In this lab, we changed from manual to open loop control of the car. At the end of this lab, our car executed a pre-programmed series of moves, using the Artemis board and two dual motor drivers.
Prelab
What pins will you use for control on the Artemis?
We recommend powering the Artemis and the motor drivers/motors from separate batteries. Why is that?
This is because the motors draw a lot of current and the Artemis does not require as much, it makes sense to use different batteries to power each one; the battery with higher energy capacity (850mAh) is used for the motors while the one with lower energy capacity (650mAh) is used for the Artemis.
Routing paths
One way that we’ve attempted to suppress noise is to twist the wires and cables. This is because the positive and negative magnetic fields cancel each other out. I’ve decided to use longer wires as compared to shorter ones just in case the sensors/board does not reach the areas I would like to place them at in the car. I’d imagine having longer ones would be easier to manage (just either cut them and use heat shrink to connect them together again or stuff them under sensors) rather than have them too short and I would need to desolder them and use a longer wire (seems like more hassle). I also assumed that the noise is not too significant to be taking it into too much consideration (that being said, I avoided using unnecessarily long wires too).
Lab Tasks
I first soldered the motor drivers to the Artemis board and made the connections as shown below:
I chose a 3.7V power supply so as to mimic the voltage of the batteries that we use (3.3V from the Artemis, 3.7V from the battery) and 5A current. I used the function analogWrite() to generate PWM signals to the 7 and 12 pins. I took the image below from the datasheet and referred to it when deciding what value to analogWrite() to each pin.
I first set pin 7 to 0 and pin 12 to 100 and got the oscilloscope output as shown below.
Changing pin 12 to 200 gives this output:
These were expected results so I did not try other combinations of PWM values. I then moved on to taking my car apart and replacing the oscilloscope connections with connections to the motor. I used the code below instead to show that the motors can run in both directions.
As you can tell from the code, the motor should run in one direction for 1s, pause for 1s, run in the other direction, and pause for another second before repeating this cycle. I was supposed to use the power supply but jumped straight to using the battery because the battery supplied a more stable current than the power supply (i.e. the motor was more jittery with the power supply). Below is a video showing the motors running with the battery. The pause in the video seems less than 1 second because it takes some time for the motors to come to a stop (which eats into the pause time).
Next, I soldered the second motor similar to the first (with the connections shown in the Prelab) and ran this code on the Artemis:
Similar to the example with one motor, the exact same thing should happen but this time, to both motors simultaneously as shown below:
Now that I knew that both motors work, I could secure everything in the car and run it on its own.
I set the motors to the PWM value of 100 and set it to stop after 3 seconds as shown below. After testing it out, I realized that the battery was not secured under the Artemis and had to add more tape to make sure it wouldn’t fall out.
I tested slower speeds on the car travelling on both carpet and the floor. The lowest speed that the car can go while still moving at the expected speed is when the PWM value was 25.
In order to determine whether the motors spin at the same speed, I set the PWM values to the lowest value (25) to make sure that the car goes slow enough to tell whether it deviates from the straight line. As you can see in the first half of the video, the robot travels along the straight electric tape on the ground and does not deviate much. I wanted to test this with a greater speed (100) to see how the car would perform, which you can see in the second half of the video. The car would travel more to its left, before realigning and going in a straight line. This is because the time it takes for each motor to reach the final speed is different (their accelerations are different). However, after it reaches the final speed, the motors spin at approximately the same speed. Thus, no calibration was needed for the motors.
Open Loop Control
Below is the code and video of the robot car demonstrating open loop control: