Encoder period/rate moving average

[vamfun]

In the SetMinRate problem thread http://forums.usfirst.org/showthread.php?t=12050 there is a lot of discussion re encoder period averaging. Apparently, the encoders are set up to do a moving average of pulses to determine the average period used to derive rate. This is defaulted to one period but Joe H. has suggested a way to extend this to be any number of pulses between 1 and 127. This is a very valuable tool to handle the noise problems seen with encoders. I would like to use this thread to develop prototype changes to the Encoder class that we all can use. I suggest we add the following simple method to Encoder() class that goes something like this:

void Encoder::SetMovingAveragePulseCount(UINT32 max_count)
{ //check if 1=<max_count<=127 TODO: should throw an error flag here with message if max_count invalid
if (max_count<1) max_count = 1;
else if (max_coount>127) max_count = 127;
if(m_counter)
{m_counter->writeTimerConfig_AverageSize(max_count, &status);} //x1
else
{m_encoder->writeTimerConfig_AverageSize(max_count, &status);}//x2 OR x4
}


Also, since the encoder pulse widths are asymmetric, using a single pulse to determine periods leads to some very large rate errors. See thread http://www.chiefdelphi.com/forums/sh...ad.php?t=74763 . I would recommend that in InitEncoder(bool reverseDirection, EncodingType encodingType) that we always average over one full period as a default. I.E.
x1: m_encoder->writeTimerConfig_AverageSize(1, &status);
x2: m_encoder->writeTimerConfig_AverageSize(2, &status);
x4: m_encoder->writeTimerConfig_AverageSize(2, &status);// This is 2 because of FPGA error which reads only B chan for timing.

This should drop the noise rates by at least a factor of 3 when using x2 or x4.

[Joe Hershberger]

FYI, there is a bug in FPGA 1.0.0 that only phase B is used for timing (position is decoded at 4x). So for the purposes of timing, it should be treated as 2x until the FPGA is updated. You can see a note about that in the WPILib source for the timer functions of the Encoder class such as SetMaxPeriod() and GetPeriod().

[kingofl337]

This is a great thread, it's too bad it is not in the library.

After reading this I creaded constant
#define MAXCOUNT 25

changed the cases to look like this

case k4X:
m_counter->writeTimerConfig_AverageSize(MAXCOUNT, &status);
case k2X:
m_counter->writeTimerConfig_AverageSize(MAXCOUNT, &status);
case k1X:
m_counter->writeTimerConfig_AverageSize(MAXCOUNT, &status);

I know it's a little dirty but I'll probably create a function after I do initial testing tomorrow.

[Joe Hershberger]

Don't forget that the 4x version needs to set on the m_encoder member, not the m_counter.

[vamfun]

Ok, ran the first tests today with the new SetMovingAveragePulseCount routine. Ran data with Pulse count = 10 and 50. The rate sigma was 2.29 and 1.95 respectively. The sigma reduction ratio is .85 . I would have expected a factor of 1/sqrt(5) or .44. So not sure this is working.

We just extended the class to include our new function. Any comments?

Here is what we are doing in the code:
class OurRobot : public SimpleRobot
{
RobodoxEncoder *leftGearbox;
}

OurRobot(void)
{


leftGearbox = new RobodoxEncoder(4,1,4,2, false, Encoder::k1X);
leftGearbox->SetDistancePerPulse((3.0*PI)/250);
leftGearbox->SetMinRate (1.);//sets minimum rate to 1 in/sec
leftGearbox->SetMovingAveragePulseCount(50);//Here is where we set the moving average pulses to 50.
leftGearbox->Start();
}


********************Encoder Source code************************

RobodoxEncoder.h

#ifndef ROBODOX_ENCODER_H
#define ROBODOX_ENCODER_H

#include "ChipObject.h"
#include "CounterBase.h"
#include "SensorBase.h"
#include "Counter.h"
#include "PIDSource.h"
#include "Encoder.h"


class RobodoxEncoder : public Encoder
{
public:
//RobodoxEncoder();
RobodoxEncoder(UINT32 aChannel, UINT32 bChannel, bool reverseDirection=false, EncodingType encodingType = k4X);
RobodoxEncoder(UINT32 aSlot, UINT32 aChannel, UINT32 bSlot, UINT32 _bChannel, bool reverseDirection=false, EncodingType encodingType = k4X);
RobodoxEncoder(DigitalSource *aSource, DigitalSource *bSource, bool reverseDirection=false, EncodingType encodingType = k4X);
RobodoxEncoder(DigitalSource &aSource, DigitalSource &bSource, bool reverseDirection=false, EncodingType encodingType = k4X);
~RobodoxEncoder();
//double GetRate();
void SetMovingAveragePulseCount(int max_count);

};

#endif

*******************source file**********************

RobodoxEncoder.cpp

#include "RobodoxEncoder.h"


RobodoxEncoder::RobodoxEncoder(UINT32 aChannel, UINT32 bChannel, bool reverseDirection, EncodingType encodingType)
:Encoder(aChannel, bChannel, reverseDirection, encodingType){
}

RobodoxEncoder::RobodoxEncoder(UINT32 aSlot, UINT32 aChannel, UINT32 bSlot, UINT32 _bChannel, bool reverseDirection, EncodingType encodingType)
:Encoder(aSlot, aChannel, bSlot, _bChannel, reverseDirection, encodingType){

}

RobodoxEncoder::RobodoxEncoder(DigitalSource *aSource, DigitalSource *bSource, bool reverseDirection, EncodingType encodingType)
:Encoder(aSource, bSource, reverseDirection, encodingType){

}

RobodoxEncoder::RobodoxEncoder(DigitalSource &aSource, DigitalSource &bSource, bool reverseDirection, EncodingType encodingType)
:Encoder(aSource, bSource, reverseDirection, encodingType){

}

RobodoxEncoder::~RobodoxEncoder(){
}

void RobodoxEncoder::SetMovingAveragePulseCount(int max_count)
{ //check if 1=<max_count<=127 TODO: should throw an error flag here with message if max_count invalid
if (max_count<1) max_count = 1;
else if (max_count>127) max_count = 127;
if(m_counter)
m_counter->m_counter->writeTimerConfig_AverageSize(max_count, &status); // Counter object within Encoder -> tCounter within Counter
else
m_encoder->writeTimerConfig_AverageSize(max_count, &status);
}

[vamfun]

Joe H. ,
Got a good way to check if we have the timer count set correctly with a simple print? Something like


int countMax = 599encoder->m_counter->readTimerConfig_AverageSize(&status); ???

[kingofl337]

Don't forget that the 4x version needs to set on the m_encoder member, not the m_counter.

oops... Copy / Paste Error

[Joe Hershberger]

Joe H. ,
Got a good way to check if we have the timer count set correctly with a simple print? Something like


int countMax = 599encoder->m_counter->readTimerConfig_AverageSize(&status); ???

Assuming you weren't accessing a private member, sure.

You should be able to call read on any register that has it defined in the header file to read the current configuration of the register in the FPGA. That can be a good debugging tool while developing library functions.

[kingofl337]

When I try to compile WPILIb I get an error saying.

'class Counter' has no member named 'writeTimerConfig_AverageSize'

So I had to edit both counter.cpp and encoder.cpp.

[vamfun]

When I try to compile WPILIb I get an error saying.

'class Counter' has no member named 'writeTimerConfig_AverageSize'

So I had to edit both counter.cpp and encoder.cpp.

we solved this with

m_counter->m_counter->writeTimerConfig_AverageSize(max_count, &status); // Counter object within Encoder -> tCounter within Counter