GB2203254A

GB2203254A - Force monitoring systems for moving objects

Info

Publication number: GB2203254A
Application number: GB08704241A
Authority: GB
Inventors: Roger Shingler
Original assignee: Davis
Current assignee: Davis
Priority date: 1987-02-24
Filing date: 1987-02-24
Publication date: 1988-10-12
Anticipated expiration: 2007-02-24
Also published as: GB8704241D0; GB2203254B

Abstract

In assembly load monitoring it is known to provide a mute or inhibit circuit which is triggered by a switch arranged to be operated just prior to a component meeting a stop, so that the loads produced on meeting the stop are ignored by a load monitoring system, but the adjustment of the switch position is troublesome. We monitor load but also look at the speed of the component during assembly, and we use the speed curve to enable parts of the load curve, associated with the abutting with the stop, to be disregarded. We can look for a predetermined deceleration or zero speed of the component as the basis for identifying the useful part of the force curve. The system can be used to measure forces required to engage and disengage gears of a gearbox.

Description

FORCE MONITORING SYSTEMS FOR MOVING OBJECTS This invention relates to a system for monitoring the force opposing the movement of a moving object particularly, but not exclusively, in situations in which the available displacement of the object is restricted by a restriction means such as a stop. The invention relates particularly, but not exclusively, to the monitoring of assembly loads associated with the assembly of mechanical components, and to the monitoring of operating forces in moving a component of a mechanical system on a test rig.

When the movement of an object is restricted by a stop a system to monitor the force opposing the movement of the object needs to take account of the fact that the force will increase significantly when the object meets the stop.

Thus, it is known in assembly load monitoring to measure the force opposing the pressing of a machine component to its assembled position, such as the pressing of a bearing bush into a blind bore in a larger component by an hydraulic assembly press, and to provide a switch which is positioned with the intention that the switch is operated just prior to the component hitting a stop, such as the bush reaching the blind end of the bore, the switch being connected to a mute or inhibit circuit associated with the load monitoring means which is arranged such that the load monitoring means does not respond to the large load which results from the component meeting the stop.

The problem with that known arrangement is that if the switch is not correctly positioned relative to the stop, the position of which may itself vary due to tolerances in the dimensions of the components, the switch may be operated at the wrong moment. If, on the one hand, the switch is positioned such that it is operated well before the components have been pressed to their fully assembled positions, then the assembly load may well rise above the load seen by the monitoring circuit at the moment that the monitoring circuit is first muted, and so the monitoring circuit does not see the maximum assembly load reached. If, on the other hand, the switch is positioned such that it has not been operated by the time that the component hits the stop, then the monitoring circuit will see the large increase in load caused by this engagement.In the latter case if the monitoring circuit is arranged to respond just to the maximum monitored load then this maximum will in most cases be the same, resulting from the engagement with the stop, even for components which have only a small assembly load due to their incorrect size.

According to one aspect of the invention a method of monitoring the force opposing the movement of a moving object comprises deriving a signal representative of the speed of the object, identifying a characteristic portion or event of the speed signal, and looking at the force which is operative during that characteristic portion, or at or prior to that event.

Thus, in monitoring assembly loads it might be decided that the important load to monitor is the assembly load at the moment that a component is moving at its maximum speed during the assembly operation, and so the maximum speed portion of the speed signal or curve would be identified, and the assembly load would be checked for that portion of the assembly cycle.

In another assembly operation it may be desirable to identify a bump or trough in the speed curve which would be associated with a particular part of the cycle, and that event would be used as the datum for determining where in the cycle to take account of the force measurements.

According to a second aspect of the invention a method of monitoring the force opposing the movement of a moving object of which the available displacement is restricted by restriction means comprises looking for the sudden change in object speed which results from the interaction between the object and the restriction means, and disregarding the forces which arise subsequent to that sudden change.

Thus instead of using a switch to inhibit the monitoring means we detect the sudden change in object speed when (and if) the object encounters the restriction means. It will be appreciated that this avoids the need to set a switch, although if desired such a switch may still be provided for other purposes.

The change in object speed can be detected in various ways: A predetermined acceleration/deceleration may be detected. A predetermined slow speed or zero speed (following some movement) may be detected. Of course, the force data and speed/acceleration data may be stored, and the analysis of the data may be performed after the cycle of object movement has finished.

When the data has been stored, a time spaced by a predetermined amount in advance of the time that zero speed is first detected may be used as the criterion for disregarding subsequent load measurements.

According to a third aspect of the invention in a system for monitoring the force opposing the movement of a moving object of which the available displacement is restricted by a restriction means, we provide a speed change means for detecting the sudden change in object speed which results from the object interacting with the restriction means, and inhibit means associated with the force monitoring means and responsive to the speed change means such that the force monitoring means is inhibited from responding to the excessive change in force which results from the moving object interacting with the restriction means.

The speed of the object can be derived, for example, from the output of a linear variable displacement transducer (LVDT) , from a potentiometer, or from a tachometer.

In assembly loading monitoring the LVDT can be used also to check that a component has been assembled to the correct position.

In one embodiment of the invention for measuring the assembly load of a bush into a blind bore using an hydraulic assembly press, the press load is measured by a load cell mounted behind the top punch, or with a pressure transducer connected to the hydraulic pressure line close to the press cylinder.

A 6809 microprocessor and 12 bit analogue to digital converter are employed. The software is arranged to provide monitoring of the analogue value of the press load and to compare the press load with pre-set trigger values indicative of excessive/insufficient assembly loads.

In a second embodiment of the invention a production test rig is arranged to check the forces required to engage and disengage the gears of a vehicle gearbox. Changing gear involves a series of forces to overcome syncromesh units and in engaging gear. A final high load occurs at the end of the selection process as the mechanism hits a dead stop. This high load can be discounted in accordance with the invention, the speed of the gear shift being measured until zero or a pre-set low speed is detected.

Alternatively, the total load curve is stored and an analysis is made of the load and speed curves on completion of the shift cycle. The software of the system can be arranged such that an analysis of the load curve is made working backwards from the datum point provided by the zero or low speed point.

Claims

1. A method of monitoring the force opposing the movement of a moving object comprises deriving a signal representative of the speed of the object, identifying a characteristic portion or event of the speed signal, and looking at the force which is operative during that characteristic portion, or at or prior to that event.

2. The method of Claim 1 applied to measuring an assembly load for a component of a mechanical assembly, which load constitutes said force.

3. A method of monitoring the force opposing the movement of a moving component of a mechanical assembly of which the available displacement is restricted by restriction means comprises looking for the sudden change in component speed which results from the interaction between the component and the restriction means, and disregarding the forces which arise subsequent to that sudden change.

4. The method of Claim 3 in which the sudden change in component speed is detected by determining the acceleration/deceleration and comparing the determined value with a predetermined acceleration/deceleration.

5. The method of Claim 3 in which the sudden change in component speed is detected by determining when the speed falls below a predetermined slow speed or becomes zero, following some movement of the component.

6. The method according to any of Claims 3 to 5 applied to the monitoring of the load required to press on a power press the component to an assembled position.

7. The method according to any of Claims 1 to 6 in which data relating to the speed and force is stored, and the analysis of the data is performed after the cycle of the object/component movement has finished.

8. The method of Claim 7 as appended to Claim 5 in which a time spaced by a predetermined amount in advance of the time that zero speed is first detected is used as the criterion for disregarding subsequent load measurements.

9. A force monitoring system for monitoring the force opposing the movement of a moving component of a mechanical assembly of which the available displacement is restricted by a restriction means, comprising a speed change means for detecting the sudden change in component speed which results from the component interacting with the restriction means, and inhibit means associated with the force monitoring means and responsive to the speed change means such that the force monitoring means is inhibited from responding to the excessive change in force which results from the moving component interacting with the restriction means.

10. A method of monitoring the force opposing the movement of a moving component of a mechanical assembly substantially as described herein with particular reference to the embodiments described.

11. A force monitoring system for monitoring the force opposing the movement of a moving component of a mechanical assembly substantially as described herein with particular reference to the embodiments described.