GB1597446A

GB1597446A - Counting the revolutions of the rotor of a compressed air motor

Info

Publication number: GB1597446A
Application number: GB522078A
Authority: GB
Original assignee: Desoutter Ltd
Current assignee: Air Compressors and Tools Ltd
Priority date: 1978-05-25
Filing date: 1978-05-25
Publication date: 1981-09-09

Description

(54) COUNTING THE REVOLUTIONS OF THE ROZT,OR OF A COPMRESSED AIR MOTOR (71) We, DESOUTTER LIMITED (for- merly DESoUlTER BROTHERS LIMITED), a British Company, of The Hyde, Hendon, London. NW9 6ND, 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:- This invention relates to a compressed air motor having an arrangement for counting the revolutions of the rotor of the motor.

There have recently been devolep a number of methods of increasing the accuracy with which pneumatic tools tighten threaded fasteners. A number of these methods (for example that shown in U.S. Patent 3,939,920) require an indication of the number of revo lotions made by the motor rotor as part of the information indicating that the desired final tension has been achieved.

The rotors of compressed air motors conventionally consist of a metal body having slots cut in it. Non-metalic vanes are received in the slots and move in and out as the rotor rotates. This rotor configuration makes it possible to use a proximity detector detecting the number of rotor slots passing the detector, from which the number of revolutions can be counted. At the present time, inductive types of proximity deetctors have ben used for this purpose. Such detectors are not ideal, because when the rotor slows down, the signal produced by the detector cannot be distinguished through the inevitable noise, and it is impossible to count the last revolutions of the rotor as the motor decelerates and stops. However the number of revolutions occurring in this period is usually crucial in ensuring that the correct final tension has been achieved.

According to the invention, there is provided a compressed air motor having an arrangement for counting the revolutions of the rotor of the motor as the rotor decelerates to a stop, the arrangement comprising a proximity detector, arranged in proximity either to the motor rotor, or to a part which co-rotates with the rotor, to produce a signal indicating the number of revolutions made by the rotor, wherein the proximity detector is a Hall Effect device which continues to produce the said signal as the rotor slows down and until the rotor comes to a stop.

The rotor may be arranged for rotation in a working chamber about a rotor shaft, and with a portion of the rotor shaft extending out of the motor working chamber, or a part arranged outside the working chamber rotating with the rotor shaft, the proximity detector being then provided adjacent said shaft portion or said part.

Once the detector and the member which it detects are arranged outside the chamber, the apparatus is not subject to any of the disadvantages of dtector arranged in the working chamber, such as variations in working pressure and damage or loss of accuracy due to the presence of ferrite particles produced by wear when the motor is running.

In addition, detectors aranged in the working chamber are different to adjust to produce the desired detector output signal magnitude.

The Hall Effect device may be arranged between the poles of a magnet, the device being stationary and detecting rotation of a castellated end surface of the motor rotor.

Alternatively, the device may be arranged between two anti-pole pieces, with a magnet arranged to rotate with the motor rotor.

The invention also relates to a tool including a compressed air motor as set forth above. The tool may be mounted in a common assembly with other tools, and may be remotely operated.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an axial cross-section through one embodiment of a compressed air motor according to the invention; Figure 2 is a view on the line II-II from Figure 1; Figure 3 is a cross section through a second embodiment of a compressed air motor according to the invention; Figure 4 is a view of the end face of the rotor; Figure 5 is a perspective view of a pole piece; Figure 6 is a cross-section through a third embodiment of a compressed air motor according to the invention.

Referring to Figure 1, there is shown therein a pneumatic motor with a rotor 1 and a working chamber 2. The working chamber 2 has an end wall 3, and an axial extension 4 of the rotor passes through this end wall. The axial extension 4 can be machined out of the same piece of material as the rotor, or it can be made separately from the rotor and subsequently be fixed thereto. The compressed air inlet and exhaust air outlet to and from the motor working chamber are not shown, but will. be positioned conventionally. The axial extension is received in bearings 5, and supports the rotor when it is rotating.

The free end face of the extension 4 Is formed with castellations 6. The height of the castellations can for example be 1.5 mm.

A proximity detector in the form of a Hall Effect device 7 is provided. In Figure 1, only the connections 7a to the device itself can be seen, because the device is arranged between the poles of a magnet 8. The Ushaped magnet base 8a is arranged on the centre line of the motor, but the pole pieces are shaped so that their tips Sb are off-set from the centre line. This is done so that, as the rotor rotates, the two tips 8b will simultaneously be aligned with a pair of the castellations 6. It will be seen from Figure 4 that there are five castellations, and so the tips of the pole pieces have to be out of symmetry with the rotor axis if they are to align simultaneously with a pair of these castellations. In the embodiment shown, the pole pieces of the magnet are made separately from the magnet base Sa, for ease of manufacture. However it is not anticipated that there would be any difficulty in manufacturing the entire magnet in one piece. AX magnet 8 and the Hall Effect device 7 are contained within a housing part 9. When the pole nieces, device 7 and magnet base Sn have been correctly positioned, and the connections to the Hall Effect device have been made, these components will be embedded in a setting resin. This combination of components will be termed a Hall Effect pickup.

The spacing between the tips 8b of .he magnet poles and the castellations 6 is critical. If the spacing is too small, there is a risk that there may be. some contact between the pole tipes and castellations which should not be allowed to happen, and if the distance between them is too great, the signal produced by the device will be too low to be useful. The clearance should be set to between approximately -1 and 5 thousandths of an inch.

To attain this spacing with the embodi- ment of Figure 1, shims of the necessary thicknes are inserted in the space 10 before the working chamber housing 2 is screwed onto the back end of the tool. In the embodiment shown in Figure 3, the housing portion 11 which contains the magnet and Hall Effect device is threadably received in the back end of the tool body and can be screwed in and out to alter the clearance. k lock nut 12 is provided to lock the housing portion 11 when the correct spacing has been reached. In Figure 1, the main air inlet to the motor is through the channel 13, and in the embodiment shown in Figure 3, the air inlet is through the junction denoted by 14.

An O-ring 15 forms a seal between the bearing cap 16 and the housing portion 11. A certain amount of air may leak from the rotor working chamber back along the axis of the motor and through the bearing into the zone of the Hall Effect device. However this pressure is not likely to be very great, and is not subject to the wide variations which occur in the working chamber itself and should not affect the device.

In operation, the axial extension 4 of the rotor rotates and the sensor remains stationary. At a certain relative position of these two parts, the two pole tips 8b will each coincide with one of the castellations 6.

When this happens, the magnetic field through the Hall Effect device drops, and the field through the extension 4 rises. A little further on in the rotation, the pole tips 8b will coincide with the open spaces between the castellations 6, and the field through the Hall Effect device will then rise. This regular increase and decrease of the field through the Hall Effect device causes the device to produce a signal in the form of a sine wave. Conventional counting methods can be used for counting the number of signal peaks which are emitted.

The Hall Effect device described produces an analog output, which is converted to a digital reading by external circuitry. As an alternative, a digital or switchable Hall Effect device could be used. In this case, the output is not a sine wave but is a definite ON or OFF signal depending on the strength of the fluctuating magnetic field through the Hall Effect device.

With the Hall Effect device, the amplitude of the signal produced is roughly constant.

It gives its maximum signal amplitude at low speeds, and will keep countingxthe rotor revolutions until the motor comes to a stop.

In the anticipated applications of the device described, it will be very important to be able to count accurately the number of revolutions of the rotor at low sped, i.e. when the motor is about to stall or a torqueresponsive clutch about to disengage. Also.

an increase in inertia of the rotor is avoided.

Inded, metal is actually removed from the rotor shaft, thus decreasing the inertia of the rotor.

It would be possible to arrange the Hall Effect device in series with the magnetic field, i.e. between a pole piece of the magnet and the magnet base.

Figure 6 shows an embodiment where a disc magnet is fixed to the end of the rotor for rotation with the rotor. Suitable disc magnets are supplied under the Trade Name "Feroba 1" by Balfour Darwins Ltd, and are istropic with one of the flat faces being magnetised with a plurality of pole pairs arranged around the circumference of the face. With this arrangement, the Hall Effect device 22 is mounted between two anti-pole pieces 23 without a permanent magnet as part of the stationary pick-up. If there are an even number of pole pairs, the tips of the anti-pole pieces will still be off-set, as shown in Figure 1. In operation, when the tips of the anti-pole pieces register with opposite poles of different pole pairs on the magnet, a magnetic field wil be set up through the Hall pick-up which will emit a corresponding signal.

Provided that the castellated part and the proximity detectors are arranged outside the working chamber, a number of different positions can be chosen. It might be possible to arrange the sensor on the gear side of the motor, rather than on the air fed side as presently shown, and to arrange the detector so as to pick up signals from a toothed gear wheel. A further possibility which we envisage is that it may be possible to produce a castellation formation on the inner race of the bearing 5, and then to make the inner race an interference fit on the shaft of the rotor 1. If this was done, it would make it possible to increase the number of castellations, because of the greater radius, and this would increase the resolution of the counter and the accuracy of the counting.

Alternatively, a large diameter plate could be fixed to the extension 4 of the rotor, so as to rotate on the left hand side of the bearing 5. Again, a larger number of castellations or other formations could be provided around the circumference of this plate.

The number of castellations 6 can be selected depending on the particular electrical circuitry to be used for counting.

WHAT WE CLAIM IS: 1. A compressed air motor having an arrangement for counting the revolutions of the rotor of the motor as the rotor decelerates to a stop, the arrangement comprising a proximity detector, arranged in proximity either to the motor rotor, or to a part which co-rotates with the rotor, to produce a signal indicating the number of revolutions made by the rotor, where in the proximity detector is a Hall Effect device which continues to produce the said signal as the rotor slows down and until the rotor comes to a stop.

2. A compressed motor as claimed in claim 1, the rotor being arranged for rotation in a working chamber about a rotor shaft, and wherein a portion of the rotor shaft extends out of the motor working chamber, or a part arranged outside the working chamber rotates with the rotor shaft, and wherein the proximity detector is provided adjacent said shaft portion or said part.

3. A compressed air motor as claimed in claim 1 or claim 2, wherein the Hall Effect device is a digital or switch able device producing an ON signal or an OFF signal.

4. A compressed air motor as claimed in claim 1 or claim 2, wherein the Hall Effect device produces an analog signal, and external electronic circuitry is provided to produce a digital count of the rotor revolutions.

5. A compressed air motor as claimed in any one of the preceding claims, wherein the Hall Effect device is arranged between the poles of a magnet, the device being stationary and detecting rotation of a castellated end surface of the rotor.

6. A compressed air motor as claimed in any one of claims 1-4, wherein the Hall Effect device is arranged between two antipole pieces, with a multi-pole disc magnet mounted on the shaft portion or on said part.

7. A compressed air motor as claimed in claim 5 or claim 6, wherein the clearance between the shaft portion or said part and the pole pieces is from 1 to 5 thousandths of an inch.

8. A compressed air motor as claimed in any preceding claim, wherein the proximity detector is carried in a threaded mounting which can be turned to vary the distance between the detector and the shaft portion or said part.

9. A compressed air motor substantially as herein described with reference to any one embodiment shown in the accompanying drawings.

10. A tool for rotating a threaded fastener including a compressed air motor as claimed in any preceding claim.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Inded, metal is actually removed from the rotor shaft, thus decreasing the inertia of the rotor. It would be possible to arrange the Hall Effect device in series with the magnetic field, i.e. between a pole piece of the magnet and the magnet base. Figure 6 shows an embodiment where a disc magnet is fixed to the end of the rotor for rotation with the rotor. Suitable disc magnets are supplied under the Trade Name "Feroba 1" by Balfour Darwins Ltd, and are istropic with one of the flat faces being magnetised with a plurality of pole pairs arranged around the circumference of the face. With this arrangement, the Hall Effect device 22 is mounted between two anti-pole pieces 23 without a permanent magnet as part of the stationary pick-up. If there are an even number of pole pairs, the tips of the anti-pole pieces will still be off-set, as shown in Figure 1. In operation, when the tips of the anti-pole pieces register with opposite poles of different pole pairs on the magnet, a magnetic field wil be set up through the Hall pick-up which will emit a corresponding signal. Provided that the castellated part and the proximity detectors are arranged outside the working chamber, a number of different positions can be chosen. It might be possible to arrange the sensor on the gear side of the motor, rather than on the air fed side as presently shown, and to arrange the detector so as to pick up signals from a toothed gear wheel. A further possibility which we envisage is that it may be possible to produce a castellation formation on the inner race of the bearing 5, and then to make the inner race an interference fit on the shaft of the rotor 1. If this was done, it would make it possible to increase the number of castellations, because of the greater radius, and this would increase the resolution of the counter and the accuracy of the counting. Alternatively, a large diameter plate could be fixed to the extension 4 of the rotor, so as to rotate on the left hand side of the bearing 5. Again, a larger number of castellations or other formations could be provided around the circumference of this plate. The number of castellations 6 can be selected depending on the particular electrical circuitry to be used for counting. WHAT WE CLAIM IS:

1. A compressed air motor having an arrangement for counting the revolutions of the rotor of the motor as the rotor decelerates to a stop, the arrangement comprising a proximity detector, arranged in proximity either to the motor rotor, or to a part which co-rotates with the rotor, to produce a signal indicating the number of revolutions made by the rotor, where in the proximity detector is a Hall Effect device which continues to produce the said signal as the rotor slows down and until the rotor comes to a stop.