I'm using a 9.6v truck with the stock battery, stock motor controller(no speed control).
The truck is just too fast. I've tried PWM, I've tried a more primitive bit bang and in the end I just ended up playing with the on/off timing and have the truck pretty much leaping to the edge of the PING sensor's range over and over after each PING sweep. It's a little barbaric for a GPS based project. I'm probably not going to use this chassis after robothon but I wanted to know if I missed an easy way that someone else out there may have so that others in the future know that there is a way (using the stock battery and harware) or to just avoid these speed machines.....
With PWM I could get the truck rolling on flat pavement at an analogWrite of around 150. This was with no torque at all. Couldn’t roll on grass and evenutally if I left it running in a straight line it would get back to top speed. I call this “crash and burn” speed due to the carnage that happens when it hits a curb.
At 200 it was still only able to run on flat pavement but got to crash and burn speed faster.
Utlimately with PWM I was only able to reduce the torque at the wheels or delay the time that it took to get to top speed. I was never able to actually reduce the running speed if I let it run in a straight line for any time at all.
This may just be a bad platform for a UAV due to the weight (it does not survive crashes well), speed (it runs too fast to correct for obstacles in time) and the gear box (apparently geared for top end horsepower not torque).
I was kind of hoping that there was a success story out there to inspire me to keep tinkering with it. I got the truck and two sets of batteries for $18 on CL so I’m not too attached to it.
It's just one of those larger Nikko 4x4 trucks with the old NiCad 9.6V battery pack.
I'll tear it down after this weekend and take a pic of it and the control board with notes.
The remote has two speeds. One behaved just like my PWM @ 200. This was for forward "low" and reverse. The other speed was "turbo" which is pretty much PWM @ 255. I think that I am going to have to build a really small arduino dynamometer to show this weird power curve.... For this weekend I've got a set of smaller wheels and tires I'm going to throw on it as a temporary speed cheat. This should lower my top speed and increase my torque a little. It's going to look a little less macho but it might survive a crash.
I swapped out the wheels/tires for a set off of a smaller Nikko truck last night and it changed the behavior completely. Between PWM @ 130 and 210 I have some ability to control the speed. Anything over 210 results in wheel spin. The great news is that it has reduced my curb smashing speed tremendously and my PING survived the evening!
It looks like I had more of an issue getting the big tires to start turning than controlling the motor speed.
Ultimately I would recommend not being a cheap a$$ like I was and look for a used RC car with an actual speed control in the first place. It was a cool platform for open parking lot racing projects but a little rough for obstacle avoidance.