Thursday, December 10, 2009

Creating Movement for the Cat Toy

Over the past couple of days I have struggled in my attempts to set up a stepper motor. Late last week my struggle continued as I tried to set-up three new stepper motors that I received for the cat that I am building. Having come home defeated I decided that the best course of action was for me to do some research regarding how stepper motors work so that I can improve my understanding and conceptual model of this component.

In the last hour I have discovered two really good overviews of how steppers work. The first is Mike Cook’s overview on his instructional blog; this is the second time that I link to Mike’s blog, he has a lot of great content for beginner's like me. This tutorial helped me finally understand how the coils are arranged and organized within the motor and how the stepping sequence is able to move the motor rotor through different positions. In retrospect it all seems obvious.

Another website that has content that is worth checking out is The tutorial here does not provide as thorough an overview of the inner workings of stepper motors. However, it does a better job at providing guidance for figuring out the proper wiring sequence of a stepper motor.

So what the hell did I learn about the topics mentioned above? Here is a brief overview but for more in-depth information check out the two links above.

Structure of the coils inside the motor
The coils in stepper motors are wrapped around a structure that surrounds the rotor. The number of times that the coils are wrapped around the rotor determines the number of steps required for the motor to make one full rotation. For example if the coils are wrapped around 48 times, then the motor would take 48 steps to complete one full rotation. Here is an image from Mike Cook’s site that illustrates this design.

To move the motor the coils are energized in sequence. Motors can be used in two different modes: full-step and half-step. When two coils are energized at any given time the motor moves in full step, which provides greater torque but less precision. When the motor is energized one coil at a time it provides greater precision of movement (twice the number of steps per rotation) but less torque. Here is another image from Mike's blog that demonstrates how full-step movement works.

Now that I understood how stepper motors work, I had to figure out the proper step and wiring sequence to get the motor to work properly. I started by re-checking all wire connections to ensure that I attached the leads from the motors to the appropriate control pins (via the transistors) and power source pins. This was a good thing because I realized that I had attached one of the power wires to a control pin.

Once I the wiring was set-up properly I was still experiencing issues with the stepper motors. They would turn on and spin for 10 to 20 seconds, then they would stop working. I met with Xiaoyang, one of ITP's residents, regarding this issue. He recommended that I test the power source voltage and amperage. The motor’s rating is 5 volts at 1 amp. Based on Xioayang’s advice and my research online, I decided that I needed to find a power source that delivered twice the current required by the motor.

I purchased a 2 amp transformer from Radio Shack that can be set to output between 3v to 7v. It is a great little tool, and it brought my motors to life! I was dancing around the table when this happened. It seems like I may actually be able to bring my cat toy to life. My next challenge was getting multiple motors to run smoothly together smoothly. The code samples that I've found and the stepper motor library are not appropriate for controlling multiple motors - more on this on my next post on this subject.

[note: most of this post was written during last weekend on December 4th]

No comments: