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News Release from: Oriental Motor | Subject: Comparing servo and stepper motors
Edited by the Manufacturingtalk Editorial
Team on 22 July 2004
Cheaper servo motors do not eliminate
'steppers'
Are cheaper servo motors ousting the stepping motor? Oriental Motor's Mark Checkley says not, and many applications remain best served by the easily controlled stepper motors.
Increasingly inexpensive servo motors could be regarded as having spelled the end of the stepping motor This is not so however, and many applications remain best served by the easily controlled brushless DC technology steppers provide, as Oriental Motor's Mark Checkley explains
This article was originally published on Manufacturingtalk on 4 Aug 2003 at 8.00am (UK)
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While some suggest that stepping motors are outdated, they are in fact a relatively recent invention that developed as digital controls became available.
Since the stepping motor is effectively a digital motor, it is ideally suited to microprocessor control.
In contrast, without sophisticated control electronics, servo motors can appear crude and unstable.
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With the introduction of a number of new models and variants, a stepper and AC motors manufacturer offers what it believes to be Europe's broadest range of two-phase hybrid stepping motors.
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The first consideration in selecting between servo or stepper systems is to examine the technology of each in the context of the application.
Stepper motors are hybrid motors, using permanent magnet technology, that 'step' one increment each time the driver gives the control electronics one pulse.
They require no position feed back if run within their limits and, when stopped, they hold their position.
Servo motors on the other hand are standard DC or brushless motors with a feedback loop - generally from an encoder or resolver.
The computer reads the position of the motor and controls the power applied to it.
Stepper motors are generally as accurate as servos but are simpler and more reliable and maintenance free in harsh or dusty environments.
The servomotor's encoder can be susceptible to dirt and vibration, causing problems.
Servo motors exhibit faster acceleration/deceleration when moving heavy loads point to point.
However, one trade off, even where these features are needed, is their higher maintenance.
In some instances stepping motor systems can be just as fast or faster than many servo systems because of the control's software algorithms.
In such cases, the maintenance and robustness issues can be a deciding factor in the adoption of steppers.
Distinct advantages - there are distinct advantages to the motion engineer that cannot be overlooked.
Which is why stepping motors remain the first choice for so many applications from printers to disk drives and robotics to packing machines as well as advanced, precision applications such as medical equipment.
These advantages take the form of size, cost and simplicity of use.
While servo motors have become cheaper in recent years, it is still the case that, for most applications requiring an equivalent performance, stepping motors are less expensive - especially when one considers all the peripheral equipment required to fulfil the needs of most precision applications.
This is thanks to the simpler construction and lack of commutation components.
The brushless construction of the motors gives them longer, maintenance-free lifetimes, and the fact that the drive electronics does not require linear amplifiers, means there is less heating and higher efficiency.
The drive modules are also less expensive than servo drives because there are no servo control electronics required since the signals originate from driver.
Steppers are also a lot less 'fussy' with regard to the driving electronics.
A motor from one manufacturer can be readily driven by a drive from another - unlike servo systems where hardware normally has to be very closely matched and coupled to ensure optimum performance.
Users can even build their own stepper controllers that fulfil the exact requirements of their application - this keeps cost and size down to a minimum and ensures that costly over-specification for lower-end applications is not an issue.
Being digital motors steppers can be positioned accurately without hunting or overshoot.
The performance of stepping motors has also continually evolved; with many modern types offer microstepping accuracy and improved torque-for-size characteristics.
Stepping motors usually operate in the speed range of 0 to 3,000 rev/min, limited normally by the driver type and its power input, whereas conventional DC and AC motors do not operate to their optimum at very low speeds.
A stepper's wide speed range often eliminates the need for a gearbox but, when one is needed, the user is not tied to a supplier and cost is not normally an issue Motor industry consultant Dan Jones, VP at Motion Media Group, suggests that motor torque density will nearly double over the next ten years.
He says magnets improve about 5% each year due to better materials, noting the University of Delaware's research on nano-composites (combinations of soft iron, rare earth elements, and composites).
Engineers also are employing finite element analysis to improve pole shaping and exploit more of the effective force of the magnet, and increasing the amount of copper in the windings.
Five years ago, the typical motor had less than 60% slot fill - today that number is in excess of 70% and rising.
Today's stepping motors are capable of producing excellent torque at low speed.
Of course, a stepping motor will also provide a holding torque when at rest, locking all mechanical components and reducing the need for clamps and brakes.
No damage to the motor will occur whilst being energised but not rotating.
From a mechanical perspective, unlike servo motors, nearly all hybrid stepper motors in the world conform to a NEMA sizing standard.
This means that an existing stepper on a machine can usually be replaced easily without redrilling holes or machining because shaft sizing, flange mounting holes length and diameter will be the same regardless of manufacturer.
Another mechanical consideration is that the stepping motor has its windings on the stator (stationary outer section) of the motor.
Heat generated in the windings can be easily dissipated through the motor casing.
DC brushed motors on the other hand have windings in the rotor (rotating inner section), so there is a much greater resistance to heat transfer.
There is also a safety benefit from steppers.
When DC motor amplifiers or servo controllers fail motors may run at full speed, out of control; if, in the less likely event a stepping motor drive fails, the motor simply stops.
In short, stepping motor drives are fail-safe.
Because stepping motors have no brushes and commutator, there is no arcing to ignite combustible materials.
This can be useful in chemical handling applications.
Also, unlike servos, modified stepping motors will operate more readily in a vacuum, whereas brushed DC motors will not.
This is useful in laboratory, aerospace, nuclear and particle physics applications.
Stepping into applications - hybrid stepping motors are most commonly found in medical and X-Y machinery such as small-scale CNC equipment and a broad range.
Non-hybrid versions see use in printers, scanners, photocopiers and other applications that require affordable and accurate positioning and/or speed control.
The maintenance-free nature of the motors means they are often preferred in continuous use applications or situations where motor failure is highly undesirable - such as safety critical industrial control and medical applications.
The overwhelming advantage of a stepper is that speed and position can be accurately controlled without a feedback mechanism.
Servo feedback mechanisms such as encoders, resolvers or tachometers may cost as much, or more, than the motor to be controlled.
This is why steppers remain the motor technology of choice for countless applications.
When to consider servo motors - there are some applications for which stepping motors are not suited.
Their power to weight ratio, while improving, is lower than for DC servo motors and because most stepping motors are just positional devices, they cannot work with errors.
Finally, at certain low speeds, it is possible for some steppers to resonate.
This can result in loss of synchronisation - but advances in technology, such as a choice of step angles coupled with commensurate changes in control frequencies, have resulted in solutions to this problem.
Applications where high torque at low speed is required can be met by servo motors if price or complexity is not an issue, otherwise stepping motors will usually be the technology of choice.
Where high torque and high speed (>2,000 rev/min) is encountered, the servo comes into its own, where speeds can typically reach 12,000 rev/min.
AC servo motors may also be considered in such applications since they are more robust and can handle higher current surges than any DC technology.
If short, rapid and repetitive moves are to be met, the stepper is an economical solution unless there are high dynamic requirements.
In positioning applications, there must be a clear distinction.
For applications where the load on the motor is determined by inertia or quantifiable friction as opposed to variable friction, select stepper motion.
This is also true for micropositioning or where intermittent duties allow a smaller servo motor to be overdriven.
Provided acceleration/deceleration rates, torque and speed fall within the range of the stepper it would remain the lowest cost option.
In summary, as with all technology, it is selection for purpose rather than fashion that must dictate the engineer's choice.
Any news of the death of the stepping motor may be premature by quite a considerable time.
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