As both an electrical engineer and a Halloween enthusiast, I decided to make my Halloween dreams (or nightmares) into reality with my first animated prop. I set out to make a skeleton lady that rocks back and forth as the centerpiece of our haunted house.
To build the animated skeleton, I started with:
- an old dress
- a skeleton head
- a remote control car toy
First we took apart the remote control car and hooked it up to an L293D motor control IC (H-bridge circuit) and we made our first prototype with styrofoam rods.
We used a micro-controller to send control signals to the IC. The H-bridge was used to control the direction of the DC motor to enable both forward and reverse movement.
After a successful dry run, we added the dress and realized we had a big mechanical problem. With the weight of the dress on top of the long rod, the torque was too much for our little remote control car motor. It could move forward, but the motor could not return to it’s starting position.
Next we tried a little servo motor. For a moment it looked like it would work, but after struggling against gravity one too many times, the gears were sheared.
The third time must be the charm, because next we tried a motor with a little more "umph" and found success! With a working prototype it was time to construct the final structure.
We used balsa wood and another foam rod to keep the frame light-weight but sturdy and secured it to a heavy base for stability. We topped it with our skeleton head and some purple hair.
We dressed the frame in its vintage gown, and found we had torque issues again. In order to lighten the design, we had to remove several layers out of the dress and the hair. Then, to give the motor just a little help against gravity, we tied a string from a support behind the frame to the arm attached to the motor. This took some weight off the motor when the frame was in full extension. That way the motor wasn’t strained to operate in reverse. Ta da!!! A successful rocking skeleton lady!
We probed the signal going into the motor to get a better understanding of what was going on electronically.
We used the servo library in the Arduino programming language to program our IC. We adjusted the angles and time of the movement, time between movements, and repetition of movement. The signal sent to the motor is digital, 0V or 5V so we suspected that we would observe a pulse width modulated signal on the oscilloscope. Servo motors, in a nutshell, consist of gears attached to a DC motor with a control circuit which manipulates potentiometers to change the motor’s movement. The resistance of the potentiometers is adjusted to send the customized movements to the motor.
Here is what we found:
A pulse width modulated signal!
Here we have the motor hooked up to the frame and are probing the signal to the motor simultaneously so we can see how the pulses coordinate with the movement. The smaller pulses are the rock forward and the large pulses correspond to the rock back – which makes sense as it probably takes a lot more power to go against gravity then with it.
All that was left was a few finishing touches and this prop is ready for Halloween and its debut in the haunted house.