Converting PULSOUT command to PWM for a PWMPAL output to BOE-BOT Servos.

JackDaniels

Hi there,

I can make the BoeBot practically do the Macarana with the pulsout command, but now I want to be able to process other things while the bot runs around.

I have added a PWMPAL to the mix, but what I can gather from the manuals isn't working.

I read that the PULSOUT command for the BS2 sends a 2 microsecond pulse for every number entered, ala 750 (used for full stop) is a 1500 microsecond pulse.

Translating this timing to the ontime-offtime command for the PWMPAL produces movement, not a full stop.

I also read somewhere in these forums that the PULSOUT command has a set time period of 20 milliseconds. I then tried the 1.5 milliseconds on, 18.5 milliseconds off, but it also produced movement.

Has anyone done this transition before?

JD

Mike Green

Be careful of what you read. The PULSOUT doesn't have a set time period. You have to create that in your program with a PAUSE statement.

Servos require a control pulse roughly every 20ms or they will shut themselves down. If you read the PWMpal documentation, you'll see that the units for the on/off time are 25us. For a 1.5ms on time, the value is 1500 / 25 = 60. The 18.5ms off time gives 18500 / 25 = 740.

If you're using continuous motion servos (as it looks), they may not be calibrated for a 1.5ms stop setting. The "Robotics with the BoeBot" tutorial discusses how to adjust the servo using a short test program. You can do the same thing using the PWMpal set for a 1.5ms pulse width.

Post Edited (Mike Green) : 9/26/2008 10:45:26 PM GMT

Mike Green

By the way, the posting of the same question / message to two different forums is not allowed. Please delete your other message in the Robotics forum. Use the X icon in the upper right hand corner of the message box.

JackDaniels

Thanks Mike, I will look into that. Sorry about posting in two places, I wasn't sure if it applied to just the stamp or to the Boe-Bot.

JackDaniels

Hi Mike, thanks for the tip about the 25 microsecond unit for the PWMPAL. I'm not sure why I just assumed it was the same 2 microsecond unit as with the BS2, but it works now.

I was also having a resetting issue with the stamp because it was plugged into a 5v dc plug.. but when I let it run on batteries it worked fine.

Thanks again.

Chris Savage

Jack,

Your duplicate thread in the Robotics Forum was removed. Please follow up in this original thread on your PWMPAL questions.

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Chris Savage
Parallax Engineering

Bruce Bates

Jack -

Attaching a copy of your program would be most helpful in determining the problem.

Regards,

Bruce Bates

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When all else fails, try inserting a new battery.

JackDaniels

Hi Bruce, thanks for reading this thread... but the code isn't the problem, as it is a grand total of 4 lines.

I serin the value I want to send to the motors and serout to the pwmpal. That's it.

I have found something interesting now, and am one step closer to getting it working.

Starting with the original conversion I made from the 2 microsecond on the Stamp Pulsout command to the 25 microsecond on the pwmpal, I setup a program that cycles through all possible combinations of bits sent to the pwmpal.

I have the OffTime byte set all the time to 255 and start the ontime bit at 0.

From 0-59, the servo runs full throttle backward.
At 60, the servo is at full stop. (This on time produces a full stop no matter what the off time is)
from 61-~120, full throttle forward is produced,
from 120-255, full throttle slows down incrementally.

I have played around with the upper byte (the pwmpal actually takes two bytes for on/off) for both the on an off times, but can't get the boe-bot servos to run more slowly in the backward direction.

Has anyone used the PWMPAL to control the Boe-Bot servos to free up the Stamp to do other things?

Thanks