Touch probes are precision devices used in e.g. CNC controlled mills to precisely locate the edges and corners of a workpiece. They have a probe arm terminated with a known diameter ball, that touches the workpiece and breaks an internal switch to indicate contact. A commercial example can be seen here
http://www.tormach.com/product_tts_digitizing_probe.html.
I am in the process of designing an IR based wireless solution for one, to avoid the need to connect/disconnect a cable for the touch indication. The cable is not really compatible with automatic tool changers and IR and radio based solutions are the norm. Radio will follow, but first i do the IR.
My problem is that the LiPo battery in the device has a strictly limited discharge/charge cycle time. Due to safety regulations, the probe must maintain constant connection with the receiver during a positioning run lest the machine get out of control in case of connection issues. This means that the IR transmitters must be active while the probe is in position and stop when contact is detected. It also means that there must be a way to conserve power when the probe is idle. Otherwise the battery will be flat in no time (roughly 1 hour).
At this time the controller is an ATTiny25 that can go to a sleep mode whenever the probe is idle. The challenge is in detecting the conditions for idling. A prototype with wire indicator connection exists and there, a simple tilt switch is used. Whenever the probe is on its side or upside down, power is off. This works nicely for everything else except carousel type toolchangers, where it doesn't work at all. The tools are always in the vertical orientation.
For the IR option i have now specified a Bosch BMA220 accelerometer. It has easily all the features needed for motion sensing but there are a couple of things.
First, the package it comes in is ridiculously small, only 2x2 mm in size with 12 miniscule non-visible contact pads. It can be soldered in an IR oven, but manually positioning that one will be a PITA. So first question is, has anybody encountered a "reasonable" sized accelerometer?
The second issue is that while the BMA220 has a simple interface for generic motion sensing, the parameters for the simple interface are not configurable. So i am a bit worried about the motion levels the chip will indicate when in that mode. If anyone has experience i would like to hear about that. The full interface works either by i2c or spi and that is one way of course. However, the ATtiny only has one serial channel that needs to be shared with the programming interface and it could get a bit messy. Not to mention that control software would be orders of magnitude more complex.
So the question really is, if anyone has done simple/reliable motion sensing lately? As you see from the above, all i would need is an indication whether there is motion or not, nothing else matters. A toolchanger will cause high accelerations easily detected but a positioning run will be done ultra smoothly with very slow motions, so that part may not be so easily detectable - subject to the sensor of course. But any ideas / experience would be welcome.