GB2150250A

GB2150250A - Rapidly arresting motion in system for a measuring machine

Info

Publication number: GB2150250A
Application number: GB08331552A
Authority: GB
Inventors: John Kenneth Dearden; Neil Rutherfordlaidlaw; David Allan Wright
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1983-11-25
Filing date: 1983-11-25
Publication date: 1985-06-26
Also published as: GB8331552D0; GB2150250B

Abstract

A braking system is provided for arresting the movement of a measuring machine probe (11) relative to a body (13) whose dimensions are to be measured. The system includes a drive shaft (14) driven by the relative movement and a clutch (17) operable to connect the drive shaft (14) through a speed-increasing drive train (19-22) to a final shaft (23). A friction brake (24) may be applied to the final shaft (23). Engagement of the clutch (17) when the probe contacts the body causes energy to be dissipated in the drive train, thus arresting the relative movement without damage to the probe. Rotation of a knob (25) attached to the final shaft (23) causes engagement of the clutch (17) so that fine adjustment of the relative position of the probe and body may be effected by the knob. <IMAGE>

Description

SPECIFICATION Braking system for a measuring machine This invention relates to a braking system for a measuring machine. In particular it relates to a system for arresting the relative movement between a measuring probe and a body to be measured in a non-powered coordinate measuring machine.

Coordinate measuring machines are well-known in which a measuring probe may be moved in two or three dimensions relative to a body to be measured. Usually the probe is power driven about the mutually perpendicular axes, and measurements are taken about a defined datum point. The measuring probes commonly used have a probe tip which is resiliently mounted and which carries switches to indicate when the probe touches the body to be measured. As well as indicating the point at which the required measurement is to be taken, a signal from the switch will, in the case of a powered machine, cause the movement of the probe to be stopped. This must be done quickly, since the probe has only slight freedom of movement and damage to the probe is undesirable.

There are clearly many ways in which a powerdriven machine may be stopped, and combinations of these are in common use.

Smaller measuring machines may be unpowered, that is the operator controls the movement of the machine arms directly. With present-day hydrostatic bearings fairly large masses may be moved with ease. However, it is necessary to stop such movement within very short distances to avoid probe damage, as stated above. Since there are no drive motors many of the usual braking techniques are not available.

It is an object of the invention to provide a simple braking system for an unpowered measuring machine.

According to the present invention there is provided a braking system for arresting the movement of a measuring machine probe relative to a body to be measured, which includes a rotatable drive shaft driven by the relative movement between the probe and the body, a clutch operable to connect the first shaft through a speed-changing drive to a final shaft, rotation of which may be prevented by a small friction torque, and control means for engaging the clutch whereby the kinetic energy may be dissipated by the final shaft and by the speed-changing drive to arrest said relative movement.

An embodiment of the invention will now be decribed with reference to the accompanying drawing. This is a schematic diagram of a braking system applied to one axis of a measuring machine.

Referring now to the drawing, a worktable 10 is movable about a horizontal axis relative to a probe 11. The tip 12 of the probe 11 forms the measuring point and is mounted with a small degree of resilient movement about the same two axes. A body 13 to be measured may be mounted on the worktable 10.

The worktable 10 will be mounted on bearings such as hydrostatic bearings, and will be provided with a suitable measuring system for measuring its movement relative to a datum.

A drive shaft 14 is driven by the movements of the worktable 10 about the said axis by means of a rack 15 carried on the worktable 10 and a pinion 16 mounted on the shaft 14. Hence any movement of the worktable 10 about the axis causes rotation of the drive shaft 14. The drive shaft is connected to a clutch 17 which may be operated remotely such as by means of an electrical signal. The disengaged clutch 17, rack 15 and pinion 16, and the bearings of the drive shaft 14 are arranged to present minimum resistance to the movement of the table 10.

The output of the clutch 17 is connected through an output shaft 18 to a speed-changing drive. This is shown as comprising two pairs of gears 19,20 and 21,22, and is connected to a final shaft 23. The speed-changing drive is a speed increasing drive from the output shaft to the final shaft. The speedchanging drive, output shaft 18 and final shaft 23 are arranged to present considerable resistance to rotation measured at the drive shaft 14, and this resistance may be augmented by a friction brake 24 acting on the final shaft. This friction brake may be permanently applied or may be remotely controlled.

The resistance to rotation may be provided by using plain bearings for the shafts and speed-changing drive, and by the form of the drive itself.

A manual operating knob 25 may be carried on the final shaft 23.

An electrical signal is produced when the tip 12 of the probe 11 touches the body 13. This signal indicates the position of the probe at the instant of contact, and is also applied to a pulse extending circuit 26. The output of the circuit 26 is applied to the operating mechanism of the clutch 17 and to the braking mechanism 24 if this is controllable. An external "clutch engage" signal CE may also be applied to the clutch as will be described later.

In operation, the worktable 10 carrying the object 13 is moved by the operator to position the tip 12 of the probe 11 in contact with a required point on the surface of the body 13. The movement of the worktable 10 drives the drive shaft 14 and part of the clutch 17 which is normally disengaged.

The signal produced by the probe 11 is applied to the external measuring circuitry and also to the pulse extender circuit 26. This causes the immediate engagement of the clutch 17 and the application of the brake 24 if this is provided.

Rotation of the drive shaft 14 is stopped rapidly in a controlled manner due to energy being dissipated at the clutch since the output shaft 23 is effectively locked. Movement of the worktable 10 is thus stopped, within the limits of resilience of the probe tip 12.

After a delay determined by the pulse extender circuit 26 the signal is terminated, disengaging the clutch 17 and the brake 24, and allowing the worktable 10 to be moved again.

The knob 25 is provided on the final shaft 23 to allow fine adjustment of the worktable position. A "clutch engage" signal CE causes the clutch to operate and translate rotation of the knob into movement of the worktable 10. The resistance to movement provided by the speed-changing drive and shaft bearings is small at this end of the gear train and is easily overcome by rotation of the knob 25.

A similar braking system may be applied to the other two axes of movement of the worktable 10. It will be realised that it is possible to move the probe 11 and keep the worktable 10 and body 13 stationary, in which case similar braking systems may be applied to the three axes of movement of the probe 11.

The speed-changing drive has been shown above as incorporating gearing. It will be understood that alternative forms of speed-changing drive may be used equally well, such as belt or chain drive arrangements. Similarly the rack and pinion drive from the worktable 10 to the drive shaft 14 may be replaced by other mechanisms such as chain, belt or wire drives, or a friction drive.

The clutch 17 may be operated electrically, hydraulically, or in any other suitable manner. The clutch may be disengaged manually after operation, rather than after a preset delay as described above.

The arrangements for measuring the extent of relative movement between the probe 11 and the body 13 have not been described. Many such arrangements are well known.

Claims

CLAIMS:

1. Abraking systems for arresting the movement of a measuring machine probe relative to a body to be measured, which includes a rotatable drive shaft driven by the relative movement between the probe and the body, a clutch operable to connect the drive shaft through a speed-changing drive to a final shaft, rotation of which may be prevented by a small friction torque, and control means for engaging the clutch whereby the kinetic energy may be dissipated by the final shaft and by the speed-changing drive to arrest said relative movement.

2. A system as claimed in Claim 1 in which the control means is operable to engage the clutch when an electric signal indicates physical contact between the probe and the body.

3. A system as claimed in Claim 2 in which the control means causes the electric signal to continue for a predetermined time after said physical contact.

4. A system as claimed in any one of Claims 1 to 3 in which the clutch is electrically-actuated.

5. A system as claimed in any one of the preceding claims which includes a friction brake capable of acting on the final shaft.

6. A system as claimed in Claim 5 in which the friction brake is caused to act on the final shaft by an electric signal.

7. A system as claimed in any one of Claims 1 to 6 which includes a manually-operated control attached to the final shaft for the purpose of providing fine adjustment of the relative position of the probe and the body.

8. A system as claimed in Claim 7 which includes means for engaging the clutch when fine positional adjustment is to be made.

9. A system as claimed in any one of the preceding claims in which the speed-changing drive is a speed-increasing drive from the output shaft to the final shaft.

10. A system as claimed in any one of the preceding claims in which the speed-changing drive includes at least one set of gears.

11. A system as claimed in any one of Claims 1 to 10 in which the probe is held stationary and the body is moveable relative thereto.

12. A braking system for arresting the movement of a measuring machine probe relative to a body to be measured substantially as herein described with reference to the accompanying drawing.