decoding quadrature encoders

[HenrikOlsson]

Hi,
If I'd venture a guess it would be that with a 360CPR encoder and 4x decoding the counter variable will "tick" 4 counts per degree but your math assumes the resolution is 0.1 degrees.

As a simple workaround try multiplying your count value by 2.5 before you enter your display routine.

' Here is were I struggle.....
enc_counter = enc_counter */ 640   ' Multiply by 2.5
'*********************************************************************************
lcdout $fe,$C0, SDEC2 enc_counter/10, ".", SDEC2 enc_counter//10     	'display enc_counter value
'*********************************************************************************

Obviously, the encoder only provides 0.25 degree resolution and no math can change that.

If it doesn't work properly then strip out the formating and display the raw value (pre or post multiplying with 2.5) and tell us between what values it ranges.

/Henrik.

[cbrun17]

Hi Henrik,

Thank you so much for the quick response. I will give your suggestion a try this evening and report back with the results.

Kind regards,

Chris

[cbrun17]

I changed the multiplier as suggested but I received very erroneous results. Using just "lcdout $fe,$C0, SDEC enc_counter"

The count results in 2,5,7,10,12,15,,,455. So 2 and 3 counts alternately.
Then if I multiply enc_counter by 250, I get exactly + 180 degrees which I think confirms 2 "ticks" Also note that by altering the value of enc_counter, I loose my negative count.

Counting enc_counter without multiplier results in ~ +/- 184.

To clarify, the shaft has a 12:00 o'clock "zero" position. Turning in either direction from center results in ~ +/- 184. Counting the number of pulses on my Fluke counter indicates the same count.

So, the way I understand this quadrature coding, it compares the A/B channels to determine the direction. So changing the post result would only be wiped away by any subsequent updates from the interrupt routine.

I would be satisfied with .25 degree increments if I could get the LCD output correctly. Showing +/- 90 in .25 increments in the form of xx.xx.

Thank you again for sharing your knowledge.

Chris

[HenrikOlsson]

So you're getting ~360 counts for 180 degrees movement. Then the interrupt code must be doing 2x decoding instead of 4x - provided the encoder is actually 360cpr.
You loose the negative value because PBP doesn't handle the multiplication properly and you're of course better off copying the enc_counter variable to a second one for scaling and display purposes, sorry about all that.

Try this (untested):

Degrees VAR WORD
Sign VAR BIT

Degrees = enc_counter   ' Degrees is now -180 to 180
Sign = Degrees.15   ' Save sign
Degrees = (ABS Degrees) * 5   ' Degrees is now 0 to 900
If Sign THEN Degrees = -Degrees   ' Restore sign, value is now -900 to 900

/Henrik.

[cbrun17]

Hi Henrik,

No worries... I learn from mistakes as well. I appreciate all your help.

I believe we're getting very close. I have the following code:


if enc_counter <> enc_counter_old then 'see if value has changed
enc_counter_old = enc_counter 'move new value to old
Counter = enc_counter
Sign = Counter.15 ' Save sign
Counter = ((ABS Counter) * 5) ' Degrees is now 0 to 900

If Sign THEN Counter = -Counter ' Restore sign, value is now -900 to 900

lcdout $FE,1, sdec Counter / 10, ".", sdec Counter //10
lcdout $FE,$C0, sdec Counter


First, the counter increments by 5.
Also, in the first LCD statement, the count increments to +90 as expected. However, the negative value shows as 6553.1, 6552.6, etc...
The second LCD out statement displays +/- 900 as you explained.

Seems the absolute value gets lost when trying to display negative after formatting.

Is there maybe a way I can parse the decimal values prior to display statement to maintain the value?

Chris

[cbrun17]

I made some changes to the formatting and I successfully display the +/- values. Displays +/- xx.x. I updated the code to:

if enc_counter <> enc_counter_old then 'see if value has changed
enc_counter_old = enc_counter 'move new value to old
Counter = enc_counter
Sign = Counter.15 ' Save sign
Counter = ((ABS Counter) * 5) ' Degrees is now 0 to 900
If Sign THEN Counter = -Counter ' Restore sign, value is now -900 to 900

If NOT sign then
lcdout $FE,1, sdec Counter /10 , ".", sdec Counter // 10
else
lcdout $FE,1, "-", sdec -Counter /10 , ".", sdec -Counter // 10
endif

This takes care of displaying properly. Now is there a way I can bring the count down to .25 increments instead of .5?

Thanks again.

Chris

[HenrikOlsson]

Hi,
Great job! The problem with the display routine is, again, that PBP doesn't handle math with negative numbers properly so the result of the Counter / 10 operation gets messed up when Counter is negative.

You can probably make that snippet a bit more efficient, somelike like this perhaps:

if enc_counter <> enc_counter_old then 'see if value has changed
enc_counter_old = enc_counter 'move new value to old
Counter = enc_counter 

Sign = Counter.15 ' Save sign
Counter = ((ABS Counter) * 5) ' Degrees is now 0 to 900

LCDOUT $FE, 1  ' Clear screen
IF Sign THEN
  LCDOUT "-"    ' Print a minus sign if the value is negative
ENDIF
LCDOUT SDEC Counter / 10, ".", SDEC Counter // 10   ' now print the value

I must admit that I can't really follow the ASM code for the interrupt but since it's using Interrupt on change I think it "should" do x4 decoding so you "should" get 1440 counts per revoultion on a 360CPR encoder. I don't know why that's not happening. Hopefully someone else is able to help with that part.

/Henrik.

[cbrun17]

Thank you Henrik.

I couldn't have made it this far without you're help. The displayed number is perfect. As you said, the counts aren't making much sense. I'll continue to experiment with this to see if I can figure out how to get the correct resolution.

Thanks again and kind regards....

Chris