Closed-Loop Stepper Motion Alternative

Most step-motor-based motion systems operate in open loop and thereby provide a low-cost solution. In fact, stepper systems offer the only motion technology inherently capable of position control without feedback. However, when step motors move loads in open loop a potential loss of synchronism between commanded steps and actual steps may occur.

02/01/2005


AT A GLANCE

 

  • Step-motor-based motion systems

  • Open/closed-loop control

  • Feedback with/without sensors

  • Missed step detection

  • Torque output maximized

Sidebars:
Closed-loop control methods for step motion

Most step-motor-based motion systems operate in open loop and thereby provide a low-cost solution. In fact, stepper systems offer the only motion technology inherently capable of position control without feedback. However, when step motors move loads in open loop a potential loss of synchronism between commanded steps and actual steps may occur.

Closed-loop control—a subset of traditional stepper motion—offers an alternative that remains cost-effective for applications requiring added reliability, safety, or product quality assurance. A feedback device or one of various indirect parameter-sensing methods 'closes the loop' in these stepper systems to verify/control 'missed steps,' detect motor stalling, and enable greater usable torque output. More recently, closed-loop control (CLC) of steppers helps to implement intelligent distributed motion architectures.

Available methods, benefits

Several techniques are available to obtain CLC of step motor position, velocity, and/or torque. In increasing degree of controllability, these include: counting steps, back electromotive force (emf) detection, and full servo (see 'CLC methods' sidebar for details).

Rahul Kulkarni, product manager-industrial control at National Instruments (NI), mentions several reasons and scenarios for applying closed-loop stepper control:

  • No tuning is required; systems are easy to setup and typically maintenance-free.

  • Enables the triggering of a camera or data acquisition device with breakpoints during a move sequence.

  • Controls positional overshoot, which simply is not an option in some applications, such as nanotech manufacturing or semiconductor fab.

  • Corrects for position slippage at the end of the move.

The last scenario refers to proper sizing of a step motor to system load and inertia requirements, which is not always the case. 'In the real world, when your application is not path critical, you may under-rate your motor slightly to save costs. This is where closed-loop stepper control can help,' says Kulkarni.

All of NI's motion controllers—NI 733x (stepper only) and NI 734x and high-performance model NI 735x (stepper/servo)—can be configured for closed-loop stepper control. In CLC mode, stepper axes use quadrature encoders or analog inputs for position and velocity feedback, he explains.

Baldor Electric Co. regards stepping motors as simple, inexpensive devices ideal for positioning a load. Reasons to use stepping motors include: simplified operation (typically used in open loop); easy to interface as they work with digital inputs; and lower cost, because normally they do not have a feedback device, explains John Mazurkiewicz, motor product manager at Baldor.

Yet, open-loop operation brings the risk of losing steps, which cause improper positioning. 'This can result in many rejected products before someone discovers the error,' he states. 'Additionally, when the stepper motor is used near its maximum torque level—to move heavier loads—or with higher acceleration rates to improve productivity, there is a high risk of stalling.' Adding feedback gets around these traditional stepper motor limitations.

Closed-loop steppers can work with lower-cost encoders, compared to those used on servo systems, which is a further benefit. 'Typically, stepper applications may use single-ended encoders. Also, electronic commutation feedback signals are not required,' adds Mazurkiewicz.

CLC steppers are especially useful for light loads and very short moves, according to Baldor. Typical industrial applications include X-Y and rotary positioning devices in NC systems, process controls, and printing and packaging.

Parker Hannifin Corp./Compumotor recommends closed-loop stepper motion to applications that require zero-speed stability of a stepper, plus position verification. 'This need is still served well by step motors and is the primary ongoing need,' states John Walewander, engineering manager. Instead of feedback devices on the motor, Parker has made large investments in sensorless technologies and developed patented digital methods that accomplish the tasks without the need for external devices.

These techniques— active damping and encoderless stall detect —are incorporated in Parker's Gemini family of microstepping drives. Walewander expects continued development of these complicated algorithms will make them more efficient so smaller and lower-cost devices also can share the benefits. 'As these features continue to get generational improvements, stall detection and anti-resonance techniques will be as common as microstepping is today,' he concludes.

Bob Parente, application engineering manager at Intelligent Motion Systems Inc., says, 'Closed-loop steppers are used in critical applications that require position confirmation.' As an example, he cites an automated laboratory chemical/blood analyzer, where numerous axes move to position the test sample under appropriate reagent dispensers. Meanwhile, other steppers retract, loading the exact amount of reagent chemicals, then move forward to dispense reagents in the proper sequence and amount—a process similar to using multiple syringes. 'Any errors in position on any of the axes can result in lost data or incorrect results,' says Parente.

Justifying added cost

An extra component or development effort means some initial cost premium. It's also a case of little extra cost for a lot more benefits.

Machine

Parker Hannifin/Compumotor applies Active Damping (a sensorless form of closed-loop control) to let step motors use more of their torque margin. Conventional stepper systems can’t operate safely in the reserved torque zone.

'Even with the added cost of the feedback device, steppers are still a cost-effective solution when compared to other closed-loop motion control technologies,' explains Parente. 'The extra cost of feedback devices is justified by the guarantee of accurate results. One failure in a critical application may cost more than the feedback devices.' An encoder or resolver represents up-front 'insurance' that provides confidence about the steppers being in the proper position.

In Baldor's view, improved part accuracy and quality comes from closing the loop, by allowing a comparison of actual measured position against desired position in a machine or motion system. 'If there is a discrepancy, the motor is moved to compensate for any missing steps,' says Mazurkiewicz. Cost and benefit of the feedback device and a closed-loop approach would be justified based on the application. 'Extra cost depends upon the required machine performance and productivity, positional accuracy, and part quality desired,' he adds.

Value of materials in the controlled process also affects the cost equation. 'Step motors are often used with very costly materials ranging from electronic components to DNA samples,' says Parker's Walewander. 'Possibility of a failure justifies the extra cost of the feedback.' Parker says sensorless technologies can match many of the benefits of sensor-based feedback for improving step-motor control at a lower cost. Walewander adds, 'As new generations of digital step motor control evolve, there will be less reliance on external sensor feedback and additional savings for machine builders.'

Market view

Incremotion Associates, a consultancy firm specializing in motion control, estimates that 8-10% of hybrid step motors use step-verification control. Dan Jones, president of Incremotion, suggests even lower current usage for other CLC stepper approaches: &1% for back electromotive force methods and only about 1% of the market for full servo control. However, 'full servo' is forecast to grow faster than the other strategies, according to Jones.

Intelligent Motion Systems shares a similar view, noting its sales to closed-loop stepper applications are holding at about 10%. To help decide a customer's inquiry about needing a closed-loop system, Parente replies with a few questions like, 'What happens to the 'product' if the stepper is in the wrong position?' 'What's the cost of scrapping bad parts produced before the problem is corrected?' If that cost is tolerable, an open-loop system will do. 'Remember, 90% of stepper systems are open loop,' he adds.

National Instruments' Kulkarni relates the growth of CLC steppers in semiconductor and biomedical applications in the 1990s to more demanding motion-control requirements in clean room environments. At that time the brushless servo alternative was still too costly. 'Hence engineers turned to closed-loop stepper configurations hoping to get best of both worlds,' he says. Cost of brushless servo drives and motors has decreased considerably since then. 'Today, closed-loop stepper applications are either flat or on a decline,' Kulkarni says.

This is a new area for Baldor. However, Baldor projects that up to about 20% of stepper motors in NEMA sizes 17, 23, and 34 might be used with encoders for future closed-loop applications.

Parker Hannifin estimates that 10-15% of 'precision stepper applications' use an encoder. 'If you include all industrial step motors the percentage would be much lower,' according to Walewander. 'Applications using hardware to close the loop are declining and those that use sensorless technology are increasing,' he adds.

Parker believes that sensorless technologies hold a promising future. As these methods evolve, there will be high-pole-count (stepper) motors and low-pole-count (servo) motors—both being applied with and without feedback. Concludes Walewander, 'Sensorless motors will be used in many dynamic applications that require servos today, and motors with sensors will be used for positioning applications.'




Closed-loop control methods for step motion

Closed-loop control of step motors employs various methods—among them—counting steps (or step verification), sensorless back emf detection, and full servo with sensor feedback, according to Incremotion Associates.

Step verification , the simplest position control, uses a low-count optical encoder to 'count' number of steps moved. A simple circuit compares commanded versus measured steps, verifying that the step motor has moved to the desired position.

Back emf , a sensorless detection method, uses the step motor's back electromotive force (emf) signals to measure and control velocity. When 'back emf' voltage drops below detection levels, the 'closed-loop' control shifts to open loop for the final positioning move, according to Dan Jones, Incremotion's president.

Full servo refers to full-time use of an encoder, resolver, or other feedback device to more precisely control step motor position and torque. Products are emerging from a number of suppliers worldwide.

Parker Hannifin includes its active damping and encoderless stall detect as variants of back emf control. The stepper drive monitors and measures the motor windings and uses voltage and current information to improve step-motor control. Active damping uses this information to damp oscillations at speed and allow more usable torque output—rather than waste torque as mechanical vibrations (see 'Torque-speed' diagram). Encoderless stall detect uses the information to detect loss of synchronous speed, which is a serious concern for open-loop stepper control.


Online Material Extra

Step-motor-based motion stays active

Sometimes overshadowed by competing servo control systems, step-motor-based motion technology continues to innovate and offer an alternative solution for certain applications. Closed-loop control (CLC) of step motors—with a feedback device or with various “sensorless” methods—is an evolving area. Interest stems from the step motor’s attractive characteristics. For instance, a step motor can supply more torque at low speed than a servo motor of equivalent size. CLC adds further benefits.

Less “reserved torque” is one such benefit. Conventional stepper systems must limit usable torque available for an application to substantially less than what the motor can produce. This reserved torque acts as a safety margin for motor stall and resonance, according to

With “Active Damping”—one of Parker’s digital sensorless CLC methods—a step motor can be applied much like a servo motor without stalling, so the recommended safety margin can be 5-10%, rather than 50%, he explains. As a result, a smaller motor might safely handle the same application.

Another aspect of the topic is the growing number of motion controllers that can handle step motors as well as servo motors. Because feedback interfaces are already incorporated into the controller design for servo requirements, this trend, at least indirectly, plays into spreading closed-loop control possibilities for stepper-based motion systems.


Other supplier companies

A significant number of manufacturers and developers offer products for closed-loop control of stepper systems, whether in the form of motors with feedback devices, motion controllers, drivers, or complete systems. The following list of supplier companies and Web site addresses is far from exhaustive. See

 

 

 

 

 

 

 

 

 

 



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