Stepper Motor Driver

In order to keep power loss low, two methods are used. A simple and popular solution is to give only as much voltage as needed, utilizing the resistance (RL) of the winding to limit the current. A more complicated but also more efficient and precise solution is the inclusion of a current generator, to achieve independence from the winding resistance. However, the supplied power required is also higher. 

Power delivered by the motor is proportional to the current in the winding. In the dynamic working order a stepper motor changes poles of the winding current in the same stator winding after two steps. The speed with which the current changes its direction in the form of an exponential function depends on the specified inductance, the coil resistance and on the voltage. Figure below shows that at a low step rate the winding current IL reaches its nominal value VL/RL before the direction is changed. However, if the poles of the stator windings are changed more often, which corresponds to a high step frequency, the current no longer reaches its saturating value because of the limited change time ; the power and also the torque diminish clearly at increasing number of revolutions.

Running a stepper motor in half-step allows its position resolution to be increased by a factor of 2. It also avoids disturbance by the motor resonance, as the course covered by the rotor is only half as long and the

system is less stimulated. These may be so strong that the motor has no more torque in certain step frequency ranges and looses completely its position. This is due to the fact that the rotor of the motor, and the changing magnetic field of the stator forms a springmass-system that may be stimulated to vibrate. In practice, the load might deaden this system, but only if there is sufficient frictional force.On the down side, the half-step system needs twice as many clock-pulses (twice clock frequency) as the full-step system, and it is only able to deliver half of the torque of the full-step. It is also possible to turn the motor in small microsteps by controlling current at each motor phase precisely. Advantages of microstepping includes better positional resolution, less resonance issues and loser audible noise.

Commands driving the motor comes from a connected microcontroller. In its simplest form, a full-step control needs only two rectangular signals in quadrature. According to which phase is leading, the motor axis rotates clockwise or counter-clockwise, whereby the rotation speed is proportional to the clock frequency. In the half-step system the situation becomes more complicated. The minimal two control signals become four control signals. In some conditions as many as six signals are needed.

A typical control circuit that reduce the number of outputs required from a microprocessor from the 6 required to 3 static and dynamic control line is shown below, 

information obtained from http://users.ece.utexas.edu/~valvano/Datasheets/Stepper_ST.pdf

The stepper motor I have is L2818S0604-T5X5 from Nanotec with a resolution of 0.025mm per step and is able to deliver a maximum thrust of 30N. Matching driver is available to purchase, there are also a variety of similar products available in the market. The maximum current/phase of the motor is 0.95A. The Cytron 3-40V, 2A Unipolar / Bipolar Stepper Motor Controller from http://www.robotshop.com/cytron-3-40v-2a-unipolar-bipolar-stepper-motor-controller.html could be an option. I have also sent e-mails to suppliers of Allegro MicroSystems in the UK to ask if a free sample could be delivered.