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Re: Serializer encoder problem

10/30/2007 12:02:02 AM

rpg

35 posts

Re: Serializer encoder problem

I attached the biggest capacitor I had lying around (220 uf) across the Serializer power terminals and that did reduce the 1-revolution count on encoder port 2 a bit. I didn't have a chance to run out to get a bigger capacitor so I went on to the other options.

Connecting my smaller motors with their built-in encoders worked great: both encoder ports report the same counts--so it does look like there is something about the bigger motors that the Serializer doesn't like, even though both big and small motors are 7.2 V and I am running the big motors under no load and low speed. (It occurred to me that encoder port 2 on the Serializer is closest to the first motor power connector, so perhaps that is why only port 2 is showing the erroneous readings.)

In any event, I'd like to try option three: connecting my Scorpion XL H-bridge/speed controller to the external H-bridge headers on the Serializer. The trouble is, I'm not exactly sure how to assign the jumpers between the Scorpion and the Serializer. In case you're not already familiar with the Scorpion, here are a couple of links:

http://www.robotpower.com/products/scorpion_XL_info.html
http://www.robotpower.com/downloads/RobotPower_Scorpion_XL_quickstart.pdf

As far as I can tell, I should jumper the Scorpion for "Tank" mode that allows independent left/right control from the microcontroller (in this case, the Serializer). When I used the Scorpion with a BrainStem controller in the past (with the Scorpion jumpered for "Mixed" mode), I connected the left/right "RC" cables (each with a 3-pin header) to two servo ports on the BrainStem. However, the Serializer has pins labeled Enable, L-Dir, R-Dir, L-PWM, R-PWM. So my question is: what would be the proper assignment of the Scorpion's "RC" leads to the Serializer's H-bridge header pins?

Thanks!
patrick

10/30/2007 11:25:33 AM

ringo

36 posts

Re: Serializer encoder problem

The scorpion looks like it is designed strictly for Rc use, so It needs a Servo type control, Not PWM and direction.

Bad News, pwm and Direction won't work.

Good news, you can use the Serialzers Servo command to control the scorpion.

Bad news, all the PID stuff (Mogo, Digo) is tied to PWm and Dir so it won't work with the servo command.

You can use the servo command and so your own PID on the higher processor, at least that will tell you if the encoders are working properly. The Encoders are NOPT tied to pwm in any way so they will still work fine.

Ringo

Ringo Davis, Hardware Technical Lead RoboticsConnection.com

10/30/2007 12:01:22 PM

rpg

35 posts

Re: Serializer encoder problem

Hi Ringo,

Thanks for the clarification. So now I'm looking at other high-current DC Motor controllers out there to see what I could use with the Serializer encoder ports and PID control. Your own motor controllers seem to require 12V or higher whereas I am trying to stick with 7.2V motors. So I looked over at Trossen and they have some SyRen and Sabertooth controllers that have the voltage/current range I need. What I can't tell from the description is whether or not they would work with the Serializer's PWM H-bridge control. Quoting one of the specs it says: "With just one SyRen driver you can control a motor with: analog voltage, radio control, serial and packetized serial." A similar statement applies to the Sabertooth. So are these compatible with the Serializer's PWM H-bridge control? If not, can you recommend a DC motor controller that is and that can drive 7.2V motors up to, say, 10A continuous (at least briefly) and, say, 15A peak?

Thanks again,
--patrick

10/30/2007 12:24:38 PM

ringo

36 posts

Re: Serializer encoder problem

If you low pass filter(resistor and capacitor) the PWM you get an analog voltage. Unfortunately what you get is 0-5V for a pwm of 0 to 100. What the controllers you mentioned want is 2.5V for no movement, then close to 5V is forward, close to 0 is reverse. You could use these H-bridges and the serializers PWM command, but again MOGO and DIGO would not work. I did some searching briefly and did not find anything that works below 12V. Try posting on Comp.robotics.Misc or some of the big club sites (PARTS, Seattle robotics, etc) and see if anybody has a recommendation. Higher Amps at a lower voltage, I don't know if anybody has anything like that.

One alternative is to use 12V, and limit the speed in your software to a max of 7.2/12 of the maximum speed, then your motors would be seeing essentially 7.2V at max.

Ringo Davis, Hardware Technical Lead RoboticsConnection.com

10/30/2007 1:07:23 PM

rpg

35 posts

Re: Serializer encoder problem

I'm definitely keen on using the MOGO and DIGO commands so I'll need a true PWM compatible H-bridge. The reason I want to stay with 7.2 V motors is that I want to use smaller 7.2 V battery packs. I have a 12V Portable PowerStation to power the mini-ITX motherboard and other logic boards (via a 7.2V regulator), but it is capable of only 5A max total output. So I am thinking of powering the motors with a 7.2V RC battery pack.

Here's a bare H-bridge from RobotPower that seems like it might do the job and work with all the Serializer PWM stuff. Would you please take a quick look and see if that is correct?

http://www.robotpower.com/products/simple-h_info.html

--patrick

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Serializer Robot Controller

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