It used to be easy to choose between a step motor and a servo. Today, the choice is less clear-cut, but the benefit is greater design flexibility
There was a day when the design decision of whether to specify a step motor system over a servo system was simple. If you needed repeated motions with good accuracy at low speed, stepper systems were the right choice. All that was necessary was some form of sensor to mark the boundaries of where movement took place.
However, if you needed reliable precise positioning, then you needed the feedback from a servo motor's position sensors. Servos were also the choice for any application requiring high speed. And if you needed a higher level of resolution, then you needed to go to a servo system.
Step motors were inexpensive and servos weren't. Steppers were simple to apply. Servo systems usually needed a systems integrator to get them up and running.
The lines separating these technologies are less distinct now. The costs of servo technology have dropped dramatically, making the price differential less of an issue. Plus, both technologies have made advances. The result: More flexibility to tailor your motion control solution.
Less is more
Step motor construction follows the same basic design today as it did a dozen years ago, but there have been many refinements. Continual advances in magnetic circuit design have reduced flux losses that rob a motor of useful torque. New magnetic materials with a stronger field help boost torque output, enabling higher torque densities. This not only lets engineers squeeze more power into a smaller space, it improves positioning precision as well.
Accurate positioning, or control of the step angle, is important for some stepper applications. Accuracy is influenced to a lesser degree by the motor because of inexact motor and stator tooth profiles plus mechanical tolerance buildup of parts. It's influenced to a greater degree by the displacement angle created by load torque. In other words, the initial step angle of a motor and the ability of motor torque to consistently overcome load torque dynamics dictate stepper accuracy. When there's little variation in friction and load torque, a stepper can precisely recreate a motion repeatedly.
In general, the higher the torque generated by the motor, the lower the displacement angle produced by the load torque and the higher the position accuracy of a step motor. Steppers, with their high pole count, have greater torque for a given volume than servos. Ultimately, they are more precise. Because of their open-loop operation, though, they don't have the repeatability of constantly monitored servos.
The lower pole count of the servo motor allows it to operate at higher speeds than step motors. Although the stepper has a greater inherent angular accuracy, the servo uses a high-resolution position feedback device for repeatable accuracy with high precision, even under changing loads.
Step drive design has also influenced stepper system capability. Microstepping, for example, lets motors run more smoothly and with less audible noise than full-step drives. These "subdivided" steps boost accuracy and repeatability, assuming a consistent load. Idle current reduction circuits reduce the heating that occurs in steppers at rest. And drives with digital control offer such features as indexed moves and electronic gearing.
Although low-cost step modules will continue to drive step motors in an open-loop control scheme for the near term, closed-loop step drives and indexers are gaining acceptance. In these systems, position feedback information, typically from an absolute encoder, feeds back to the step drive. The result of closing the position loop is low-cost repeatable position accuracy.
There are slight but important differences in position feedback between servos and closed-loop steppers. Position feedback enables a servo loop to react quickly and constantly in near-real time to operate the motor. Steppers, on the other hand, use feedback in a delayed mode to confirm that the move arrives at the expected end location. It's important to note that the feedback here is not real time. Adding a feedback loop will slightly increase stepper system cost.
Servos gain ground
Ten years ago, installing servo motors required motion control "wizards," system integrators who had the experience necessary to set up and tune them for reliable operation. These engineers' servo tuning "black magic" relied heavily on oscilloscopes and calculus to tweak high-strung feedback loops into useful submission.
Today's advanced digital electronics have changed all that. Servo tuning can now be done by system software, making it possible for anyone to optimize a servo system.
Digital electronics and recent servo motor construction developments have all but eliminated low-speed jerkiness, or cogging, that often plagues servo systems. A digital approach also eliminates the drift that was a hallmark of analog circuits.
Developments in servo motor design incorporate new magnetic materials and new manufacturing methods to reduce cost. In addition, servo motor design has migrated from super high performance down to a level appropriate for most machine designs, and thus benefitting from the resulting economies of scale. Today it is possible to find a servo system priced in the range where simple stepper systems were just a few years ago.
Software programming of digital servo drives has also evolved. The new drives are easier to use. In the same way that servo motors have changed to suit a larger cross section of motion control needs, servo drives have found a middle ground between "dumb" drives that function as torque or speed controls and fully programmable drives. The new drives can execute simple preset move profiles or move at the command of a controller or outside input.
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Pitfalls to watch
Step motor and drive design have pushed the frontier of system speed over the past few years, but stepper system speed is still lower than the speeds possible from servo systems. Plus, steppers can still skip steps because the majority of systems have no position feedback. The motor may stall for several counts, and the machine won't recognize it.
Another problem is resonance. All mechanical systems inherently have regions of resonance at certain speeds, which must be addressed. Both stepper and servo systems battle resonance. Stepper systems try to muscle their way through the resonance region. Servo systems sense the resonant vibration and try to counteract it – sometimes too aggressively, adding to the problem.
Step motor resonance occurs in almost all open-loop stepper systems. It happens when the motor loses synchronization, causing noise and vibration. Usable torque often falls to zero, and the step motor stalls. Solutions to this problem include mechanical dampers, viscous couplings, and most recently, active electronic damping.
One active damping system uses a comparator circuit to continually monitor for fluctuating winding currents at all motor speeds. At the earliest sign of changing currents, an indicator of resonance, the drive alters the timing of stepping pulse rates almost instantly. This maintains a motor's average speed and corrects the instability before torque falls off.
In servo systems, machine components reflect mechanical system resonance to the servo loop. But as speeds increase or decrease through naturally resonant regions of the mechanical system, any vibration may interact with the servo loop to magnify resonance. This is where the experts made their living, with oscilloscope and little screwdriver in hand, sequentially tweaking potentiometers and manipulating the PID process to minimize the impact of mechanical vibration on servo operation.
Now, software-based adjustable notch filters resident in some new servo drives improve servo compensation, enabling a machine to operate at almost any speed regardless of the mechanical resonance that occurs.
Name that tune
Tuning improvements have helped increase machine throughput by allowing the servo to compensate for resonances typically introduced at higher speed operation. Better tuning allows higher speeds without compromising settling time.
The latest generation of servo drives make good use of software to aid and automate the servo system tuning process. So much so that in many cases, all you need to know is the motor part number and a few facts about the intended operation of the machine – the utility software takes it from there. Software development also has made significant strides in system troubleshooting. From walking users through system difficulties, to providing software-based oscilloscopes to monitor and document system performance, software makes it easy for anyone to apply the technology.
The important differences
The differences between stepper and servo systems are less distinct now. The price of servo systems has dropped due to increasing use of microelectronics and the economies realized through expanding market acceptance. Servos are easier to use because of gains in software development that automate setup, tuning, programming and troubleshooting. Steppers have become quieter and more compact. Steppers also have closed the position feedback loop, giving them the repeatability of servos, albeit at the cost of reduced machine throughput because of the feedback delay.
But, technology hasn't advanced to the point where steppers and servos are completely interchangeable. There are still some black-and-white guidelines.
Steppers continue to be more economical in situations requiring high torque in a small package at speeds typically less than 1,500 rpm. The paper feed in a line printer is a good example. A small motor allows an enclosure compact enough for placement on a desktop. Robust torque accommodates different paper types. The machine doesn't catastrophically fail if the motor stalls - you simply clear the paper jam, and start the page over. The paper indexes a line at a time, and motor speeds stay low.
Servo systems are suited for applications that use a wide range of speeds and any machine that needs high speeds. And servos are the only choice where torque control is important.
Consider the control of a high speed web of material – an application commonly found in printing, converting, packaging, and paper, plastic, and metal-making and handling operations. Maintained proper torque provides the appropriate tension necessary to avoid damaging the web material. At the same time, the web is moving through its process at high speed. Throughout the travel of the web material, adjustments must be made to control registration of the material, to coordinate machine processes.
Servo technology is gaining favor with designers looking to improve flexibility and throughput. Multiple servos ganged together through a common control can replace a complicated mechanical system, like a mechanical line shaft arrangement. Substituting a series of programmable servo systems allows changing the function of the machine simply by changing a software program, rather than swapping out hardware components.
Dan D'Aquila is product manager at Pacific Scientific Co., Rockford, Ill.