There are two common winding arrangements for the electromagnetic coils: bipolar and unipolar (Fig 4).The described stepping sequence utilizes the bipolar winding. Each phase consists of a single winding. By reversing the current in the windings, electromagnetic polarity is reversed. A unipolar stepper motor for sale has one winding with center tap per phase. Each section of windings is switched on for each direction of magnetic field.Since in this arrangement a magnetic pole can be reversed without switching the direction of current, the commutation circuit can be made very simple for each winding.
Aside from the “one phase on” (aka Wave Drive) and “two phases on” (aka Full Step Drive) stepping mechanisms,there are also Half Stepping (1 & 2 phases on) and Microstepping (Continuously varying motor currents). HalfStepping is a driving method that alternates between “two phase son” and “one phase on”. It therefore doubles the steps. Microstepping is a more advance stepping mechanism where the coil windings are no longer fully on or fully off. What is commonly referred to as microstepping is often “sine-cosine microstepping” in which the winding current approximates a sinusoidal AC waveform. Sine-cosine microstepping is the most common form, but other waveforms can be used. Theoretically, this method can position the rotor direction anywhere between phases. As the microsteps become smaller, motor operation becomes smoother. To better understand these mechanisms, Fig. 4 shows the drive currents on 4-phase unipolar stepper motor.
Stepper motor Driving and Control:
Stepper motors require some external electrical components in order to run. These components typically include a cnc power supply, logic sequencer, switching components and a clock pulse source to determine the step rate. Many commercially available drives have integrated these components into a complete package. Some basic drive units have only the final power stage without the controller electronics to generate the proper step sequencing. Common drives include Unipolar Drives, Bipolar Drives, L/R Drives, Chopper Drives, Microstepping Drives.This tutorial will not discuss in depth the driving of stepper motor.
Most commercially available stepper motor drivers takes pulses as inputs. The amount of rotation of the stepping
motor is proportional to the number of pulses given to the driver. The relationship of the stepping motor’s rotation
and input pulses is expressed as follows.
The speed of the rotation is then proportional to the speed of the pulses. The relationship of the pulse speed (Hz)
and motor speed (r/min) is expressed as follows: