Servos and steppers clock in

Sept. 1, 2008
Once relegated to light duty in the most protected environments, servo and step motors are lacing up their steel-toed boots and venturing out onto the

Once relegated to light duty in the most protected environments, servo and step motors are lacing up their steel-toed boots and venturing out onto the plant floor. Here, they are setting productivity records, but not without some serious forethought. Find out from our experts how steppers and servos can take your manufacturing systems to the next level.

Stepping up productivity

How does motor selection contribute to productivity in today's motion-centric automation environment?

Bob • IMS: This is like Goldilocks and the Three Bears. If the selected motor is too big, it will slow the point-to-point move time because of the additional rotor inertia. Depending on how oversized the motor is, this will add tens or hundreds of milliseconds to the overall motion profile, resulting in fewer parts per minute and lower productivity. If the selected motor is too small, it will accelerate slower, move slower, and settle out for a longer time at the end of the move, again resulting in fewer parts per minute. But if the selected motor is “just right,” with proper consideration for load friction and inertia, it'll produce snappy move profiles with fast settling times, resulting in maximum parts per minute.

Sus • CMC: Correct servomotor selection is essential to improving productivity, and it goes beyond matching speed-torque characteristics. Designers must understand load mechanics, process physics, and environmental factors, and then choose a motor that best complies with all requirements.

Joe • AD: Choosing the correct motor type is also an important consideration. Designers must determine whether absolute, incremental, or no feedback will lead to ease of integration and/or better performance. Some other factors to think about include compatible rotor inertia and cooling. Also, keep in mind that a properly sized motor will use less energy than one that's oversized.

Mindy • Lin: Meeting project objectives on time and under budget is essential, and is one reason why proper motor selection is so important. Both electrically and mechanically, step motors are technologically advanced products that come in various shapes and sizes. For some folks, choosing the right motor might entail browsing through a catalog and making a purchase. For most designers, however, this isn't the case.

Although most steppers might mechanically look the same, internally they are quite different. Making simple adjustments to the magnet wire gauge or number of turns per coil, for example, can dramatically alter electrical characteristics such as resistance and inductance. Understanding how these things affect overall performance is vital to the motor selection process.

Servo, stepper sizing wisdom

What's your best advice on specifying, sizing, and applying steppers or servos where productivity is the main goal?

Bob • IMS: A load-to-rotor inertia ratio of 1:1 is classified as “ideal.” Applications with this ratio produce the best move times and quickest settling time. A load-to-rotor inertia ratio of greater than 1:1, but equal to or less than 5:1, is classified as “good,” producing decent move and settling times. A load-to-rotor inertia ratio of greater than 5:1, but equal to or less than 10:1, is classified as “okay.” Move times and settling times are slower, but can be reasonable. Avoid load-to-rotor inertia ratios greater than 10:1 unless move and settling times are not critical.

Sus • CMC:

  • Carefully consider environmental conditions (dirty, high ambient, high shock and vibration, washdown)
  • Establish a clear productivity goal and develop a motion profile (based on accel/decel rate and rms torque and speed) to meet or exceed it
  • Select motor frame size based on calculated rms torque and speed
  • Select motor material to withstand environmental conditions
  • Select motor winding corresponding to terminal voltage to match torque and speed requirements for the optimized motion profile

Joe • AD: Use sizing tools, when available, to make calculations and system models more accurate, and don't forget to leave enough headroom for growth and miscalculations. Estimating all incidental forces in a system, such as friction and stiction, is sometimes very difficult. So, for high-volume applications, you should build or purchase a prototype system, measuring actual speed, acceleration, inertia, and other performance-related variables.

Mindy • Lin: Steppers are sound and robust instruments that have a long lifecycle if manufactured under the correct tolerances. Do some research on stepper performance in order to better understand their capabilities, and don't be afraid to call a stepper specialist and ask questions.

Motor mayhem

What's the worst that can happen if a stepper or servo motor is not specified or installed correctly?

Bob • IMS: If it's not specified properly, the motor won't move the load the way it's supposed to. If it's not installed properly, it could fail catastrophically. In one case, a customer purchased and installed a servomotor system, and immediately had problems with it. An on-site application review revealed that the customer had specified the torque requirement in inch-pounds when it should have been foot-pounds. The motor was 12 times smaller than it needed to be.

The cost to correct the error was significant. It included a partial credit for the incorrect equipment, ordering new and more expensive equipment, a significant time delay in completing the machine build because we had to wait for the correct equipment, building new mounting brackets for the larger motor, purchasing a new shaft coupling, and modifying the existing machine frame to allow the larger motor to fit.

Mindy • Lin: Selecting the wrong motor happens frequently. Although steppers look relatively simple, it's important to understand that they really are an engineering marvel. Pending the specific manufacturer's capabilities, the final product can be of extremely high quality with the ability to take smooth, miniature steps with a great deal of accuracy. When the wrong stepper is selected, however, things might not turn out so well.

One such situation involved a motor that was selected for a pill dispensing application. Although the motor met with size constraints and initial performance requirements, it overheated after a few move cycles. Our technical support group discovered that the motor was getting too much voltage, causing it to quickly heat up. An excessive supply voltage was used to push more torque out of the motor; when decreased to normal levels, the pills would no longer dispense.

By determining the required torque and subsequently altering the internal windings of the motor, our engineers were able to maximize torque output corresponding to the speed at which the application functioned best. With the newly altered windings, the motor was able to successfully output an adequate amount of torque without generating excessive heat.

Sus • CMC: In a manufacturing environment, producing flawed parts, for any reason, is about as bad as it gets. Not producing anything, say if a machine fails, is second worst, followed by producing parts too slowly or with too much associated labor and cost.

Serving up the future

If you could create the ideal stepper or servo motor, what would it look like?

Bob • IMS: The ideal stepper motor would be an integrated motor-drive combination. Mounting the motor automatically installs the drive because it's one piece. The overall footprint of the machine can be reduced because the equipment cabinet can either be reduced in size or completely eliminated.

Sus • CMC: In an ideal world, we would like to create an extremely responsive servomotor with very high torque density and overall efficiency approaching 100%. This would not only help achieve more challenging motion profiles to improve productivity, but would also save energy to assist the U.S. in becoming energy independent.

Joe • AD: The ideal motor would be low-priced and high performance. It would have field-installable replacement parts and options, such as bearings, encoders, and a drive with swappable power and control sections. Another feature would be the ability to add torque as needed to the drive. By giving motors “add-on” capability, users could insert additional stator and rotor “stacks,” improving application flexibility.

Industry expertise

Bob Parente

Intelligent Motion Systems Inc. (IMS)
(860) 295-6102

Sus Datta

Cleveland Motion Controls Inc. (CMC)
(800) 321-8072

Joe Kimbrell

AutomationDirect (AD)
(800) 633-0405

Mindy Cheng

Lin Engineering
(408) 919-0200

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