Saturday, March 24, 2012

Using the DF Robot 4WD (SKU ROB0025) with the Arduino (Part II): Qt user interface.

In the last post, I put together the robot base, wired the motors, and wrote a simple test program to make all the wheels move. The next step is to make a simple user interface to control the robot to get it to drive around. The point really is to enable manual control to drive the sensors around and record measurements. Then given the measurements, develop a better controller to improve localization estimates of the robot over time. After that is done, I will add in other sensors that add information about the scene (i.e., video) that will enable rudimentary SLAM capability.

In a previous post, I had written a UI in Qt ( Qt is a UI toolkit that comes with a lot of examples, is open source, and fairly straight forward (as widget toolkits go) to use. I realize many people might think of developing user interfaces as a daunting task. There are several basic tutorials out there that can be uncovered with a little googling (

I have taken a pretty simple approach. Use OpenCV ( to grab camera images from a web cam and display them in a Qt OpenGL widget. Then have a second widget that enables robot motion control. I thought for awhile about the easiest widgets to use for control. Sliders and push buttons came quickly to mind. After some thought, I decided to use mouse movement in a 2D cartesian coordinate system. This enables easy mapping of left and right motor speed to a single point in the 2D grid. Moving the mouse around changes the ratio and spin speed of the wheels on both side of the robot. I figure this will let me figure out the robot dynamics and eventually build a fairly good controller.

Here is a screen shot. On the left is the webcam image, on the right is the 2D wheel speed grid. To send the commands, hold down the control key and move the mouse in the 2D grid. Releasing the key will stop the motors. This allows a virtual "kill switch" if the robot gets too out of control.

I also downloaded some code to interact with the Arduino over a serial connection. Most of the libraries I found were overly complicated for what I needed. Eventually I came across the four functions I was looking for at the site I wrapped this code in a simple threaded Qt class that has a signal to connect to receive data from the Arduino. The data can be written through a slot (with a mutex around to prevent collision).

On the device side, I downloaded some code to parse incoming messages from the device. There is a heavyweight library called Shine (see This seems like overkill for the first implementation. There is also a library called SimpleMessageSystem (see Like the name says, this seems like exactly what I was looking for .... simple. After some more digging, a newer version of the library exists called CmdMessenger (see Has a similar type interface in that you send a command id (in this case I am using 4 as the motor control command) followed by comma separated values, terminating in a semi colon.

I set things up so command 4 controls the motors, and the next two numbers are the speed of the left and right motors respectively. This means sending "4,100,100;" will cause both wheels to move forward and "4,-100,-100" will cause them to move backwards. In playing around with the speeds it seems that nothing much below 100 causes the wheel to turn. Looking on the web, a few places have suggested using an extra capacitor to smooth out the motor power. I will look into that once everything is up and moving.

So the code to receive the commands and send the speeds to the motors is located in:
Once that is uploaded, then the UI program can be started. The code for that is located at:
The serial port to connect to is hard-wired in the code. That is something I will need to fix. When this runs, holding down control and moving the mouse around causes the wheels to move. Here is a movie of the wheels moving:


Next up is hooking in the xbee and the wheel encoders. I also need to wire up a battery connection so that it can drive un-tethered. Luckily my Mega board arrived today so I will switch over to using that to gain access to some much needed ports.

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