DFRobot 4WD Arduino Mobile Platform
I just got my DFRobot 4WD Arduino Mobile Platform from RobotShop (Arduino not included).
The whole kit was pretty simple to put together, and DFRobot throws in a couple of nice extras like a toggle switch for power, and a female receptacle you can use for your charging system. All in all, it seems like a pretty sweet deal at ~$50.
I’m using the platform with an Adafruit Motor Shield and an Arduino to control the four included gear motors. This probably isn’t the optimal solution, because the yellow motors that come with the platform have a stall current of 1.5 amps at 3 volts. The Motor Shield has a max current of 1.2 amps per motor. I did some testing and couldn’t get the motors to pull more than 700mA so I should be OK, but it is something to consider.
The biggest hurdle in getting the platform working with the Motor Shield was the fact that the motors would not run consistently: they would stop and start at random intervals. After half a day of research, I came across this post describing exactly the symptoms I was experiencing. My solution was to solder .1uF caps across the leads on each motor. They’ve worked without a hiccup ever since.
My other big concern was load capacity. I’ve managed to carry about two pounds with the platform, but any more weight and it just won’t move.
My original steering plan for Pedro2 also would not work here. I had initially planned to rotate the robot by spinning the left and right wheels in opposite directions, like a tank. Due to the spacing of the tires, the four wheels actually work against each other when this is done and the tires will eventually work themselves right off the gear motors (they just press on to the motor nubs). My solution was to only slow the motors down on the side that I want to turn in. The turning radius is greater (no more turning on a dime), but the tires won’t pop off and all four motors will push the robot through the turn now.
So it wasn’t quite a plug and play solution, but it does look like this will be an adequate base for Pedro2. Here’s a quick shot of the upper level chassis I’ve put together paired with the mobile platform:
Hi Scott, I found your post about the DFRobot very helpful. I’m thinking about getting one soon and this was full of great info. I’m still a newbie so I have a couple of questions…
In your post, you say “The Motor Shield has a max current of 1.2 amps per motor. I did some testing and couldn’t get the motors to pull more than 700mA so I should be OK, but it is something to consider.”
Can you elaborate on that? Specifically, 1) do you want each motor to pull more than 700mA and if so why? 2) Why is it something to consider? 3) Is the lower amperage affecting the bot’s ability to carry a heavier load or move faster?
One other question… what is the possibility of using stronger motors?
Thanks for your insight!
Hey Doug, I’m glad you found the post helpful!
I’m a newbie to much of this myself, but I think I can answer your question: Basically, the less amperage the motors pull the better. Ideally you want to remain as far below the current limit of the motor shield as possible. As you start to get closer to the 1.2A peak (and 600mA operating) limit, you run the risk of burning out the motor driver chips on the shield. I mentioned it as something to consider because although it appears to be working well for me, I am getting close to the current limit of the motor shield and your mileage may vary.
There’s definitely a relationship between motor speed and voltage. Basically the higher the voltage the faster the motors spin, but the trade off is that the motors draw a proportionately larger amount of current. Push it too far, and the motor shield simply won’t be able to handle it and the driver chips on board will burn out.
As I said, I’m a bit of a newbie myself so I can’t offer anything more scientific than that. I discovered what worked best for me by basically piling some weight on the base, and powering the motors with my variable voltage power supply. I checked to see if it could carry the weight, and use a multimeter to check how much amperage was being pulled by the motors at different voltages.
If your interested, I’ll be posting over the next few weeks / months about the build of my robot here:
Pedro2 build log
any ideas on how to keep the wheels on? mine keep falling off…..smoothing out any jitter has helped and not letting the robot yaw too much helps but fed up with the wheels coming off.
I would think that weight may be an issue with turn, I’ve got stationary turns and moving turns working as a tank ie opposite direction for stationary or slow dow one side speed up the other for moving turns. But I am using newer solarbotics standard motor with 6.5 V to the motor driver.
I would also advise on suppressor caps on the motors, they will also add some life to the motors. I’d be aiming at 6- 7.5 Volts for the voltage to the motors. Some drivers will drop some and so the voltage at the motors is less than suppplied.
Have fun with Pedro!
Dave
i was wondering what code you used to make one side go slower then the other i am trying to do the same with a tank style, at current it rips the treads off…
Hi Scott, great article. What are you using for power? I’m having a heck of a time getting these motors to work on battery power.