LAB 4
Characterize Your Car

The fourth lab was different from the other labs up till now, in that it did not require any major implementation of hardware or software, but instead involved observation and characterization of the RC stunt car in the lab kit. We will be using this car throughout the course for our robot with the original chassis, wheels, and motors, so it was important to study and document some features of the car so that we can use that information while designing our own control system for the car using the Artemis board.

Simple Measurements

The first part of the lab involved documenting some parameters of the car like its physical dimensions, battery capacity, etc. The dimensions of the car are as follows:

  • Length: 18 cm (7 1/8 ")
  • Width: 14.2 cm (5 5/8 ")
  • Height: 8 cm (3 1/8 ")
  • Wheelbase: 10 cm (4 ")
  • Ground clearance: 1.5 cm (3/5 ")
Figure: Length, width, and wheelbase measurements of the car.

In order to determine the battery capacity in a standard manner, I turned on the robot and ran the wheels continuously until the robot stopped moving. This gives an estimate of the battery life to be around 5 mins at full speed. Once the battery was completely discharged, I put it to charge and it took about 25 mins to come back up to 3.9 V.

Experimental Measurements

What is the range of speed of the car?

In order to measure the speed of the car, I manually created a straight path for the car to follow, and measure the time it took for the car to cover different distances. The car took on average (over 3 readings) 0.96 seconds to cover a distance of 2.5 m, and 1.12 seconds to cover a distance of 3 m. This gives an average speed of about 2.6 m/s.

What stunts can it do?

The car, being an RC stunt car, is meant to do a variety of stunts, and is also made to be durable enough to withstand all forms of bumps, jumps, jerks, etc. The easiest ones I was able to perform were with combinations of forward and backward motions. Due to the high speed of the car, stopping the car abruptly can cause its front/back wheels to lift up, resulting in stunts like “wheelie”, “stoppie”, and vertical flips.

The built-in “Cyclone” mode in the car (activated by left finger button on the controller) is set up to perform a sequence of jumps on all its wheels. Once the button is pressed, the car accelerates straight for almost 3 m and then suddenly starts tossing in the air out of control. I was unable to recreate this stunt with manual control, but it would be a cool sequence to perform.

While I was testing the car and trying to perform various stunts on the car, my cat was also curiously and fearfully watching and tried to contribute with its own stunts here.

How reliably can the robot rotate around its own axis?

The robot can almost perfectly rotate on its axis due to the differential pair drive. This results in a turning radius of 16 cm (6 ¼ “).

How well can you operate the car without manual control?

It is quite difficult to control the car manually, especially when running the motors at full speed by holding down the control buttons. For example, if trying to stop before a wall while going forward towards it, the only way is to suddenly press the back button, which mostly results in a flip and collision with the wall anyway. If we try to control the car speed by repeatedly holding down and leaving the button (essentially manual PWM), it can be slightly easier. Moreover, the motors on the car are not of the exact same quality. While trying to go straight in a line, the car always drifts a little to the right, implying that the left motor is slightly stronger than the right.