise or anti-clockwise direction of a rotational movement with speed indication
    • G—PHYSICS
    • G08—SIGNALLING
    • G08C—TRANSMISSION SYSTEMS FOR MEASURED VALUES, CONTROL OR SIMILAR SIGNALS
    • G08C19/00—Electric signal transmission systems
    • G08C19/16—Electric signal transmission systems in which transmission is by pulses
    • G08C19/28—Electric signal transmission systems in which transmission is by pulses using pulse code

Definitions

  • This invention relates to a speed measurement device for measuring the speed of rotation and/or determining the angular position of a rotating member such as, for example, a shaft.
  • a number of shaft encoders are available and are designed to provide speed and/or positional information relating to a rotating shaft in either absolute or incremental terms.
  • a shaft encoder typically comprises an encoder disc mounted on the shaft and a number of sensors mounted at a static location nearby to the encoder disc, and optionally includes simple processing electronics to amplify the outputs from the sensors prior to transmission over a transmission medium.
  • the encoder disc includes markings arranged in a predetermined pattern which may be on one or both sides of the disc. The markings are detected by the sensors which produce signals corresponding to the markings and these signals can be interpreted by a control system to gain information relating to the speed and/or position of the shaft.
  • Figure 1 shows an example of typical signals which are produced by such a shaft encoder.
  • Two pulse train signals A and B of equal frequency, but with a phase displacement of 90 are produced having a frequency proportional to the speed of rotation of the shaft.
  • the 90° phase displacement enables the direction of rotation of the shaft to be determined. For example, if A leads B the shaft may be defined to be rotating clockwise and if B leads A the shaft may be defined as rotating anti ⁇ clockwise-
  • the encoder is arranged to produce a predetermined number of A and B pulses per revolution of the shaft. In addition to the two pulses A and B the encoder may include a once per revolution marker pulse M.
  • the marker pulse M can be generated in a similar manner to the A and 3 pulses and may be used to determine when the shaft is in a predetermined position.
  • A, B and M signals dictates the best method of detection of the signals as being edge detection, that is the detection of a quick change in condition from on to off or from off to on.
  • edge detection systems are particularly sensitive to noise when the signals are transmitted over any distance. Thus, if noise is allowed to corrupt the signal, any glitches generated by noise in the signal are likely to be detected as additional pulse edges. Any additional pulse edges thus detected in the signal result in the speed as measured by the control system being incorrect; leading to inaccurate control of the shaft.
  • shielding the cables is a simple matter theoretically.
  • the power supply to a motor driving a shaft to be independent from the power supply to the signal and control system, in which case there will be separate ground planes for the motor power supply and the signalling system; and it is all too easy to inadvertently connect the cable shield to the power ground instead of the signal ground in which case the cables will not be protected by the shield but will instead be subject to additional noise injected via the shield.
  • the abovementioned techique of information transmission requires up to four twisted-pair cables all of which must be shielded to prevent corruption of the signals by electrical noise.
  • the cost of installing these cables in a system where the controller is any great distance from the shaft it is controlling is very high.
  • repeaters placed at suitable intervals along the length of the cable may be required if the length of the cables exceeds about 100m.
  • Figure 2 shows how such a data packet may be arranged during transmission.
  • the three-bit data field holds information about the three signals A, B and M.
  • the start field contains a unique -4-bit data pattern which can be identified by the controller as signifying the start of a data packet.
  • a 1-bit stop field identifies the end of the data packet.
  • This arrangement requires an 8-bit data packet to transmit 3 bits of information (the sampled values of A, B and M) .
  • the A, B and M signals must be sampled at least twice as fast as the highest frequency of the signals, i.e. greater than twice the pulse frequency corresponding to the highest speed of revolution of the shaft, if information is not to be lost.
  • the sample rate is greater than twice the frequency corresponding to ' the maximum speed of revolution of the shaft and may be greater than eight times the speed of revolution.
  • the maximum controllable speed of the shaft (N) is calculated from the equation
  • B is the Bandwidth of the transmission medium, that is the maximum data rate in
  • P is the encoder resolution in pulses per revolution of the shaft
  • the maximum controllable speed of the shaft is limited by the bandwidth of the transmission medium, the resolution of the encoder, and the bit length of the data packet.
  • a data packet transmitted over a twisted-pair cable may include additional error checking bits to compensate for any detrimental noise effects. Additional error checking bits increase the bit length of the data packet resulting in a corresponding reduction in the maximum controllable speed of the shaft.
  • the present invention aims to overcome these and associated problems by providing a secure medium for pulse transmission over which the three information signals A, B and M are transmitted and by providing an electronic circuit to transmit data relating to the three signals along the secure medium.
  • the invention provides a method of compressing the information signals into a narrower bandwidth whereby an increased amount of information may be transmitted.
  • the encoder in an encoder arrangement deriving A, B and M signals as hereinabove described, the encoder can be arranged such that there are only a limited number of A, B, M combined states so that, rather than having to process Information representative of each A, B, and M signal which requires an extended bandwidth, it is necessar»y in accordance with the invention to process only information representative of a relatively low number of states.
  • A, B and M signals can each be either "on” or “off” and the combinations available for A, B and M can have only five unique states namely 000, 010, 100, 110 and 111.
  • the embodiment thus proposes to incorporate into the shaft encoder the transmission electronics necessary for formulating a pulse length signal from the three A, B and M signals from the encoder disc sensors, and this signal can be decoded at a remote receiver simply by detecting the pulse length.
  • a fibre optic transmission is utilized between the encoder/transmitter and the remote receiver.
  • Figure 1 shows three signals A, B and M which contain information relating to the speed and direction of rotation of a rotating shaft
  • Figure 2 shows a data packet for transmitting data serially according to the prior art
  • Figure 3 shows an encoder state truth table in accordance with the invention
  • Figure *4 shows a signal format used in the transmission of state data from an encoder to a controller
  • Figure 5 shows a system according to a first embodiment of the invention
  • Figure 6 shows a schematic diagram of a custom chip included in a second embodiment of the invention.
  • the first embodiment employs fibre optic cable as a secure means of transmission, and employs an electronic transmitter circuit at the shaft encoder to multiplex the three signals onto one optical fibre and an electronic receiver circuit to demultiplex the three signals at a remote receiver.
  • the second embodiment employs a microprocessor adapted to preprocess the pulses A, B and M and derive the speed of the shaft within the encoder itself.
  • fibre optic cable may be used as a secure transmission medium between the encoder and the remote receiver.
  • the encoder can in accordance with the invention, be arranged so as to provide only five different combinations of the signals A, B and M, that is to say only five different states.
  • a and B There are four possible combinations of A and B namely 00, 01, 10 and 11. If M is gated by the electronics such that M is high only when A and B are both high, this limits the maximum number of states to the five states shown in Figure 3-
  • the start field was required to uniquely identify the beginning of the data packet. Because at least four bits were required to uniquely define the start of the data packet, at least 50 of the data packet is taken up with a start field the data in which, apart from its use in identifying the start of the packet, is redundant.
  • each data packet will contain less redundant information and will therefore be more efficient.
  • the length of the data packet can be reduced, the amount of information transmitted and hence the maximum controllable speed of the motor will increase in accordance with equation 1 discussed hereinabove.
  • Figure -4 shows a format for encoding the five states of Figure 3-
  • the format includes a single start bit which is active low to define the start of a data packet.
  • the subsequent bits in the data packet represent the state of the shaft.
  • the data frame always goes high immediately after the low start character.
  • the time for which the data frame remains high indicates the state of the shaft.
  • the end of the fifth bit period signifies the end of the data packet.
  • the need for a stop-bit to positively indicate the end of a data packet is eliminated.
  • the receiver automatically synchronises its own reference clock so that the state can be determined from the length of the data frame.
  • the format shown in Figure 4 cannot use the classic serial link decoding scheme since there is no unique start pattern for the receiving electronics to lock onto. Instead, the receiver is adapted to measure the length of the transmitted pulse and thus to determine the state of the input data. However, the system must still lock onto the start of the message frame and derive a sampling clock from the transmitted bit rate if the state Is to be correctly determined.
  • Figure 5 shows a system . which is adapted to transmit data serially along a fibre optic link using the format shown in Figure 4.
  • the system Includes an encoder module 1 which is mounted at a static location near the rotating shaft and comprises an encoder 2 operating as previously described and supplying output pulse packets to a six bit shift register 3- a receiver circuit 4 connected to a control module (not shown) located remote ⁇ ly from the encoder module, and a fibre optic link 5 along which signals from the encoder module are passed to the receiver module which decodes the signals from the encoder module and passes them to the control module for further analysis and interpretation.
  • the decoder module 4 includes a phase locked loop (PLL) circuit 6 which is used to lock onto the 0 to 1 transition of the message as the first data bit is received.
  • PLL phase locked loop
  • the receiving circuit By using a divide-by-six circuit 7 in the feed back loop of the PLL the receiving circuit generates a sampling clock which is equal in fre ⁇ quency to the bit rate and which is locked onto it. Thus, even if the transmission frequency changes the re ⁇ DCving circuit 4 will automatically compensate so that the data will always be sampled in the centre of each data bit.
  • the sampled data is collected in a six bit shift register 8.
  • the corresponding data bits in the shift register are latched into four bit latch 9 as the data is received.
  • Data from the latch is then decoded by decoder 10 back into A, 3 and M thus re ⁇ constituting the encoder waveforms.
  • a demonstration system has been built using plastic fibre optic cable.
  • the rated maximum transmission frequency of this cable is 3Mbits/Sec.
  • Using an industry standard encoder disc of 1000 pulses/rev encoder shaft speeds of 3750 RPM can be measured.
  • the encoder signals may be transferred over distances of up to 60m using the chosen fibre optic cable; longer transmission distances may be obtained by using repeaters. It should be noted that the present limit of 3750 RPM is due to the maximum transmission frequency of the chosen fibre optic cable system, using fibre optic cables with higher transmission data rates will proportionately increase the maximum speed of the encoder shaft in accordance with equation 1.
  • the second embodiment of the shaft encoder according to the invention is based on a microprocessor controlled system.
  • the pulse trains transmitted from an encoder disc and sensors mounted on a rotating shaft are normally processed at the receiving end by the control system.
  • This processing normally consists of a counting process; by counting the frequency of the received pulses over a fixed period of time a measure of speed may be obtained. If the system is adapted to count the total number of pulses during a given control operation the position of the shaft can also be determined.
  • the preprocessing of the pulses, direction detection and counting functions executed on the received signals by the control system are common to most control applications. If these functions are performed in an encoder much of the processing overheads and hardware will be removed from the control system. Furthermore, if the processed data is transmitted over a serial link, the security of that link will be enhanced and its complexity reduced since the processed signals from the encoder can be adapted to include error checking and correcting formats such as, for example, Hamming code.
  • the pre ⁇ processing and counting functions are performed in a cus ⁇ tom chip 20 which is shown schematically in Figure 6.
  • the custom chip comprises preprocessing circuitry 21 , a counter 22, two latches 23 and 24, and a control section 25.
  • the control section 25 receives instructions and the like from the micro-processor (not shown) and data is re ⁇ turned from the two latches (A and B) along a data bus 26 to the microprocessor.
  • the preprocessing unit 21 receives the three signals A, B and M, in the raw form in which they are produced, from the encoder sensors.
  • the preprocessor unit 21 is provided to change the signals from this form to a form which is acceptable to the rest of the system; this form could be for example TTL logic voltage levels.
  • the preprocessed signals are then ana ⁇ lysed logically to determine direction of rotation (DIR) and to count the total number of pulses (XOR) in the A and B signals.
  • the counter 22 is arranged to count the number of pulses in the signals A and B and to store an instantaneous indication of the direction of rotation of the shaft.
  • the microprocessor requests data from the counter 22, the data is loaded into latch A and the counter is then reset to zero.
  • the data is simultaneously transferred from latch A to the microprocessor via the data bus.
  • the. transfer sequence is transparent to the micro-processor, the transfer sequence execution being supervised by the control section 25 of the custom chip.
  • the counting capacity of the counter 22 determines how frequently it must be read by the microprocessor to prevent overflow, and thus the intervals at which it must be regularly serviced. As mentioned hereinabove, loading latch A with the data held in the counter results in the counter being reset, thus losing information relating to any previous condition of the shaft.
  • Latch B is provided to enable the microprocessor to read the instantaneous contents of the counter without the counter being reset to zero. Latch B may be used to read the counter during, for example, positioning applications. In response to a predetermined event, for example a status flag within the microprocessor resetting, latch B can be loaded with the instantaneous data in the counter.
  • the data in latch B may subsequently be used in determining the position of the shaft by acting as an offset value to the data transferred to latch A.
  • the data read from the counter via either latch A or latch B is converted into a suitable format or serial transmission, for example, the format described hereinabove with reference to Figure 4.
  • the microprocessor accumulates pulse counts and sends data packets over a fibre optic cable at predetermined intervals, whereby the frequency of transmission of the data is determined by the algorithms controlling the operation of microprocessor.
  • the use of a microprocessor in the encoder module introduces flexibility into the system and enables the encoder to be configured in different arrangements depending upon the required application.
  • the maximum speed of the shaft is not limited so significantly by the bandwidth of the transmission medium since additional algorithms can be included to provide further signal analysis so that, for example, the transmitted data contains information relating only to changes in speed of rotation, position and direction.
  • the flexibility offered by the invention as described hereinabove enables the encoder to be connected directly to a wide range of control and processing equipment from dedicated control systems to personal computers. Moreover, the invention is not limited to the control of rotating shafts and may be adapted to measure desired parameters •_ any rotating component.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Transmission And Conversion Of Sensor Element Output (AREA)
  • Arrangements For Transmission Of Measured Signals (AREA)

Abstract

Pour rendre les signaux développés par un codeur de position angulaire moins exposés aux altérations dues au bruit de transmission, la présente invention décrit un processeur de signaux disposé dans le codeur de position angulaire, qui traite les signaux bruts provenant des transducteurs du codeur de position angulaire et les transforme en un état plus approprié pour permettre une transmission sur des distances relativement longues. Dans un mode de réalisation, un codeur de position angulaire produit trois trains d'impulsions de sortie séparés fournissant des informations sur la fréquence et la phase représentatives de la vitesse de la direction de rotation ainsi que de la position instantanée d'un axe. La présente invention décrit également un processeur de signaux qui détecte des états logiques uniques existant le long des trois trains d'impulsion et produit un signal modulé en correspondance par la largeur des impulsions pour permettre une transmission, de préférence via une ligne de transmission à fibres optiques, vers un décodeur récepteur. Dans un autre mode de réalisation, les trains d'impulsions de sortie du codeur de position angulaire sont traités sous le contrôle d'un microprocesseur par un processeur de signaux disposé dans le codeur de position angulaire et un signal donné de sortie est développé dans un format adapté à une transmission à distance et comprend des moyens de contrôle et de correction d'erreurs.
EP19880904253 1987-05-15 1988-05-16 Ameliorations apportees a des codeurs rotatifs Withdrawn EP0363382A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB878711559A GB8711559D0 (en) 1987-05-15 1987-05-15 Shaft encoders
GB8711559 1987-05-15

Publications (1)

Publication Number Publication Date
EP0363382A1 true EP0363382A1 (fr) 1990-04-18

Family

ID=10617425

Family Applications (1)

Application Number Title Priority Date Filing Date
EP19880904253 Withdrawn EP0363382A1 (fr) 1987-05-15 1988-05-16 Ameliorations apportees a des codeurs rotatifs

Country Status (3)

Country Link
EP (1) EP0363382A1 (fr)
GB (2) GB8711559D0 (fr)
WO (1) WO1988009026A1 (fr)

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Also Published As

Publication number Publication date
GB8928096D0 (en) 1990-02-21
GB2226414A (en) 1990-06-27
WO1988009026A1 (fr) 1988-11-17
GB8711559D0 (en) 1987-06-17
GB2226414B (en) 1991-10-16

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