Special Machines Questions and Answers

Answer: A

Correct answer: through a fixed angle for each input pulse

A stepper converts a train of pulses into a corresponding number of discrete angular increments, so counting pulses counts position. This is what allows open-loop positioning: the controller knows where the shaft is because it knows how many steps it has commanded, provided none has been lost.

Answer: B

Correct answer: 1.8 degrees

\[ \frac{360}{200} = 1.8\ \mathrm{degrees}. \] The 200-step motor is the industry standard, and 1.8 degrees is the step angle almost every general purpose stepper offers. Halving it to 0.9 requires 400 steps, achieved by doubling the rotor teeth rather than by anything done in the driver.

Answer: C

Correct answer: toothed soft iron with no winding or magnet

The variable reluctance rotor is simply a shaped piece of soft iron, which makes it cheap, robust and light - a low inertia that suits rapid stepping. The price is that with no magnet there is no detent torque, so the rotor is free to turn when the machine is de-energised.

Answer: D

Correct answer: detent torque when de-energised

The rotor magnet interacts with the stator iron even with no current flowing, so the shaft resists displacement and holds its last position without power. This is often a useful safety property; where a free shaft is wanted when de-energised, the variable reluctance type is chosen instead.

Answer: A

Correct answer: a permanent magnet with a toothed rotor structure

The hybrid takes the fine step angle of the toothed variable reluctance rotor and the detent torque and higher torque density of a permanent magnet, with the magnet magnetised axially between two toothed end caps offset by half a tooth pitch. It is the commonest type in use because it is better than either parent on both counts.

Answer: B

With two phases on, the rotor settles between two full-step positions, so the sequence gives twice as many positions per revolution. The torque alternates between the one-phase and two-phase values, which is a drawback.

Answer: C

Driving sinusoidally weighted currents in the two phases places the resultant field at any angle, so the rotor can be held anywhere between full steps. Smoothness improves greatly and the resolution becomes a matter of the driver rather than the machine.

Answer: D

Correct answer: needed to displace it while its windings are energised

Holding torque is measured with the windings carrying rated current and the rotor at rest, and it is the figure that decides whether the shaft will stay put under load. Detent torque is the corresponding figure with the windings de-energised, and it is very much smaller.

Answer: A

Correct answer: the load torque exceeds what it can develop at that stepping rate

A stepper’s available torque falls as the stepping rate rises, because the winding inductance limits how fast the current can be established. If the demand exceeds what is available the rotor fails to follow and steps are lost - and since the machine is running open loop, nothing detects or corrects the error.

Answer: B

The rotor oscillates about each new step position, and at rates near that natural frequency the oscillation builds up and can stall the machine. Damping, microstepping or simply avoiding the affected range are the remedies.