To have accurate measurements during a DCC measurement sequence, the angular table position must remain stationary between commanded movements. In the more-common "non-external" implementation of a rotary table, the machine controller handles reading the table position. When a probe trigger occurs, it latches the position of the table as well as the XYZ position and returns all of that information. Therefore, PC-DMIS knows the actual table position that corresponded to the XYZ coordinates. If the table tends to drift slightly before the next probe trigger occurs, it doesn’t cause a problem. This is because the machine controller returns the new table position along with the new XYZ coordinates.
With an external rotary table, the machine controller is unaware of the presence of a rotary table, and therefore no real-time latching of the table position occurs when a probe trigger occurs. In terms of real-time machine controls, there is a substantial delay between when a probe trigger occurs and when PC-DMIS could get a reading of the table position. When the probe trigger occurs, the machine controller must send the XYZ result to PC-DMIS over a communications channel (usually a serial port), PC-DMIS must recognize that event and read the values. It would then have to send a position request to the rotary table controller over a different communications channel and wait for its response. Any table drifts during that interval would result in a perceived table position that is not the true position at the time the XYZ positions were recorded. This causes inaccurate measurement results that cannot be corrected.
Due to the above concern, PC-DMIS assumes that the table position remains stable between commanded moves. When PC-DMIS commands the external rotary table to move to a given position, it waits for a response before it proceeds. The rotary table controller should not respond until the table position is stable, and then it should respond with the actual position. This position can vary a little from the requested position with no impact on the accuracy, given the following:
The controller responds with the actual position.
That position does not change until a new movement request occurs.