Use incremental rotary encoder to measure the absolute position and velocity

Question

Hi all,
I faced a problem by using incremental rotary encoder to measure the absolute position of the rotation. 
I'm not using NI-Daqmx but NI-motion module. In NI-motion module, I use read encoder position.vi to read the position of the encoder. 
However, the readings is in incremental form. I'm doing the feedback control, so I need the absolute position and velocity.
Is there any functions or vi in the labview that can be used to transform the information of incremental encoder to the absolute one?
Thanks
Jun Wong

Answer 1

Jun,
1. The incremental encoder provides incremental position. After switching on the encoder power the encoder counts upwards or downwards. For the absolute position you need the index (R or I) signal. I don't know which type of incremental encoder you have but there should be A, B and (I or R) signals. The index signal should reset the counter to zero setting this way a pseudo-absolute-position (which is lost after the first switch of). Most of the motion controllers have a mode, just after switch on, in which the controller search for the Index. This mode is called Homing. 
2. Velocity. It's very simple. You sample the position with a fix sampling clock. Let's say: 10kHz. The speed is: (Actual Position - Previous Position) / Sampling Period. Pos[n] = 10.000inc, Pos[n-1] = 9.900inc. Speed = 10.000 - 9.900 / (1/10kHz) = 1.000.000 inc/sec. If one increment is 0.0001mm then the speed is 100mm/sec.
Paul