Renishaw Linear Encoder (Find HOME)

[Controlink_Systems]

I have Renishaw RGH22X Series Readhead optical linear encodes on the X&Y Axis. The encoders provide 25,400 pulses per inch (1000/mm) and are about 16 inches in length. I am working in "Open Loop Pulse Mode" sending step/direction.

These encoders share a forward and reverse limit switch. Basically, a 0.25 VDC output is measured if the drive is between the limit switches, but if the drive hits a limit (either one), the voltage drops to zero.

The encoder is also equipped with a "reference" switch which provides a single pulse between X and Xnot. The limit swith is suppose to be used to accurately "home" the axis. Unfortunately, I am having trouble trying to "home" the axis. The X or Xnot trigger has not been sensed when wired to t
he "home" input on the UMI-7764. I have verified, that if a standard 5 VDC Signal, or 0 VDC depending on the Axis setup, is used, the "home" state is sensed. However, the reference pulse is not being seen (either X or Xnot) when wired directly to the "home" input.

I tried to find a "reference" pulse to home the drive but the drive needs to move further than a single rotation since the refernce pulse is located on a single end of the encoder. I have not been successfull using this approach and then learned that find reference is not supported in Open Loop Stepper Mode.

Is it possible to "home" this system using the signals available from the Renishaw encoders? What must be done to establish home?

Regards,

Guy

[mhaag]

Guy,

Thank you for contacting National Instruments. I believe that you were able to speak with another engineer over here to get that question resolved, but I just wanted to post the results in case you had other questions or other users are interested.

Because the encoder you are using is very fine, it returns a large number of pulses every second; this obviously will make the pulse width very small. If you look at the specifications for the Home inputs of the 7344, then we can see where it requires a 1 ms minimum pulse width. In your current setup, the system is moving fast enough to where that pulse is coming in too quickly for the motion controller to detect.

In order to correct this, we can develop an application where we slow down the velocity of the mo
tor. By doing this, we should be able to successfully detect the pulse and reliably find the home switch in your system.

Regards,
Michael
Applications Engineer
National Instruments