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How to Determine Stepper Motor Size Requirements

Stepper motors can be conscripted into a variety of duties. The versatility ranges from ink jet printers to robot mechanisms and everything between. For hardware developers and hobbyists, the right size of stepper motor will help dictate successfully completion of the project. Demand from industry and consumers has lead to a vast array of stepper motors. Although the physical dimensions of the motor must be taken into account, the functional characteristics will play a greater role when deciding on the necessary size requirement for a stepper motor.

Things You'll Need

Stepper motor specifications
Project specifications
Scientific calculator

Instructions

- 1
  Determine how fast the motor must turn. Each pulse applied to the stepper motor makes it turn only a fraction of a full rotation. The required number of pulses can vary. Fewer pulses can allow the motor to turn faster. However, if your project needs greater resolution, more required steps per rotation will allow for greater control of the motor shaft position, but that may translate to a slower motor.
- 2
  Calculate your torque requirements. If the stepper motor you select has insufficient torque, you incur the risk of stripping the motor armature and subsequently burning out the motor. The typical unit for motor torque is in ounce-inches. To calculate the torque requirement, use the equation, T = Fr, where F is the force measured in ounces to move a mechanism and r is the radius of rotation in inches. Multiply the force by the radius with your scientific calculator and even consider doubling that number to account for any possible spikes in torque demand.
- 3
  Calculate the load inertia. This will depend on whether the motor will need to turn a hollow cylinder or a solid cylinder. Typically, a wheel for a robot can be modeled as a solid object, so I = (W * r ^ 2) / 2 will suffice. For this case, the weight of the cylinder, W, is measured in pounds and the radius of rotation is in inches. If the cylinder is hollow, use I = [W * (r1 ^ 2 + r2 ^ 2)] / 2, where W is the weight of the hollow cylinder in pounds, r1 is the inner radius of the cylinder in inches and r2 is the outer radius in inches. As a reminder, the unit for I is reported in pounds-inches-squared.
- 4
  Consider the required step angle. The step angle is measured by the full 360 degrees of rotation divided by the number of steps required for a full rotation of the shaft. This is usually reported in degrees.
- 5
  Know the acceleration and deceleration time needed. The acceleration time and deceleration time respectively refer to the amount of time required for the motor shaft to reach the determined speed and come to rest. This values are typically reported in milliseconds.
- 6
  Know the type of driver to be used. You will have to review the specifications of the stepper motor driver that will connect to the selected motor. Among the factors to consider are the connector required to connect the motor to the driver and the driving current delivered to the motor.
- 7
  Factor in the size and weight considerations. Although you should have calculated for plenty of torque for your application, you make sure the motor has enough space to fit and that the weight of the motor will be supported by the structure.

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