GB1591419A - Method of measuring the speed and/or direction of rotation of a rotatable shaft - Google Patents

Description

(54) A METHOD OF MEASURING THE SPEED AND/OR DIREGlION OF ROTATION OF A ROTATABLE SHAFT (71) We, NATIONAL MARINE SERVICE, INC., of 1750, Brentwood Boulevard, Saint Louis, Missouri 63144, United States of America; a corporation organized and existing under the laws of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and -the method by which it is to be performed, to be particularly described in and by the following statement : The present invention relates to a method of electrically measuring the speed and/or direction of rotation of a rotatable shaft utilising a plurality of magnetic elements and a pair of magnetic pickup elements.

Magnetic pickups are used with various types of electronic tachometers and counting devices for speed and/or count sensing. These pickups generate an electrical pulse each time a piece of magnetic material passes close to the pickup head.

When used in determining the speed of a rotating shaft, a gear is normally affixed to the shaft in such a manner that the gear teeth pass close to the magnetic pickup head as the shaft is rotated. Each gear tooth passing the head then generates an electrical pulse. The rotational speed of the shaft can then be determined from the knowledge of how many teeth are on the specific gear and the rate of the pulses generated. The gear teeth or other magnetic material passing the magnetic pickup are usually referred to as the magnetic exciters.

In determining the speed of a slowly turning shaft with a magnetic pickup, uniform spacing of the magnetic exciters around the shaft is desired so that a constant pulse rate is obtained when the shaft is rotating at a fixed RPM. This allows the pulse rate sample time (the period for which the pulses are counted) to be short, allowing a fast response of the electronic readout while still providing an accurate speed indication. If the exciters were not spaced-uniformly, more pulses would be counted in one sample period and less in the-next as the shaft is rotated. This uneven spacing would therefore produce a changing speed indication because of the varying pulse count, even though the shaft would be turning at a fixed RPM. Longer sample times, counting pulses for many revolutions of the shaft, reduces the errors caused by uneven exciter spacing. Longer sample times are not desirable in many applications as they proportionately slow down the response of the readout to actual changes in speed. Also, it is desirable that a fixed number of these exciters be spaced around the shaft so that the electronic readout unit can have a fixed calibration based on a fixed number of pulses per revolution of the shaft.

In the application of reading shaft RPM where accuracy, fast response and no field calibration of the readout is desired, the magnetic exciters must be of a fixed num ber with uniform spacing. Shaft diameters have a wide range of variation due to horsepower ratings of engines and specific applications. To accommodate magnetic pickups on varying size shafts, split gears of a known number of teeth have been precisionally machined and mounted to the shafts. A second current method is to affix a belt drive from the shaft to a separate pulley of the same diameter as the shaft.

This pulley then drives the magnetic exciter gear of a fixed number of teeth.

These methods require a precision gear or pulley machined and fabricated for each individual shaft diameter.

Finally, while magnetic pickups are capable of providing speed signals to tachometers, the direction of shaft rotation usually must be obtained -by different means, because the magnetic pickup is insensitive to direction of shaft rotation.

In our co-pending application No.

3102/78 (Serial No. 1591418) there is described and claimed an exciter for use with a shaft speed magnetic pickup comprising: (a) a flexible strip of material having free ends and being uniformly stretchable at least along its length; and (b) a plurality of separate magnetic elements secured to the strip along its length, the magnetic elements facing towards one side of the strip and being spaced along the strip so that they are all uniformly spaced when the strip ends are connected together.

Such co-pending application also discloses a method of measuring and indicating the speed of the rotating shaft utilising such exciter in combination with a rotatable shaft.

Such co-pending application requires the presence of a flexible strip of material which. in use, is wrapped about and secured to the periphery of the shaft.

According to the present invention there is provided a method of measuring the speed and/or direction of rotation of a rotatable shaft comprising: a) providing circumferentially about the shaft in uniformly closed spaced order, a plurality of individual magnetic elements that each generate a magnetic field; b) placing a pair of magnetic field responsive pickups adjacent the shaft in the vicinity of the arc of rotation of the magnetic elements, the pickups being spaced from each other circumferentially about the shaft a distance less than the circumferential spacing between adjacent magnetic elements, the pickups generating a pair of electric pulse signals having a phase relationship relative to each other when the magnetic elements pass closely thereby and being connected to suitable control and processing circuitry for processing the electrical pulse signals into signals representative of shaft speed and/or direction of rotation; c) driving the shaft in rotation; and d) processing the electrical pulse signals resulting from the magnetic elements passing the pickups into signals representative of shaft speed and/or direction of rotation by using the timing and phase relationship parameters of the pair of electrical pulse signals.

Preferably, the sequence of electrical pulse signals generated by the magnetic pickups is used to produce the signal representative of direction of shaft rotation.

The method of the present invention may also include processing the electrical pulse signals from the magnetic pickups to produce a visual readout indicative of shaft speed and/or rotation.

Additionally, the method of the invention may also include affixing the magnetic elements to the shaft by wrapping in tensioned condition a uniformly stretchable strip of flexible material to which the magnetic elements are affixed about the shaft, and securing the strip and magnetic elements to the shaft by at least one suitable band of relatively non-stretchable material such as metal.

The method of the present invention can be carried out on apparatus the same that disclosed in co-pending application No.

3102/78 (Serial. No. 1 591 418) except, of course, that the flexible strip need not be present.

In the accompanying drawing: Fig. 1 is a plan view of a magnetic exciter; Fig. 2 is a side elevational view of the exciter shown in Fig. 1.

Fig. 3 shows the exciter assembled to a shaft; Fig. 4 shows a view taken along line IV-IV in Fig. 3, along with a magnetic pickup and speed readout schematically illustrated; and Fig. 5 shows a shaft speed and direction sensing arrangement using the exciter in combination with a dual magnetic pickup in accordance with the present invention With reference to the drawings, Figs. 1 and 2 show the exciter 10. A flexible strip 12 that is uniformly stretchable longitudinally (left and righmt in Figs. 1 and 2) has links or bars 14 or magnetized magnetic material fastened to one face 16 thereof, the bars generating their own respective magnetic fields. Preferably strip 12 is a rubber sheet and links 14 could be adhesively bonded or embedded in the strip 12, or could otherwise be secured thereto by any convenient means. Preferably, the links 14 extend substantially across the width of the strip 12, but they do not need to be as long as they are illustrated. They are uniform in magnetic properties and are of equal dimensions, particularly along the length of the strip 12. That is to say, the links 14 are of generally uniform width in the direction along the length of strip 12.

Slots 18 are provided for receiving a small fastener such as a metal clip 20 (Fig. 4) used to hold the strip to a rotatable shaft 22, the speed of which is to be measured. Other methods of fastening the ends of the strip to the shaft are envisioned also, such as snaps, "Velcro" (Registered Trade Mark) strips, or any other form of fastener suitable for the intended purpose.

The links 14 are evenly spaced along the strip 12, and the Dosition of each end link on the strip is calculated so that when the strip is stretched and wrapped about a shaft 22 and secured thereto in assembled position, all the links, including the end links, are evenly spaced circumferentially about the shaft 22.

The strip 12, being flexible and stretchable along its length, can be wrapped about a range of shaft diameters in the manner illustrated in Figs. 3 and 4. Re gardless of the shaft diameter, the strip 12 will stretch within limits over its circumference and the links 14 will remain evenly spaced about the shaft. This is important to ensure proper signal generation at a magnetic pickup head 24 shown in Fig. 4. Pickup 24 generates an electrical pulse signal whenever a piece of magnetic material passes close to it. Stainless steel bands 26 are finally wrapped about the strip 12 and links 14 to hold the exciter securely about the shaft 22.

In Fig. 4, the magnetic pickup head 24 is connected by electrical lines 28 to a control and signal processing apparatus 30 provided with an electrical power supply 32. The output from apparatus 30 drives a digital tachometer 34 which optically displays the speed of rotation of shaft 22 by using the processed signal from apparatus 30.

Control and signal processing apparatus 30 is generally conventional, and includes usually a provision for receiving multiple inputs from multiple magnetic pickups; means for selecting specific inputs; an input gate for feeding pulse signals from pickup 24 to a counter for a precise sampling period; a sequence controller for generating required timing signals for the display unit; a clock for providing a time base; a counter for counting pulses received from the input gate during a sampling period; a storage register, comparator and dead band circuit for holding the signal value received from the counter, comparing the value in the counter to the value of the storage register, and prohibiting update of the storage register unless the difference is greater than the dead band; and a readout driver for causing the speed reading from the storage register to be displayed in the display unit.

In Fig. 5, the exciter strip 12 with links 14 spaced apart circumferentially a predetermined amount 42 is used in combination with dual magnetic pickups 36, 38.

The gap 40, which represents the circumferential spacing of pickups 36, 38 relative to the shaft 22, between the pickups is less than the circumferential space 42 between links 14. The output from pickups 36, 38 is fed to the control and processing unit 44 via electrical leads 48. A power supply 32 is provided, and the output of unit 44 is conducted to a digital tachometer 50, which also includes a display for direction of rotation of shaft 22. Control and signal processing unit 44 in this instance includes a sequence indicator (not illustrated) for determining whether the magnetic pulse resulting from links 14 passing pickup 36 leads or lags the pulse generated at pickup 38. This provides a signal that indicates whether shaft 22 is turning in one direction or the other, depending whether a lead or lag signal is processed through unit 44.

WHAT WE CLAIM IS:- 1. A method of measuring the speed and/or direction of rotation of a rotatable shaft comprising: a) providing circumferentially about the shaft in uniformly closely spaced order, a plurality of individual magnetic elements that each generate a magnetic field; b) placing a pair of magnetic field responsive pickups adjacent the shaft in the vicinity of the arc of rotation of the magnetic elements, the pickups being spaced from each other circumferentially about the shaft a distance less than the circumferential spacing between adjacent magnetic elements, the pickups generating a pair of electrical pulse signals having a phase relationship relative to each other when the magnetic elements pass closely thereby and being connected to suitable control and processing circuitry for processing the electrical pulse signals into signals representative of shaft speed and/or direction of rotation; c) driving the shaft in rotation; and d) processing the electrical pulse signals resulting from the magnetic elements passing the pickups into signals representative of shaft speed and/or direction of rotation by using the timing and phase relationship parameters of the pair of electrical pulse signals.

2. A method as claimed in Claim 1, wherein the sequence of electrical pulse signals generated by the magnetic pickups is used to produce the signal representative of direction of shaft rotation.

3. A method as claimed in Claim 2, including also processing the electrical pulse signals from the magnetic pickups to produce a visual readout indicative of shaft speed and/or rotation.

4. A method as claimed in Claim 1, including affixing the magnetic elements to the shaft by wrapping in tensioned condition a uniformly stretchable strip of flexible material to which the magnetic elements are affixed about the shaft, and securing the strip and magnetic elements to the shaft by at least one suitable band of relatively nonstretchable material such as metal.

5. A method of measuring the speed and/ or direction of rotation of a rotatable shaft, as claimed in any preceding claim, substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. gardless of the shaft diameter, the strip 12 will stretch within limits over its circumference and the links 14 will remain evenly spaced about the shaft. This is important to ensure proper signal generation at a magnetic pickup head 24 shown in Fig. 4. Pickup 24 generates an electrical pulse signal whenever a piece of magnetic material passes close to it. Stainless steel bands 26 are finally wrapped about the strip 12 and links 14 to hold the exciter securely about the shaft 22. In Fig. 4, the magnetic pickup head 24 is connected by electrical lines 28 to a control and signal processing apparatus 30 provided with an electrical power supply 32. The output from apparatus 30 drives a digital tachometer 34 which optically displays the speed of rotation of shaft 22 by using the processed signal from apparatus 30. Control and signal processing apparatus 30 is generally conventional, and includes usually a provision for receiving multiple inputs from multiple magnetic pickups; means for selecting specific inputs; an input gate for feeding pulse signals from pickup 24 to a counter for a precise sampling period; a sequence controller for generating required timing signals for the display unit; a clock for providing a time base; a counter for counting pulses received from the input gate during a sampling period; a storage register, comparator and dead band circuit for holding the signal value received from the counter, comparing the value in the counter to the value of the storage register, and prohibiting update of the storage register unless the difference is greater than the dead band; and a readout driver for causing the speed reading from the storage register to be displayed in the display unit. In Fig. 5, the exciter strip 12 with links 14 spaced apart circumferentially a predetermined amount 42 is used in combination with dual magnetic pickups 36, 38. The gap 40, which represents the circumferential spacing of pickups 36, 38 relative to the shaft 22, between the pickups is less than the circumferential space 42 between links 14. The output from pickups 36, 38 is fed to the control and processing unit 44 via electrical leads 48. A power supply 32 is provided, and the output of unit 44 is conducted to a digital tachometer 50, which also includes a display for direction of rotation of shaft 22. Control and signal processing unit 44 in this instance includes a sequence indicator (not illustrated) for determining whether the magnetic pulse resulting from links 14 passing pickup 36 leads or lags the pulse generated at pickup 38. This provides a signal that indicates whether shaft 22 is turning in one direction or the other, depending whether a lead or lag signal is processed through unit 44. WHAT WE CLAIM IS:-

1. A method of measuring the speed and/or direction of rotation of a rotatable shaft comprising: a) providing circumferentially about the shaft in uniformly closely spaced order, a plurality of individual magnetic elements that each generate a magnetic field; b) placing a pair of magnetic field responsive pickups adjacent the shaft in the vicinity of the arc of rotation of the magnetic elements, the pickups being spaced from each other circumferentially about the shaft a distance less than the circumferential spacing between adjacent magnetic elements, the pickups generating a pair of electrical pulse signals having a phase relationship relative to each other when the magnetic elements pass closely thereby and being connected to suitable control and processing circuitry for processing the electrical pulse signals into signals representative of shaft speed and/or direction of rotation; c) driving the shaft in rotation; and d) processing the electrical pulse signals resulting from the magnetic elements passing the pickups into signals representative of shaft speed and/or direction of rotation by using the timing and phase relationship parameters of the pair of electrical pulse signals.

2. A method as claimed in Claim 1, wherein the sequence of electrical pulse signals generated by the magnetic pickups is used to produce the signal representative of direction of shaft rotation.

3. A method as claimed in Claim 2, including also processing the electrical pulse signals from the magnetic pickups to produce a visual readout indicative of shaft speed and/or rotation.

4. A method as claimed in Claim 1, including affixing the magnetic elements to the shaft by wrapping in tensioned condition a uniformly stretchable strip of flexible material to which the magnetic elements are affixed about the shaft, and securing the strip and magnetic elements to the shaft by at least one suitable band of relatively nonstretchable material such as metal.

5. A method of measuring the speed and/ or direction of rotation of a rotatable shaft, as claimed in any preceding claim, substantially as hereinbefore described.