How can I find the position of a robotic arm with a cFP-CTR card?

Question

Hello,
I use a CTR-500 to measure the pulses of 3 encoders.
The encoders are related to the 3 axis (x,y,z), to which a robotic arm is moving.
Each encoder has 2 channels (A,B) which go to each pair of Count Inputs of the CTR card.
When the robotic arm is moving on one axis, the corresponding pair of Count Inputs is increasing, but there is a phase between the one Count Input and the second one. What is more, the Count Inputs do not decrease when the robotic arm is moving to the other direction, but they still increase.
So, I cannot find easily the position of the robotic arm.
What I have done, is to try finding the position, by using only one Counter Input for each axis. This means that I use the value of the encoder as a counter, and I calculate the position with some software tricks. But for some reason, it does not work properly.
I have heard that the method I use is not proper. Instead, I have been told that I must use the phase of the 2 Count Inputs, in order to find the direction to which the robotic arm is moving. But, the Count Inputs of the CTR are augmenting in parallel, when the robotic arm is moving forward or backward.
How can I find the position of the robotic arm, as it is moving like a CNC ?
Thank you very much.
Message Edited by nikosfs on 08-21-2009 12:27 PM

Answer 1

You don't want to use a cFP-CTR-500 card.  What you have are quadrature signals.  You should be using the cFP-QUAD-510.  It has 4 channels that can handle both the A and B signals of a quadrature encoder.  It has the built in circuitry do determine the phasing of A vs. B to know when to count up vs. count down.