Stepper motors: advanced techniques

In a previous article, Gerard Bush of INMOCO explained the fundamentals of stepper motor design and usage. Here he delves into some advanced control techniques that improve their motion smoothness and stability.

All stepper motors show some degree of cogging in their motion. Motion smoothness is least-good in full-step stepper motors, better in half-step and even better in microstepping - but it is still not perfect. Some stepper designs address this with "˜skewed rotors' designs to smooth the magnetic field interactions, so if your application requires a high level of smoothness, motor selection can make a contribution.

The current controller, or chopper, also plays a part. Ideally, the phase generator outputs a continuously varying desired current through each coil, and the current controller drives the coil to precisely match the actual current with the desired current command. In reality this is seldom the case, there is some element of mismatch and therefore some cogging.

A full four-quadrant controller is the gold standard, but expensive. A "˜fixed off time' PWM (Pulse Width Modulation) controller provides two-quadrant control, and is used for most applications. The theory behind current control is very complicated, but an expert can help you select a suitable amplifier for each application.

For high-accuracy applications (camera tracking, XY tables, microscope stations, semiconductor equipment, etc) the controller can "˜shape' the power supply's sinusoidal waveforms to alter the microsteps' size. The general idea is to use a high-accuracy encoder to measure the motor response to a standard sinusoidal lookup table. Then the lookup table can be modified by slightly advancing or retarding the values, compensating for the non-linear response of the stepper motor. Current control techniques are also used to address mid-range instability. The simplest approach is adjusting the trajectory profile to accelerate through the unstable frequency range and thus zoom past the problem frequency. Other straightforward approaches include adjusting the drive voltage, the winding configuration of the stepper motor, and adding mechanical damping. There are also amplifiers which incorporate anti-resonance control.

Getting lost

When operated without an encoder, the bane of stepper motors is lost steps. This can be due to a loss of synchrony or if the motor encounters momentary drag. The simplest remedy is to install a sensor somewhere on the driven machine to compare the theoretical step count with the actual position. This may allow corrections to be made on the fly, an alarm activated, or shut down of the system. This solution, though simple. has the problem of not reacting immediately, which may be an issue in some applications.

The next approach is "˜automatic step loss detection' in the amplifier. This approach is implemented in the amplifier and relies on the fact that the voltage/current profile of the motor's coils looks different under normal operating conditions than it does under anomalous conditions such as when steps are lost. Broadly speaking, this technique "˜knows' what the anticipated motor current feedback waveforms should look like, and detects when they go out of tolerance. Some amplifiers need to be tuned for this; others, particularly those that are mated to a specific motor, come ready to operate.

None of these techniques are infallible. For perfect step loss detection an encoder is required. In practice many stepper motor applications use an encoder, fortunately nearly always of the low-cost quadrature variety.

There are a number of ways you can use encoders in combination with trajectory moves. The simplest is to make the move and then compare the final position provided by the encoder with the anticipated location. The vast majority of applications use this approach.

A more sophisticated approach is to specify an error window or tolerance, measured in encoder counts, and continuously compare the commanded step position with the received encoder position.

There are also more elaborate ways to use an encoder with a stepper motor, but they compromise the simplicity of stepper systems, so you may be better off with a servo approach.

Hybrid step/servo

There is a hybrid motion control approach, where a servo-like speed adjustment is implemented in an open loop stepper system. This is bespoke to individual applications, so generalisations are not appropriate and the best advice is to seek specialist support.

Another approach is to operate the stepper motor as if it were a brushless motor, and commutate it actively based on encoder feedback. In this approach, the torque requested of the stepper motor is not fixed (as it is for a normal step motor driver), but rather is continuously adjusted by the controller. This technique has the advantage of running your motor more efficiently, as the torque is matched to demand rather than always at maximum.

Operating a step motor as a brushless DC motor is by no means common, perhaps because it requires an expensive high-resolution encoder. But the motor run more quietly and its motion is smoother.

Summary

Advanced electronics have significantly improved what you can do with a stepper motor, and many of the techniques add little or even no cost to the controller. Ultimately, however, you may find that stepper motors are not suitable for your application and that a servo is the best answer.

For further information about stepper motor design, usage and control techniques, please visit the Inmoco website at www.inmoco.co.uk/.

Inmoco

4 Brunel Close
Drayton Fields
Daventry
NN11 8RB
UNITED KINGDOM

+44 (0)1327 307600

info@inmoco.co.uk

www.inmoco.co.uk

More from Inmoco

Miniaturisation in motion control

Posted 4 days ago

New compact actuators deliver high force

Posted 1 month ago

Pumping technologies for precise fluid handling

Posted 2 months ago

High force integrated actuator reduces footprint

Posted 7 months ago

Rod-style electric actuators deliver 222kN thrust

Posted 8 months ago

Smooth motion arc segments, 360-deg rotary direct-drive motors

Posted 9 months ago

Agility Series direct-drive motors feature zero cogging

Posted 10 months ago

Voice coil motors offer precision motion in compact package

Posted 11 months ago

Inductive angle encoders are simple, robust and ultra-precise

Posted 1 year ago

Servo systems cables with improved connectors and labelling

Posted 1 year ago

Low-voltage DC servo motors for high-performance machines

Posted 1 year ago

Inmoco offers alternatives for motors no longer available

Posted 1 year ago

What does the future hold for motion control?

Posted 1 year ago

Simple Co-Engineering options for Kollmorgen AKM 2G servo motors

Posted 1 year ago

New stepper motor controller from Inmoco

Posted 2 years ago

Next-generation servomotors raise machine performance

Posted 2 years ago

Next-generation servo motors deliver 30 per cent more torque

Posted 2 years ago

Component compatibility can make or break a good system

Posted 3 years ago

High-performance actuator range is compact and rugged

Posted 3 years ago

Helicopter actuators may also transfer to other applications

Posted 3 years ago

Miniature encoders help advance surgical robot capabilities

Posted 3 years ago

Compact actuators improve aircraft motion systems

Posted 3 years ago

Encoders step up to help miniaturise electronics

Posted 3 years ago

Linear actuator design enables automated maintenance

Posted 4 years ago

Advanced motion cards provide precise actuation

Posted 4 years ago

Compact DA series actuators deliver quality and reliability

Posted 4 years ago

DA99 linear roller screw actuators increase reliability

Posted 4 years ago

DA Series high-performance electric actuators

Posted 4 years ago

New high-performance actuators available in UK

Posted 4 years ago

Servo motor with Ethernet interface connects directly with CNCs

Posted 4 years ago

Heavy duty linear actuator has high-performance controls

Posted 4 years ago

Single cable, multi-axis servo systems gets more power

Posted 5 years ago

Precision motion chips for dedicated controllers

Posted 5 years ago

Hygienic motor range has new feedback and connector options

Posted 5 years ago

Inmoco launches KOLLMORGEN safety feedback option

Posted 5 years ago

MC58113 Series programmable motion control chips

Posted 5 years ago

MicroE Veratus Series encoders: smart and reliable

Posted 5 years ago

Smaller, smarter encoders from Inmoco

Posted 5 years ago

Servo controller doubles power

Posted 5 years ago

Maintenance-free linear motors delivery high power density

Posted 5 years ago

PCB-based options are mighty minis for motor control

Posted 5 years ago

Vacuum-rated encoders from Inmoco

Posted 6 years ago

Minutes to motion - linear positioning explained

Posted 6 years ago

Servo users should not be seduced by power density alone

Posted 6 years ago

Inmoco introduces hygienic motor-gear combination

Posted 6 years ago

Servo technology is moving to a new paradigm

Posted 6 years ago

Compact washdown motor offers excellent performance

Posted 6 years ago

Maintenance-free lifting columns are easy to fit

Posted 6 years ago

Electric actuator optimises machine design and saves energy

Posted 6 years ago

Encoder offers value and performance plus flexible mounting

Posted 6 years ago

User-friendly compact encoder gets new side mount option

Posted 6 years ago

Reduce cabling with distributed servo drives from Inmoco

Posted 6 years ago

New functions make servo super-safe

Posted 7 years ago

Power storage modules act as servo UPS

Posted 7 years ago

Inmoco expands indexer family with motion features

Posted 7 years ago

Motion card integrates amps with full motion control

Posted 7 years ago

Wizard aids easy set-up of embedded motion controllers

Posted 7 years ago

EtherCAT master module for CTC 5300 Blue Fusion controller

Posted 7 years ago

OPC server gives instant access to automation controllers

Posted 7 years ago

Atlas single-axis motion amplifier is multi-talented

Posted 7 years ago

Rotary encoder establishes absolute position at start-up

Posted 7 years ago

Inmoco goes back to BASIC with new programmable servodrive

Posted 7 years ago

Mid-performance, high torque per pound servomotor

Posted 7 years ago

How to calculate point-to-point SCARA movements

Posted 8 years ago

Trombone servo drive range now has increased power

Posted 8 years ago

M4-X DC servomotor range is rugged and high performance

Posted 8 years ago

Achieve smaller, faster, smarter motion control

Posted 8 years ago

Expandable motion control system for extreme conditions

Posted 8 years ago

Elmo Gold Drum HV extends servo drive range to 65kW

Posted 8 years ago

B-Hornet military servo for high-power burst applications

Posted 8 years ago

O.P.S. Optical Encoders can be customised

Posted 8 years ago

ORMEC XD indexer family of servo drives expanded

Posted 8 years ago

Free automation systems control app for iPhone and iPad

Posted 8 years ago

Inmoco's Elmo Gold Servo Drives gain EtherCAT compliance

Posted 8 years ago

Inmoco introduces Elmo's DC Trombone digital servo drive

Posted 8 years ago

Robust linear actuators with integral force measurement

Posted 9 years ago

Energy-efficient IP67 stainless steel servo motors

Posted 11 years ago

Servo drives can be networked with IEEE 1394b

Posted 11 years ago

Wider choice of combined motion control and PLC units

Posted 11 years ago

Electronic cam controller suits labelling and capping machines

Posted 11 years ago

High-resolution encoder offers exceptional versatility

Posted 11 years ago

Elmo Duo controller integrates twin Whistle servo drives

Posted 11 years ago

Compact Guitar servo drive delivers 5kW of power

Posted 11 years ago

Ethernet/IP and Modbus options for multi-axis network manager

Posted 11 years ago

Elmo Solo Whistle saves servo application design time

Posted 12 years ago

New three-axis stepper modules for programmable controller

Posted 12 years ago

Electric linear actuators incorporate force measurement

Posted 12 years ago

Miniature ChipEncoder measures just 7x11mm

Posted 13 years ago

Cost-effective alternative for linear positioning and control

Posted 13 years ago

Rotary actuator includes servo motor, amp and position control

Posted 13 years ago

CANopen and absolute encoder options for digital servo drives

Posted 13 years ago

Programmable encoders feature miniature sensors

Posted 13 years ago

MicroE Systems' Mercury II digital encoder 'sets new benchmark'

Posted 13 years ago

Inmoco introduces ServoWire Motion and Logic Controller

Posted 13 years ago

Inmoco now offers Servotecnica slip rings

Posted 14 years ago

Inmoco launches "world's smallest" 50-nanometre encoder

Posted 14 years ago

High-resolution encoder is claimed to be the world's fastest

Posted 14 years ago

More technical articles
3 days ago
Boxing clever
A new chainflex cable box means big shipping cost savings for customers
3 days ago
New industrial vision online channel sees the light of day
Industrial vision provider IDS is making its expertise as a manufacturer of digital industrial cameras available free of charge and readily accessible on a new platform
3 days ago
Ball screws help protect buildings from earthquakes
Ball screws from NSK are helping to protect structures and people in earthquake zones
3 days ago
ATEX certification for new pneumatic valve island
Pneumatic process control offers numerous benefits, especially in hygienic applications. Bürkert says that its new Type 8652 AirLINE valve island provides users with improved safety features, communications and diagnostics
3 days ago
ABB launches condition-based maintenance service for robots
New service enables users to plan ahead and optimise production performance
3 days ago
What the UK-EU trade deal means for UK vehicle manufacturers
Now the UK has left the EU, the transition period has ended and the two parties have established a new trading relationship, UK-based vehicle manufacturers have some clarity over what they need to do to serve three key markets: Great Britain (England, Scotland and Wales), the EU and Northern Ireland
3 days ago
Farnell publishes Industry 4.0 ebook
New ‘Industry 4.0 Interviews’ ebook is available for free download from Farnell and showcases the opinions of leading global experts on the future of IIoT and Industry 4.0
3 days ago
Can DC motors be used at high temperatures?
There is no need for a heated discussion about it as maxon’s motor expert Andrew Gibson offers a view on the subject
4 days ago
Switching up a gear
Phoenix Contact is expanding its line of unmanaged switches
4 days ago
New Intertronics metering, mixing and dispensing system
Adhesives supplier Intertronics has introduced a new 2-K-DOS metering, mixing and dispensing system, said to be a compact, benchtop unit suitable for materials including silicones, polyurethanes and epoxies