Arduino/Motor Control/steppermotors

Stepper Motors
This was written for the purpose of familiarizing you with stepper motors and subsequently how to program their operation into Arduino. Arduino is a prototyping program that allows the hobbyist or student level user to perform operations of more advanced circuits or microchips through the use of an easy to use and modular computer code. Though the code is simple it can still be challenging to many who are not familiar with computer code or the devices that a computer code can control.

A stepper motor, much like a DC motor has a rotating permanent magnet propelled by stationary electrical magnets, however the motion is divided into a number of steps around the rotation of the rotating magnet. It does so by having several teeth on the rotating magnet that line up with specific locations around the stationary charged magnets. When voltage is supplied to a specific magnet or specific sequence of magnets the motor will rotate, or step to that position and hold.

Stepper motors can spin like a regular DC motor, however they can also stop on a position like a servo motor. This gives them many uses. Some basic places where you will find stepper motors are in disc drives, printers, faxes, slot machines, clocks, intelligent lighting, and automotive gauges.

Stepper Function
There are also some differences you will see internally on a stepper motor as opposed to a regular DC brushless motor. One main feature is the stepped rotor as opposed to a smooth magnetic rotor. Because of these differences the Arduino Uno can only be used as a logic module for a stepper motor circuit. A driver module will be needed to complete operation of a stepper motor. This is because to hold the rotor in individual positions a map of different voltages needs to be executed on the magnets to accurately rotate the motor. Without this your motor will most likely not work or will just spin weakly.

Attached are some diagrams of what a basic stepper motor wiring diagram will look like. Also attached is a chart that displays how a full voltage to position map must be created to operate a stepper motor.



This is what actually gives the motor its ‘steps’. By charging and releasing magnets on the stator in sequence you can get the position of the rotor to line up on the individual steps around a single revolution of the motors movement. Each ridge on the stepper rotor is an individual step and the number of steps a motor is capable of is the number of ridges around the circumference of the rotor.

This creates a difference in how a stepper motor is controlled vs a DC motor. While a stepper motor can accept straight current to turn in either direction this is not its most effective mode. If all you need is for the motor to turn a DC motor would be better as the steps on a stepper motors rotor give less magnetic area and therefore a stepper motor of similar size to a DC motor will have less power. But as stated a stepper motor can stop and hold a position.

There are a few more things that need to be known about stepper motors before we can jump into the programming side. A stepper motors measurement is not only is torq production or max RPM:

1. Pull in Torq: This is the torq provided by the motor when it is holding a position while power is being applied. This is how powerfully the motor can hold a step while it us under power. This is useful when a stepper motor is holding a spring loaded device like a small catapult or a an arm on a robot when it is pre-loaded.

2. Pull out Torq: This is the amount of torq that is needed to make the motor stop or miss steps when it is rotating under power. This is useful to know if you plan on using the motion of this motor against a force. A good example of this would be in a CNC machine or on a large tooling lathe.

3. Detent Torq: This is the resistance torq provided by the motor when it is not under any charge. This is a measure of the magnetic force inherent in the motor and not a measure of friction of any bearings in the device or the force it takes to move the mass of the rotor alone. This is important to know, however know that the detent torq will be negligible on motors with soft iron cores. This knowledge is good to have so you are aware of how much power it will take to move the motor when it is not under power. This is important in devices where several motors will be connected a single device and movement will be based on which motor is running. Or in any spring loaded device where you depend on a spring working against the motor to make the device return to its original position.

Stepper Code in Arduino
As stated above, because a stepper motor is not most efficient running off of a standard motor code other commands have been built into programs in order to operate a stepper motor properly. Arduino has special commands to operate stepper motors. On the following page is an example of a stepper motor program that can be found on Arduinos website here: http://www.arduino.cc/en/Tutorial/Stepper.

Read over this code. Please note that this program would require the arduino to be attached to either an arduino motor shield, or at least a motor driver chip, in-order to work correctly. The exact circuit layout would depend upon the type and voltage of the stepper motor. This particular program utilizes only two pins to drive the stepper, which would require resistors and/or transistors, making it a little more complex than a typical 4 pin set-up, again, depending upon the stepper used.

Here's a breakdown of the parts of the sketch:

 # include  initializes the stepper library in the Arduino. This library is loaded in-order to simplify the coding required to use the stepper.

 # define motorSteps 200 is what tells the Arduino how many steps to expect out of the stepper motor. This is where you would input how many steps your specific stepper motor has. The three lines after that, specify which pins on the arduino is controlled by each of the listed variables. Notice that the ledPin is using pin 13, which is where an indicator LED is built-in on most of the newer arduino systems. The LED acts as an indicator only and is not a function of the stepper motor.

void setup  is a required function in all arduino codes. The code within it's { and } brackets, will run only once.

myStepper.setSpeed(60) ;, as you can see, controls the speed at which the motor will operate in RPMs or Revolutions Per Minute. In this portion of the code you can control the speed of the motor.

From there a serial port is initialized so that outputs of the function can be read at a baud rate of 9600, and then the "sketch" (program) calls the blink function located at the end of the sketch, telling it to blink 3 times, indicating that the end of the setup function has been reached. Neither the blink function, nor the serial commands are needed to run the stepper, but are merely there for the user to be informed about what the program is doing, for debugging purposes; It could be useful to know that the stepper is being driven forward or backward, to give an indicator that clockwise rotation is forward or backward depending upon how you've wired the stepper.

In the required loop function of the sketch, everything within the brackets will repeat over and over again. Here, we begin to see the stepper motor receiving commands.

myStepper.step(100) ; commands the Arduino to make the motor move 100 steps forward. In this line you can change the number of steps or enter a variable based on a sensor input. To make the stepper motor rotate in the opposite direction, you'd use a negative number instead, i.e. (-100). delay(500) ; tells the system to wait 500 milliseconds (or .5 seconds) until initiating the next command. Serial.println is used to send display information to the serial monitor, in this case, indicating what the motor is doing at that particular time.

The next 'myStepper.step command uses a negative number, causing the stepper to rotate the opposite direction.

This last bit of code is a function the author decided to name "blink", and they used it to indicate to the user that the Setup section of the "sketch" (program) was completed. This will only run when the sketch calls for it, because it is after, and outside of, the setup and loop functions. As noted, this function is called once, from the last line of setup. It simply flashes the LED located at Pin 13 (built-in on many arduinos) to alert the user that it is done with setup, and is about to start moving the stepper. If your arduino doesn't have a built-in LED, you'd attach one to pin 13.