|Publication number||USRE43250 E1|
|Application number||US 12/662,143|
|Publication date||Mar 20, 2012|
|Filing date||Mar 31, 2010|
|Priority date||Jan 27, 2005|
|Also published as||CN101107580A, CN101107580B, EP1844371A1, EP1844371B1, EP2221691A2, EP2221691A3, EP2221691B1, US7350310, US20060196066, WO2006079794A1|
|Publication number||12662143, 662143, US RE43250 E1, US RE43250E1, US-E1-RE43250, USRE43250 E1, USRE43250E1|
|Inventors||Peter G. Lloyd, David S. Wallace, David R. McMurtry, James L. Chase, David Jones|
|Original Assignee||Renishaw Plc|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (50), Non-Patent Citations (9), Classifications (11), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates to an articulating device and method of moving such which enables the relative positioning of two relatively moveable bodies in one of a plurality of defined, or indexed, positions.
A known type of articulating mechanism comprises two sets of detent elements, which are mutually engageable in a plurality of indexed, repeatable relative positions. One such mechanism is described in U.S. Pat. No. 4,168,576 which includes a circular array of hemispherical detent elements on one body and three elongate cylindrical detent elements on a relatively rotatable body. At each indexed position, each of the cylindrical elements engages the mutually convergent surfaces of an adjacent pair of hemispherical elements. The number of indexed positions provided by such a mechanism corresponds to the number of sets of convergent surfaces in the circular array.
When a movement to a different indexed position is required, the elements are disengaged, the bodies relatively moved and the elements re-engaged. The exact locations of the two bodies when they re-engage is affected by the motion that has been undertaken. This can be mitigated, in the case of motorised articulating mechanisms, by using a relatively precise, backlash free servo mechanism but this is expensive.
In a further type of articulating mechanism, there are no indexed positions and the two relatively moveable bodies can locate at any relative position. A potentiometer or encoder is used to indicate the relative positions of the two bodies. Again, the exact locations of the two bodies when they come to rest is affected by the motion that has been undertaken.
The present invention provides a method of moving an articulating mechanism for a measuring device, comprising the steps of:
The particular conditions includes setting constraints on the movement for example, that the movement is always from the same direction. Limiting the movement to coming from one direction means that any positioning error due to overshoot or backlash as a result of the movement will be congruous thus reducing any errors in the positioning of the probe. The particular conditions also includes constraining the movement such that certain criteria are always met such as the speed of the movement being a certain value. The criteria or constraints can be that one or more of the movement conditions are fixed or that the same movement conditions are used for the particular conditions.
Preferably, the particular conditions includes controlling the movement at a position adjacent the desired position. In one embodiment, the bodies are moved to the desired position and then removed to an adjacent position and back to the desired position
Preferably, relative movement prior to re-constraining at the desired position is as part of a lock up or re-constraining procedure.
In a preferred embodiment, the step of:
In a second aspect the invention provides a method of moving an articulating mechanism for a measuring device comprising the steps of:
Controlling the motion at an adjacent position by carrying out said movement under particular conditions means that any change in positional error which results from the motion having different speed, and higher derivatives such as acceleration, are mitigated. The particular conditions may include stopping the motion i.e. introducing a pause. However, instead of stopping, it may be preferred that the motion is slowed down or maintained at a consistently used speed as this provides a smoother transition. The adjacent position need not be the immediate neighbouring position but should always be the same distance from the desired position in order to reap the most benefit from the invention.
In a further embodiment, prior to step (b), the two bodies are moved relative to one another until the desired position is reached.
Preferably, the step of:
According to a third aspect, the invention provides a method of positioning an articulating mechanism for a measuring device which is controlled by a potentiometer comprising:
Preferably, the particular conditions, includes approaching the new position from the same direction.
According to a fourth aspect, the invention provides a controller for a measuring device comprising:
Preferably, the particular conditions includes that the movement is always from the same direction.
The invention extends to a measuring device including a controller.
According to a fifth aspect, the invention provides a measuring device comprising:
Preferably, particular conditions includes that the arrival is from a certain direction.
Preferably, the measuring device is a probe. The invention also extends to a measuring machine comprising such a measuring device.
The invention will now be described by example with reference to the accompanying drawings, of which:
The indexable probe 16 connects to the pivot mounting 14 via an articulating mechanism, in this case a kinematic location 22 which enables the probe 16 to be rotated about a horizontal axis 100. The pivot mounting 14 also connects to the probe head 12 via a kinematic location 24 which enables rotation about vertical axis 102. Thus, in this example the probe is rotatable about two axes at the same time.
Referring now to
A further example of an articulating mechanism comprises a belt which includes a plurality of teeth on one of the bodies.
The difference in position between the resultant locked or re-constrained locations of the articulating mechanism when moved in opposite directions produces an error from the true desired position which may be around double the error than when only one direction is used. Thus by only approaching a desired position from a certain direction positional error is reduced.
A further advantage of the invention is that because the desired position is approached from the same direction the position in which the articulating mechanism is locked can be designated as the actual position.
When the probe tip 252 is moved with respect to the quill 254, the elongate arm 256 will bend. To mitigate any errors that would be introduced by such bending, a consistent acceleration is used when the probe tip approaches the new position. Additionally, it is preferred that the new position is approached from the same direction each time a movement is effected.
To protect the equipment from damage, the touch probe is provided with a breakout connection 258. In the event that any part of the probe, contacts a surface with more force than is acceptable i.e. a crash occurs, the breakout connection preferentially breaks before any fragile or expensive components are damaged. The use of such a breakout connection can be a source of error in the positioning of the probe tip, especially when high speed movement is used as such movement can cause the probe arm to bend and this is transferred back to the breakout connection. For this reason as well, consistent acceleration or constant velocity is used when moving to a new position.
Two ways in which the position of the articulating mechanism of
Potentiometers have a tendency to drift over time so the voltage signal which is associated with a particular index location or position will change. Also, as potentiometers do not vary linearly, a range of voltage values is given for each location. This can result in movement of the articulating mechanism to the wrong location. The invention provides a method of identifying this drift and a method of positioning the articulating mechanism so that rather than giving each location a range for the voltage signal, an exact value can be assigned. To enable this, for each articulating mechanism, each position must be approached in the same manner according to the first aspect of the invention i.e. under particular conditions, for example, from the same direction and preferably the same distance. This will give an absolute voltage reading for each position when locked which is recorded in a table and used either manually or electronically each time the articulating mechanism is moved to a new position. This table may be updated every time you look into a position. Alternatively, on a subsequent movement under the same conditions to a position, a signal or reading is obtained, and if there is a difference between that reading and the recorded reading, the articulating mechanism is moved until that reading corresponds to the recorded reading.
Any difference between the recorded or tabular reading and the present or indicative reading may be flagged. Preferably, the difference is flagged only when the difference between tabular reading and actual reading reaches a certain amount or value i.e. when drift of the potentiometer has reached an-unacceptable amount.
In the event that the articulating mechanism is moved to a location which is between recorded positions, the recorded position is interpolated. The recorded positions may be 1° or 5° apart for example. Obviously, if the conditions of the movement are changed, a new table is required.
If the voltage signal received when the articulating mechanism is locked does not tally with the expected result then this can be flagged to an operator and the equipment tested.
In the case of an encoder, a similar method is used. Each position of the articulating mechanism is visited under the same particular conditions and an exact position count recorded. These position counts are subsequently used when moving back to this position some time later.
The value of X is selected as a nominal amount, for example 3° or 66 mV depending on the technique used to establish location.
A controller controls the movement of the articulating mechanism and one way to establish whether a target position has been reached is to use a comparator which compares the current position of the articulating mechanism with the target position. When there is no difference, the motion is stopped or paused.
In the case of probes, the different positions of the articulating mechanism are typically assigned an angular value. In this situation, X will be of constant value regardless of whether the probe is initially moved in a clockwise or anti-clockwise direction.
In other situations, the movement of the articulating mechanism may be to move a certain number of degrees. In this case, if the direction from which the articulating mechanism moves to the new position is set as clockwise, then when there is less than, 180° of a circle to travel to reach the new position, X will have a negative value so the mechanism pauses before it reaches the new position. If there is more than 180° of the mechanism to traverse to reach the new position then either one clockwise movement can be made having X as negative or, a shorter anti-clockwise manoeuvre can be made. However, in this circumstance X is positive thus, the mechanism will move passed the new position and return to it moving in the clockwise direction.
In the case where the articulating mechanism is used to move a probe, the controller (
In this example, use of the invention is described with respect to one axis only. In reality, and for the examples describes with respect to
In a further embodiment, the invention pan be carried out as part of the lock up or engagement procedure for the articulating device. In this embodiment, the articulating device is moved to the new or desired position and then, during the locking process, prior to the re-engagement of the device, a relative movement between the two parts of the device to an adjacent position and back to the new position occurs. Thus, after unlocking the mechanism 310, an initial target position is set as the new position 302 and the articulating mechanism is moved towards this initial target position. When the mechanism has reached the initial target position 302, the target position is changed 304 and set as the first target position 320 which is the new position plus a value X. The procedure then continues as described above.
The CMM 400 is provided with a rotary table 410 which sits on the base 402. The rotary table 410 is an articulating mechanism which has two parts 410a,410b which when disengaged (not shown) are relatively rotatable, using power from motor 414, enabling rotation of a sample 412 placed thereon with respect to the probe 406. The rotary table 410 is, for example, a Hirth coupling comprising interlocking gear teeth.
Throughout this specification, a measuring device includes machines such as coordinate measuring machines, machine tools, lathes, measuring machines, manual coordinate measuring aims, non-Cartesian mechanisms and other parallel kinematic-machines (such as tripods and hexapods), robots, for example work inspection robots, and single axis machines.
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|U.S. Classification||33/559, 33/556, 33/503|
|International Classification||G01B5/20, G01B5/00, G05B19/404|
|Cooperative Classification||G05B19/404, G05B2219/39179, G05B2219/41048, G05B2219/41081|