Actuators & Control · Study deck

Stepper Motors: Precision and Reliability

The motor follows a planned move on the bench, but open-loop motion can still lose position under load.

Motor Max is your guide for this deck.

steppermotors

After studying this chapter

Learning objectives

You will be able to:

Explain: Holding torque is how hard an energized stepper resists being turned off its commanded position; it clamps the load in place with no motion, which a basic DC motor cannot do by itself.
Explain: Microstepping gives smooth, fine motion, but the incremental torque per microstep is small, so microsteps improve smoothness and command resolution far more than they improve absolute positioning accuracy.
Explain: Energize its coils in the right sequence and the rotor snaps to the next aligned position; do it repeatedly and the shaft advances one precise increment per pulse.
measure stepper precision and missed steps

Major section

Deep Dive: Step Counts, Torque Margin, and Missed Steps

That counted motion is why steppers are useful for open-loop positioning.

Energize its coils in the right sequence and the rotor snaps to the next aligned position; do it repeatedly and the shaft advances one precise increment per pulse.
Microstepping makes the command grid finer.
Sizing starts from travel.

Stepper motors turn counted electrical pulses into fixed angular steps; the driver translates MCU step and direction signals into the coil sequence that moves the rotor.

Major section

Deep Dive: Step Counts, Torque Margin, and Missed Steps (continued)

A common motor has a 1.8-degree step angle, which is exactly 200 steps per revolution.

Counted commands estimate angular motion only while current, acceleration, and load leave enough torque for every alignment step to occur.
If a controller commands 400 steps on a 200-step motor, the shaft should move 400 / 200 = 2 revolutions.
The screw needs 18 / 1.5 = 12 revolutions.

Major section

Deep Dive: Step Counts, Torque Margin, and Missed Steps (continued)

On a 2 mm-pitch lead screw, that becomes 2 rev x 2 mm/rev = 4 mm of carriage travel.

The calculation is simple, but it is only true while the motor has enough torque margin to follow every commanded step.
At 1/16 microstepping, a 200-step motor has 200 x 16 = 3200 commanded microsteps per revolution.
Torque drops as speed rises, which is why steppers need acceleration ramps.

Major section

Deep Dive: Step Counts, Torque Margin, and Missed Steps (continued)

On the same 2 mm lead screw, one microstep is 2 mm / 3200 = 0.000625 mm, or 0.625 micrometers.

If an axis is told to move 10,000 microsteps and it misses 80 during a fast acceleration, the controller still believes the target was reached.
Holding torque is how hard an energized stepper resists being turned off its commanded position; it clamps the load in place with no motion, which a basic DC motor cannot do by itself.
Detent torque is the small residual holding you feel when the motor is unpowered, from the permanent-magnet rotor.

Major section

Deep Dive: Step Counts, Torque Margin, and Missed Steps (continued)

Microstepping gives smooth, fine motion, but the incremental torque per microstep is small, so microsteps improve smoothness and command resolution far more than they improve absolute positioning accuracy.

With a 200-step motor at 1/8 microstepping, each revolution is 200 x 8 = 1600 microsteps, so the move is 12 x 1600 = 19200 step pulses.
If the controller can issue 4800 pulses/s after ramp-up, the constant-speed portion alone takes about 19200 / 4800 = 4 s.
Acceleration and deceleration add time, so a real profile may be closer to 5 s.

Major section

Deep Dive: Step Counts, Torque Margin, and Missed Steps (continued)

If the phase current limit is 1.2 A and the winding resistance is 2.4 ohm, copper heating in one energized phase is I^2R = 1.2^2 x 2.4 = 3.46 W.

With two phases active, the motor can dissipate roughly twice that, so thermal rise is a design constraint even while the shaft is stationary.
Reducing hold current after a move can save heat, but only if the load cannot back-drive the axis.
A phase winding is inductive, so at higher step rates there is less time for current to reach the requested value.

Major section

Deep Dive: Step Counts, Torque Margin, and Missed Steps (continued)

If a phase has L = 4 mH and R = 2 ohm, its electrical time constant is L / R = 0.004 / 2 = 0.002 s, or 2 ms.

At 200 steps/s, one step interval is 5 ms, or 2.5 time constants.
At 2000 steps/s, one step interval is 0.5 ms, only 0.25 time constants, so the phase current has much less time to build.
Good drivers handle this with current chopping: they apply a higher supply voltage briefly, sense winding current, then switch the output rapidly to hold the requested current limit.

Major section

For Kids: Meet the Actuator Crew!

"It's like running," the battery added. "You can't sprint from a standstill -- you have to accelerate gradually.

"Why do you move in little ticks instead of spinning smoothly like DC Danny?" asked the LED.
"But I have a weakness," Stella admitted. "If something pushes against me too hard while I'm stepping, I can miss a step.
My count will be off from then on.".

Deck summary

Key takeaways

That counted motion is why steppers are useful for open-loop positioning.

A common motor has a 1.8-degree step angle, which is exactly 200 steps per revolution.
On a 2 mm-pitch lead screw, that becomes 2 rev x 2 mm/rev = 4 mm of carriage travel.
On the same 2 mm lead screw, one microstep is 2 mm / 3200 = 0.000625 mm, or 0.625 micrometers.
Microstepping gives smooth, fine motion, but the incremental torque per microstep is small, so microsteps improve smoothness and command resolution far more than they improve absolute positioning accuracy.

Retrieval practice

Recall check 1 of 5

Motor Max says: answer from memory, then check your reasoning.

Q1Why does a stepper move need an acceleration profile when the target rate is beyond its reliable instant-start rate?

ABecause acceleration profiles always reduce power use at every step rate.

BBecause an instant full-rate start can miss steps and lose rotor position.

CBecause stepper drivers need a warm-up period before receiving pulses.

DBecause a ramp is needed only to make the moving motor sound quieter.

Show answer

Answer: B Answer: B) The rotor can miss steps and lose position.

Retrieval practice

Recall check 2 of 5

Motor Max says: answer from memory, then check your reasoning.

Q2What does 1/16 microstepping achieve on a NEMA 17 stepper with 200 full steps per revolution?

A200 command positions per turn, with an automatic increase in motor torque.

B3,200 commanded microsteps per turn from 200 full steps times sixteen.

C16 command positions per turn, replacing the motor's 200 full steps.

D200 command positions per turn because microstepping changes only noise.

Show answer

Answer: B Answer: B) 3200 microsteps per revolution.

Retrieval practice

Recall check 3 of 5

Motor Max says: answer from memory, then check your reasoning.

Q3A telescope mount uses a 200-step/revolution motor (1.8 degrees per full step). It needs nominal command increments no larger than 0.1 degrees; actual pointing accuracy will be checked separately. Which listed change meets the command-increment target?

AReplace it with a 400-step/revolution motor and use full steps

BUse microstepping at 1/32 or higher subdivision

CAdd a gearbox with a 10:1 ratio to reduce the step angle

DRun the motor at very low speed so each step covers less distance

Show answer

Answer: B see answers page

Retrieval practice

Recall check 4 of 5

Motor Max says: answer from memory, then check your reasoning.

Q4A 3D printer using open-loop steppers prints fine slowly but shifts layers when run fast. What is a likely cause?

ACurrent cannot build fast enough, so torque drops

BMore microsteps add enough torque at high speed

CEncoder drift causes the layer shift

DDetent torque is too high at speed

Show answer

Answer: A Stepper torque falls with speed because winding inductance limits current build-up per step; overload then causes undetected skipped steps in open loop.

Retrieval practice

Recall check 4 of 5

Motor Max says: answer from memory, then check your reasoning.

Q5Place each stepper responsibility where it lives so you can predict motion and find where commanded steps become lost physical position.

AStep, Direction, and Ramp

BStepper Driver

CCoil Energization Sequence

DMechanical Step and Position

Show answer

Answer: A predict motion and find where commanded steps become lost physical position.

Retrieval practice

Recall check 5 of 5

Motor Max says: answer from memory, then check your reasoning.

Q6Complete the stepper motor control sequence:

Astep_sequence = [[1,0,0,0],[0,1,0,0],[0,0,1,0],[0,0,0,1]]

Bstep_sequence = [[1,1,1,1],[0,0,0,0],[1,1,1,1],[0,0,0,0]]

Cstep_sequence = [[1,0,1,0],[0,1,0,1]]

Dstep_sequence = [[1,1,0,0],[0,0,1,1]]

Show answer

Answer: A Stepper motors use a 4-phase sequence where each coil is energized in turn.

Print reference

Answers 1 of 2

Answer key.

B · Answer: The rotor can miss steps and lose position.
B · Answer: 3200 microsteps per revolution.
B · At 1/32 microstepping, nominal command spacing is 0.05625°; verify actual pointing accuracy under load. A 10:1 gearbox gives 1.8°/10 = 0.18°, so it misses the 0.1° command-resolution target. Microstep command spacing does not guarantee positioning accuracy, and motor speed does not change the full-step angle.

Print reference

Answers 2 of 2

Answer key.

A · Stepper torque falls with speed because winding inductance limits current build-up per step; overload then causes undetected skipped steps in open loop.
A · predict motion and find where commanded steps become lost physical position.
A · Stepper motors use a 4-phase sequence where each coil is energized in turn.