GB2044482A - Compensating gear transmission errors in positional control apparatus - Google Patents

Abstract

A positional control apparatus for a machine tool includes a movable member, such as a slide 10, and a means such as a motor 18, for driving said member through a transmission 13, 15. Means 17 measures the movement of slide 10 and further means 20 measures the movement at the motor 18. The measurements are compared and the result used in a control unit to correct the position of the slide 10 by movement of the motor 18. <IMAGE>

Description

SPECIFICATION Positional control apparatus for machine tools This invention relates to a positional control apparatus for a moving member on a machine tool. An example of such a moving member is a tool slide on a lathe or boring machine, the slide being movable linearly by means of a source of power such as an electric motor, the drive from the motor being transmitted to the slide through transmission means which, inter alia, converts the rotary motion of the motor into the required linear motion of the slide.

It is however to be understood that the source and the member may be capable of movement in any mode, which may be the same in both cases or which may differ, as in the example of the slide and motor referred to.

In machine tools it is well known that error tends to occur between the source and the member which it drives, this being due, in the intervening transmission means, to backlash in gears and lead screws, bending, elastic deformation or other causes. Gear trains are subject to backlash between each pair of meshing gears, and lead screws are subject to backlash inter alia, between screw and nut and also be bending and to torsional wind up.

It is also well known that the longer the transmission between the source of power and the member being driven, the greater the potential error.

Machine tools include many precautionary arrangements against the effects of these causes of error and usually it is arranged that the transmission means is positively loaded, by application of the power, so that the parts do not remain stationary but tend to move forward and backward beyond the limits of the backlash. If this movement is of too high an order, the phenomenon known as hunting occurs.

It is also accepted that the extent of the error should be measured in order to allow appropriate correction to be made. The measurement of the error in numerically controlled machines involves feeding a corrective signal to the control device which actuates the power source.

Two methods have been proposed. In one case, measurement of the error at the member being driven is involved and the other case involves measurement at the source. These are both feedback or closed loop systems.

Both systems however are compromise systems taking only indirect account of the backlash in the transmission means, which is the principle source of the error.

It is the object of the invention to provide a positional control apparatus for a machine tool in which causes of error in position between a source of power and a movable member are measured, so as to provide an accurate feedback which allows the position of the member to be corrected.

According to the invention there is provided positional control apparatus for a machine tool which includes a movable member and a source of power for driving said member, through intermediate transmission means, the apparatus including means for obtaining a measurement of movement at the source and further means for obtaining a measurement of movement at the member and a device for comparing these measurements and for influencing a control unit whereby the power source is controlled.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a simplified illustration of a machine tool slide incorporating the present invention, and Figure 2 is a diagrammatic representation of the control system for the slide.

Fig. 1 shows a tool slide 10 mounted on a lathe support 11. The slide 10 is carried on a structure 1 2 on which is journalled a lead screw 1 3 mounted in bearings 1 4. The lead screw engages a nut 1 5 which in turn is connected to the slide 1 0. This assembly is merely a simplified illustration of a tool slide for a machine tool and is not intended to be a definitive example of such a slide.

Carried on the structure 1 2 adjacent to the slide 10 is a scale 1 6 with which co-operates a linear movement measuring device 1 7.

At one end, the lead screw 1 3 is connected to an electric drive motor 1 8. The motor incorporates a brake and is connected to the lead screw through a flexible coupling 19.

An extension of the motor spindle remote from the lead screw 1 3 carries an angular movement measuring device 20.

In this example furthermore, the device 20 is connected to the motor 1 8 through a slipping clutch device 21.

Fig. 2 indicates the slide at 10 and the lead screw and nut are indicated at 13, 1 5. The electric motor 1 8 is shown mechanically connected to the lead screw and nut. Also shown are the linear and angular measuring devices 1 7 and 20 as well as the slipping clutch 21 between the motor and the angular measuring device. The clutch, being an alternative element, however is shown in dotted lines. In Fig. 2, mechanical connections are indicated by broad lines. Remaining connections are electrical. These connect the linear and angular measuring devices 1 7 and 20 with a device 22 which co-relates signals from the respective devices 1 7 and 20 providing an output signal to a control unit 23 whereby the electric motor 1 8 is controlled.Between the control unit 23 and the electric motor 1 8 is shown a positional error store unit 24 which contains memory circuits for retaining error signals for application to the electric motor to provide that it correctly controls the position of the slide 10.

The linear measuring device 1 7 may be of any convenient kind for measuring relative linear movement between the device itself and the scale 1 6 thus giving an accurate record in the form of an electrical signal of the progress of the slide 10.

Likewise, the angular measuring device 20 provides a measure in the form of an electrical signal of angular movement of the lead screw and thus of the motor 1 8.

In one example of the apparatus no slipping clutch 21 is included and the measuring device 20 therefore provides a continuous measurment of the progress of the lead screw 1 3.

Where a slipping clutch 21 is included, however, the angular measuring device is allowed only limited angular travel in oppposite directions. This may be limited by mechanical stops or electrical switches or mechanically operated fluid control valves. The clutch itself may be electro-magnetic or fluid actuated or it may be a mechanical spring loaded friction clutch.

In use, in this arrangement, if the motor 1 8 is rotating, in one direction, the angular measuring device will cease to operate when it reaches one of the limits. Consequently, the device 22 will receive a fixed signal from this source, whereas it can receive variable signals from the linear measuring device 1 7 so that only this source will have any effect upon the control unit 23 through the device 22.

if, however, the motor 1 8 is stopped but factors in the transmission system between the slide 10 and the motor 1 8 produce some movement of the motor, the angular measuring device 20 will provide a signal in proportion to the extent of such movement which will be within the zone between the possible limits on the device 20.

In this case both angular and linear measurements will be applied to the device 22 and thence to the control unit 23.

It may be desirable in some cases to ensure that the feedback signals from the linear and from the angular measuring devices 1 7 and 20 are of different order of magnitude to ensure against equality of opposing signals which may cancel one another out, thus producing no error correction signal, though one may be required.

The electrical signals may be in the form of digital pulse or of phase shift and combination and comparison of them may be by any appropriate method.

Although described in relation to a linearly movable slide driven by a rotary electric motor, the apparatus may be applicable to other mechanisms capable of other movements or combinations of movements. For example, an electric motor may drive a movable member angularly or alternatively a linear motor may be used to drive a movable member linearly or in another mode. The apparatus is principally concerned with tool slides on lathes but may also be applicable to other equivalent apparatus where error can occur between a driving source and a movable member driven from that source.

Claims (8)

1. A positional control apparatus for a machine tool including a movable member and a source of power for driving said member, through intermediate transission means, the apparatus including means for obtaining a measurement of movement at the source and further means for obtaining a measurement of movement at the member and a device for comparing these measurements and for influencing a control unit whereby the power source is controlled.

2. Apparatus as claimed in claim 1 wherein the means for obtaining measurements comprise respective measuring devices arranged to provide signals to the control unit.

3. Apparatus as claimed in either claim 1 or claim 2, applied to a machine tool with a linearly movable slide actuated by a rotary motor wherein one of the measuring means is a linear measuring device arranged to measure linear measurement of the slide and the other measuring means is an angular measuring device for measuring rotational movement of the motor.

4. Apparatus as claimed in claim 3 in which the angular measuring device is connected directly to the motor and provides a continuous measurement of a lead screw drivingly connected to the motor, for driving the slide linearly.

5. Apparatus as claimed in claim 3 in which the angular measuring device is connected to the motor through a slipping clutch, said angular measuring device being allowed only limited angular travel.

6. Apparatus as claimed in any one of the preceding claims wherein the two measuring means provide signals of different order of magnitude.

7. Apparatus as claimed in any one of the preceding claims wherein the control unit has a comparator means for comparing measurements derived from said two measuring means and providing an output for control of said power source.

8. A positional control apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.