CA1170745A - Velocity sensing system - Google Patents

Abstract

Velocity Sensing System Abstract of the Disclosure A gear speed measuring system which is capable of measuring speeds down to zero, which is capable of use in a rugged environment, and which is relatively insensitive to gear-sensor spacing. The system comprises an electromagnetic sensor connected into the tank circuit of an oscillator to produce an FM signal in which the frequency shifts occur in synchronism with the rotation of a gear or the like. A detector having a reference frequency input converts the FM signal to a digital signal. A microprocessor adjusts the reference frequency as necessary to keep it between the high and low limits of the sensor output. The speed measuring system is applied to the control of the drive unit for a heavy off road vehicle such as a tractor or earth mover.

Description

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Descri~tion Velocity Sensing System Technical_Fleld This invention relates to apparatus for sens-ing the velocity of metallic parts such as gears andparticularly to a velocity sensing system in which a fre-quency modulated ~FM~ signal is generated as a result of relative motion between the monitored parts and a sensor.
Velocities are detectable over a wide range extending on the low end to zero.

Background Art A determination o e the speed and direction of motion of a rotating or linearly translating body such as a drive gear or gear rack is obtainable through the use of a number of prior art devices and techniques;
for example, electro optical devices, Hall-efEect de-~ vices and electromagnetic sensors can be used to gen-- erate pulses as the part moves past the sensor and the pulses can then be processed to yield the desired data.
Optical devices require a relatively clean environment;
Hall-effect devices are expensive and often technically complex. These factors would seem to favor the elec ro-magnetic sensors; however, a practical disadvantage of the electromagnetic sensor is its sensitivity to spacing variations between itself and the translating part, and the dependence of signal strength on the speed of the translating part; i.e., since the signal generator in the sensor is responsive to the change in magnetic flux per unit time, very low~speeds of translation render the sensor signal substantially useless in the subsequent data processor. Accordingly, electromagnetic sensors are not generally used where speeds at or near zero are to be detected. One approach to wheel or gear speed ' `
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measurement in which electromagnetic sensor disadvantages are reduced is disclosed in U.S. Patent No. 4~050,747 issued to Ruhnau et al. on September 27, 1977. That device utilizes counters and data registers responsive to positive and negative transitions of a sensor signal to provide a digital speed indication.
The present invention is directed to overcoming one or more of the problems as set forth above.
Di~closure of the Invention In one aspect of the present invention, a velocity sensing system has an electronic oscillator controlled by a tank circuit, a magnetic sensor in which the magnetic flux intensity vari~s according to the physical presence of the part, means for varying the frequency oE the oscillator output betwe~n two frequencies as the part moves relative to the sensor, a second means for producing a reference signal between the two frequencies of the oscillator output, comparing the reference frequency to the oscillator output frequency and producing one of two outputs as the oscillator output is greater than or less than the reference signal, and means for determining the velocity of the part according to the time interval between the two outputs.

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Brief Description of Drawings Fig. 1 illustrates an embodiment of the invention as applied to the measurement of gear tooth velocity.
Fig. 2 is a graphical representation of certain signal frequencies under various operating conditions of the embodiment of Fig. 1.
Fig. 3 is a plan view of a preferred electromagnetic pickup design Eor use in the motion sensor portion of the system of Fig. 1.
Fig. 4 is a block diagram of an embodiment of the invention in a speed control system for off-road vehicles.

Best Most for Carrying Oùt the Invention Fig~ 1 illustrates an embodiment of the invention utilized to méasure the peripheral velocity of teeth 16 in a rotating gear 12. The embodiment com-rises a pickup 20 placed closely adjacent the gear 12 such that when the gear 12 moves in the direction of the ~, arrow the end of the sensor body is alternately adjacent the gear teeth 16 and troughs 18. The sensor 20 com-~' prises a body 21 of non-magnetic material such as epoxy or a suitable moldable polymer holding in place a pair of ferromagnetic cores 50 and 52 which are spaced apart ~, in the direction of gear tooth motion by a distance which is substantially less than the circumferential . `

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distance between adjacent teeth 16. The body 21 of the sensor 20 also carries therein a unidirectional 1ux source in the form of a permanent magnet 54 which is oriented such that the flux pattern passes through the cores 50 and 52 and emerges from the end of the body adjacent the gear tooth proile being monitored. It will be apparent to those familiar with electromagnetic phenomena that the amount of flux actually linking the cores 50 and 52 at any given time is closely dependent upon the reluctance of the flux path which, in this case, is a function of the proximity of the gear 12 which is presumed to be constructed of a ferromagnetic material; i.e., a greater flux intensity links the cores 50 and 52 when a tooth 16 is adjacent the sensor body 20 and a lesser flux intensity links the cores when a trough 18 is immediately ad;acent the sensor end.
Core 50 is linked by a conductor 56 to ~orm a tank circuit for a conventional oscillator 5~. Simi-larly, core 52 is linked by a conductor 60 to form a tank circuit for conventional oscillator 62. Accord-ingly, the output signal fre~uencies of the oscillators 58 and 62 vary between relatively higher and lower values as the gear 12 translates past the sensor 20, the rela-tively high frequency components representing the prox-imity of a gear tooth 16 and the relatively low fre-quency components representing the proximity of a trough . 18. Again the signals which are produced by the oscil-: lators 58 and 62 can be made substantially identical but phase shifted by an angle dependent upon the physical spacing between the cores 50 and 52. For signal proces-sing purposes the physical spacing between the cores is preferably less than half of the distance between .~ adjacent teeth 16.
The variable frequency output of oscillator 58 is connected via line 64 to the first input of a ~;, phase detector 66 which forms part of the FM to digital .. ~ : ' " . -', " " '' ' ' ' ~' ,`' , s converter circuit 30. Phase detector 66 is a conventional component which may comprise, by way of example, a phase locked loop MC 14046B integrated circuit. A second input to phase detector 66 is derived from a voltage controlled oscillator 68 which produces a reference frequency signal which, for normal operating conditions, is set to the mean or average of the high and low frequency outputs of oscillator 58;
i.e., the reference frequency signal is preferably hal~-way between the high and low frequency components. The phase detector 66 compares the inputs thereto and produces an output signal which is a function of the difference between the phases of the two inputs. Thus, the output of th~ detector 66 is a signal comprising a series of very closely spaced pulses during that portion o~ the input cycle when the fre~uency on line 64 is less or more than the output frequency of the voltage controlled oscillator 68. The output signal is applied to an integrator 70 to produce a digital signal which alternates between relatively higher and lower amplitude levels representing the frequency changes in the signal from oscillator 58.
This digitized signal is connected back to the microprocessor 48 which controls the reference frequency of the voltage controlled oscillator 68 through a digital to analog converter 74 for initialization and recalibration purposes as hereinafter described. The digital signal is also connected through a latch circuit 72 to the output f for processing by subsequent circuitry in accordance with the disclosure of Fig. 4.
The phase shifted signal from oscillator 62 is connected to the FM to digital converter 34 which, like converter 30 comprises a phase detector 76, a voltage ,, , ; ;'' ' .~
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-5a~ 745 controlled oscillator 78, and an integrator 80 to produce a digital output signal f2. A latch circuit 84 is disposed in the signal line for initialization and recalibration purposes to be described. Again, the digital signal is also connected hack to the micro-processor 48 for purposes of controlling the reference frequency of voltage controlled oscillator 78 through a control channel which comprises a digital-ta-analog converter 82.

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Although the operation of the circuit of Fig. 1 is believed to be apparent from the foregoing, a brief review will now be made. As the gear 12 rotates, the presence of a tooth 16 immediately adjacent the sensor 20 results in a tuning oE
oscillator 58 to the high frequency level and, in delayed phase relation J a similar tuning of oscillator 62 to the high frequency level. The high frequency level is detected by phase detector 66 to be greater than the reference frequency from voltage controlled oscillator 68 and accordingly a high digital level is generated at the output of converter 30. As the gear continues to move, sensor 20 senses a trough 18, the tank circuit 56 retunes oscillator 58 to the low frequency component, the low frequency level is detected by frequency detector 66 to be less than the reference Erequency from oscillator 68 and, accordin~ly, a low digital level appears at the output of converter 30. As the gear 12 continues to rotate detector 66 in converter 30 continues to switch back and forth between the high and low conditions producing a digital signal, the levels of which occur at a rate which is synchronous with the rate of appearance of the gear teeth 16 adjacent the sensor 20. The signal level is in no sense dependent upon the rate at which the gear 12 translates past the sensor 20, hence there is no speed limitation imposed on the subject device by the manner in which the electromagnetic sensor 20 operates. Converter 34 functions similarly to produce a second digital signal identical to the first but shifted in phase by a small amount relative thereto.
If all of the physical and electrical para-meters of the circuit of Fig. 1 are well known it is theoretically possible to predetermine the high and low ,. . ..

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~3, 7~,E 7J~ , 68 and 78. However, as a practical matter, variations in installation procedure, temperature, gear teeth profile, sensor spacing, and other contributing factors suggest that the reference ~requencies be set on an empirical basis by means of a process which is readily carried out by the microprocessor 48 in the embodiment of Fig. 1. The primary steps in the process are now described with reference to the diagram of Fig. 2.
The overall objective of the process is to identify the high and low frequency components which are generated by oscillator 58 as ~he gear 12 passes by the end of the sensor body 20 and to set the fre-quency of the voltage controlled oscillator 68 between them. It is presumed that neither the high fre~uency value nor the low ~requency value is initially known.
The first step involves setting the voltage controlled oscillator to a very low frequency, as shown on line ~ of Fig. 2 such that it is substantially below the ~requency oE the lowest expected value of either the low or the high frequency component from oscillator 58. Under these circumstances, a high digital signal level is generated and presented to the microprocessor 48 on signal line 86. The frequency of the voltage con-trolled oscillator 68 ~fvco) is repeatedly increased by increments of about 1 kHz by an appropriate signal from microprocessor 48. The microprocessor 48 re-examines the output signal of converter 30 after each increment ~!~ to determine if the frequency from oscillator 58 ~Fig.
2-Al) ~as exceeded the fre~uency from vco 68. As shown on line B of Fig. 2, when the incremen~al increase occurring at tl raises fvco above the unknown output of oscillator 58, a transition occurs at the output of - the integrator 70. This transition identifies the loca-tion of fLO within the resolution permitted by the increment size but does not provide enough information to determine whether the identified fre~uency is fhi or , , '-, ;~ ~ , . .

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fI ~ Therefore, the identified frequency value is temporarily stored in memory locations corresponding to both fhi and f~O-The output o the voltaqe controlled oscil-lator 68 is stepped back and forth across the identifiedfre~uency (Lo) for as long as it remains unchanged.
Each such step produces a transition at the output o~
integrator 70. When the signal frequency from oscil-lator 58 changes either increasinglv or decreasingly, the incremental reciprocations of fvco produce no further transitions. The output of integrator 70 thus remains either high or low, depending on the direction of change in the signal being identified. The level at which the si~nal from integrator 70 remains deter-mines whether the signal fr~quency ~ust ~oun~ is eitherfhi or fLO; e.g, if it remains high, the signal i!3 fhO~
The output o~ oscillator 68 is now incremented or decremented toward the remaining unknown fre~uency ; rom oscillator 58. In the illustrated case t the ~' 20 remaining unknown is fhi and the persistent high signal state after t2 positively declares this to be the case.
~ Accordingly, the output from VCO is incremented until ; at t4 another transition in the output o integrator 70 occurs. This final transition identifies fhi and the value thereof is caused by microprocessor 48 to be stored in the fhi memory location, replacing the fLO
signal previously in that location. Finally, fhi and fLO are averaged and fvco is set exactly hal~-way Z
between the two freqwencies, as shown on line F of Fig.
2. Under these conditions the output of oscillator 58 alternates back and forth between fLO and fHi to produce an alternating digital signal at fl of Fig. 1.
Substantially the same method is programmed for periodic utilization in the system of Fig. 1 for recalibration purposes except that the starting point is the last stored value in the microprocessor 48 ' ~ : ' ~ :, . :, :: ' ': ' ', ' : ' ' - .

instead of the very low fre~uenc~ generated at initial calibration. This method is preferably carried out on a recurring basis and between transitions of the digital output signal Pl to account or gear eccentricities, wobble, temperature shifts and other changes in operat-ing~conditions which might otherwise disturb the out-put frequencies enough to generate false transitions.
To prevent the artificial shifts in fvco from generat-ing transitions during the recalibration process the 10microprocessor 48 latches the circuits 72 and 84 to hold the last generated digital signal level until the recalibration method is complete.
`~ Referring now to Fig. 3, the preferred form of a sensor 20 is illustrated adjacent the teeth 16 o lS a gear 12 which is adapted for rotation about a central axis 88~ The sensor 20 comprises a body in the ~orm of an epoxy bar which has been molded around a small l permanent magne~ 5g and a pair of ferrite cores, only `~ core S0 being apparent in Fig. 3. Core 50 is prefer-ably of elongated configuration with the aperture receiving winding 56 being o~fset away ~rom the terminal end of the sensor body. With this construction the sensor 20 may be set very close to the periphery of the gear teeth 16, much closer than is indicated by the relative spacing in Fig. 3, with the confidence that accidental contact due to eccentricity or shifting under load will merely wear away the elongated end of the core 50 with little or no appreciable change in magnetic characteristic and without breaking or alter-ing the close loop nature of the core 50.

~`Industrial Applicability Fig. 4 illustrates a system embodying theinvention as applied to the control of the drive unit for a heavy off road vehicle 10 such as a tractor or earth mover. The drive unit of the vehlcle 10 includes . .
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a differential 14 with a bevel gear 12 whose speed and direction of rotation is to be determined through the embodiment of the invention. Gear 12 exhibits periph-eral gear teeth 16 separated by troughs 18 to mesh with matting gears, the teeth 16 and troughs 18 representing relatively well defined areas o alternately opposite physical characteristics.
Disposed adjacent the periphery of gear 12 6 and suitably mounted on the~vehicle 10 is an electro-magnetic sensor 20 having individual pickups of which the first is connected by circuit 22 to an FM signal generator 24. The output of generator 24 is a fre-quency modulated signal which varies discxetely and abruptly between relatively higher and lower frequency lS levels as the gear 12 rotates about its axis so as to ; cause a translation o~ the gear profile past the sensor 20. In the illustrated embodiment, the presence o~ a gear tooth 16 immediately adjacent the sensor 20 results in a relatively high fre~uency output signal from FM
l 20 signal generator 24 and the presence o a trough 18 `l immediately adjacent the sensor 20 results in a rela-tively low frequency signal. The generator 24 is designed such that the shift between reguency levels ~ is relatively abrupt and generally synchronous with the `'~ 25 linear translation of the gear 12.
The second pickup in sensor 20 is connected ~- ~ by line 26 to a second FM signal generator 28, the out-.
put of which is identical to the output of signal gen-erator 24 but shifted in phase by a phase angle which is related to the physical distance between the pick-j~ ups measured along a line tangent to the periphery of gear 12 immediately in the area of the sensor 20.
The frequency modulated output signal from ~j generator 24 is connected to an FM to digital signal converter 30 which functions in the~manner of a fre .
quency detector to produce a digital output signal ~i ~

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at 32, the amplitude levels o~ which are representative of the relatively high and low frequency components of the FM signal from generator 24. In brief, the con~
verter 30 digiti2es the signal from generator 24 so that it may be processed in later circuitry to yield speed and direction data.
FM signal generator ~8 is similarly connected to a converter unit 34 which is identical to unit 30 to produce a second digitized signal at 36. This signal is identical to but phase shifted from the signal at 32. Both of the signals at 32 and 36 are connected as inputs to a speed and direction calculator 38 which is a pxior art device. ~s will be apparent to those skilled in the art, speed may be readily determirled b~
a simple process of countin~ clock pulses between transi~ions o~ the digital signal and direction may b~
determined by comparing the phase shift between t:he signals at 32 and 36. The speed and direction calcula-tor unit 38 may be interconnected with a microprocessor 48 which is a small limited capacity digital computer carried on board the vehicle 10 for speed and direction control purposes. The microprocessor 48 is also interconnected with the first and second phase con- i verter units 30 and 34 for initialization and recalibra-tion purposes to be described hereinafter in greater detail.
~ The data from the drive calculator 38 is con-; nected by a suitable multiconductor bus to a drive con~
trol unit 44 to provide control signals to the transmis-sion unit which ultimately supplies power to gear 12.
The drive control unit 44 is a feedback system in which the control signal on channel 46 drives to zero any differences between operator input commands and actual speed and/or direction signals from the unit 38.
The velocity sensor of the invention may be applied as described above to control the speed of ,'.: J

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vehicle drive comPonents. In addition it may be used to monitor gear speed in man~ other industrial applica-tions such as position control systems~ automatic mil-ling machines, die set accelerators, length control systems for tube mi:lls , conveyors and tachometers.
In addition, the invention may be applied to the count-ing and~or monitoring of gear rack teeth and discrete parts such as castings and stampings which move lin-early rather than angularly.
10It is to be understood that the invention has been disclosed with reference to illustrative embodi-- ments and that various modifications and additions will be apparent to those skilled in the art. It is to be further understood that the invention may be adaE?ted to applications other than vehicle drive systems.
Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.

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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A velocity sensing system for parts such as gear teeth having relatively well defined areas of alternately opposite physical presence comprising:
an electronic oscillator having an output and a tank circuit, said tank circuit controlling the frequency of signals on said output;
a magnetic sensor disposed adjacent the part and defining a flux path which links said part and in which the magnetic flux intensity varies according to the physical presence of the part immediately adjacent the sensor;
said tank circuit linking said flux path to vary the frequency of the oscillator output signal alternately between a high and a low frequency as the part moves relative to the sensor;
means for producing a reference signal having a frequency between the high and low frequency of the oscillator output signal;
means for comparing the oscillator output signal to the reference signal and producing digital outputs of first and second levels as the frequency of the oscillator output is greater than and less than, respectively, the frequency of the reference signal;
and, means for determining the velocity of said part relative to the sensor according to the time interval between the first and second level digital outputs.

2. Apparatus as defined in claim 1 further comprising:
means for varying the reference signal frequency according to variations in the oscillator output frequency to maintain the reference signal frequency between said high and low frequencies for substantially all operating conditions.

3. Apparatus as defined in claim 1 wherein the magnetic sensor comprises a core of ferromagnetic material, a source of flux, and a support body supporting the core and the flux source in magnetic juxtaposition; and, the tank circuit comprising a coil linking the core to vary the inductance thereof with flux passing through the core.

4. Apparatus as defined in claim 2 further including a second oscillator, sensor, and detector operatively connected for producing a second detector output signal which is identical to but phase-shifted from the first detector output signal.

5. In a motion detector for parts such as gears which have relatively well defined areas of alternately opposite physical characteristics:
signal generator means for producing an alternating signal;
sensor means adjacent said part and connected to said generator means for alternately varying said signal between higher and lower frequencies as the opposite areas of said part are adjacent said sensor means;

detector means connected to the generator means for receiving the variable frequency signal and producing a digital signal having first and second levels representing the alternately higher and lower frequencies of said signal; and, means for determining the velocity of said part relative to the sensor means according to the time interval between said first and second levels of said digital signal.

6. Apparatus as defined in claim 5 wherein the detector means comprises:
comparator means for receiving the variable and reference signals, and output means for producing a first digital level in response to the frequency of the variable signal being higher than the frequency of the reference signal and a second digital level in response to the frequency of the variable signal being lower than the frequency of the reference signal; and, said apparatus further comprising means for maintaining the frequency of the reference signal between the high and low frequency levels of the variable signals.

7. Apparatus as defined in claim 6 wherein said means for maintaining the frequency of said refer-ence signal includes a digital processor for resetting the frequency of said reference signal to compensate for shifts in the frequency of said variable signal.

8. Apparatus as defined in claim 7 further in-cluding latch means having an input and an output, the input being connected to receive the digital signal; and the latch means being responsive to a control signal from said digital processor to maintain on said output a signal equal to one of the digital levels irrespective of variations in said levels at said input.

9. A method of operating an FM velocity detector for relatively moving parts characterized by recurring areas of varying physical presence comprising the steps of:
generating a data signal which alternates between relatively low and high frequencies as the areas translate past a sensor location, generating a reference frequency signal, comparing the data and reference signals to generate an output signal having level transitions corresponding to frequency crossings between said reference and data signals, and being indicative of the velocity of the moving parts, identifying the low and high frequencies, and maintaining the reference frequency mediate the low and high frequencies.