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Originally Posted by bharrison6
Yes we want to decode it and use it rather than use analog because it will be more precise.
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How did you determine that it will be more precise? e.g. Did you consider the tolerances in the encoder datasheet as well as the tolerances in the decoding methods?
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I think all I need to do is figure out how to read the duty cycle of the PWM input.
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"all I need to do"... How do you plan to do that?
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Since you ask, do you know how to decode the [PWM] signal
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I've heard the FPGA sampling frequency will be bumped up from ~153KHz (in 2014) to ~1MHz (in 2015). Maybe the FPGA will be programmed in 2015 to read duty cycle (detect and report consecutive rising-to-falling and falling-to-rising edge pairs). If so, you could decode the PWM quite accurately using that. Otherwise, you'd probably need to use high-priority interrupts to detect and time-stamp the edges. I don't know what the interrupt latency will be in the roboRIO with LinuxRT and WPILib and LabVIEW.
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just put a low-pass filter on it and run it as analog.
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How much filtering would be required to reduce the analog noise to an acceptable level, and how much phase lag would that introduce?
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will using a pwm w/ low pass filter effect the quality of the analog signal vs just buying a regular analog absolute encoder?
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My guess would be yes, but I haven't looked at it in depth.