Case Closed for Servomotors

Several Issues Must Be Accounted for Before a Servomotor Can Be Deemed Best for a Particular Automation Solution

By Hank Hogan

For servomotors, the most fundamental question is open and shut. After that comes consideration of the ability to supply the necessary torques at the right speeds. Finally, the motor's operating environment and other factors come into play. All of these issues have to be accounted for before a servomotor can be deemed best for a particular automation solution.

That first point, open versus closed loop, determines if a servomotor is the correct solution. Servomotors feed position information back to drives and controllers, with the data impacting the rest of the automation system. Hence, servomotors tend to be used when the constant location of a load is important.

SEE ALSO: Should We Switch to Servos?

"Do you need absolute positioning or just incremental steps?" asks Chris Gottlieb, product manager for low-inertia servomotors at Rockwell Automation, of the first decision point in selecting a motor.

Servomotors are best suited for cases in which there's a positioning requirement to know where a load is at all times. Servos are also good when it's necessary to reverse motor direction often. In applications without these demands, a servomotor might be overkill.

In addition to a position feedback system, servomotors usually can have more magnet pairs than less-expensive stepper motors. The basic materials are the same between the two types of motors, but their motion is not. Stepper motors, as the name implies, step incrementally while servomotors offer smoother movement.

Servomotors typically offer higher rates of acceleration and deceleration, the ability to easily move either forwards or back, and accurate positioning data. This combination of characteristics means that properly sized servomotors can bring a load to stop within milliseconds and thousandths of an inch of a desired spot.

However, these capabilities come at a cost, compared to steppers. Hence, there must be a requirement that justifies spending the extra money. That requires understanding what servomotors are good at, says Gordon Ritchie, technical training manager for Kollmorgen. "Servomotors provide highly dynamic performance," Ritchie explains. "They accelerate at high rates, position the load to arc-seconds, and repeat it all over again."

As for the second basic selection criteria, having to meet a torque at speed specification applies to any motor. Determining just what that number needs to be can be a complicated affair since several speed/force combinations could be demanded as a motor drives a load along a trajectory.

"Often both the peak and RMS torque of the motor need to be considered," says Joseph Profeta, director of the Control Systems group at Aerotech.

Peak torque is the maximum amount of force a motor must exert, which typically occurs when acceleration or deceleration are at their highest. RMS, or root-mean-square, can be thought of as a steady-state torque, and is impacted by the load and duty cycle.

With regard to any speed specification, the load being driven by a motor has to make it from a starting point to a stopping point. How long that takes — and, hence, how high the speed must be — is determined by the allowable process cycle time.

Being able to hit required torque at speed points is not enough. The motor needs to be able to supply the torque/speed combination without overheating. Heat can be a problem for the motor or the rest of the automation system, and perhaps the process or product. If that's the case, then selecting a larger motor could be necessary because it will minimize heat generation.

Motor size also comes into play in another way. Servomotors always try to maintain a given position. This means they can hunt back and forth when directed to stop at a location. The amount of hunting is dependent, to a degree, on the inertia mismatch between the motor and the total load.

It's best to keep the inertia ratio at 10:1 or less. That is, the inertia of the motor should be at least a 1/10 that of the load. Generally, the closer the match between motor and load, the finer the possible positioning and the faster the settling time.

Failing to keep the motor's inertia at least 10% of the load can mean excessive hunting by servomotors as they try to hone in on a spot. That wastes energy, which actually can be heard.

"It can start humming," says Terry Bell, product manager for linear and servomotors at Baldor. "It'll reflect all the way through your mechanical devices."

In considering torque, another important factor to remember is gravity, says Greg Dieck, motion solutions engineer at Omron Automation and Safety. Moving a load vertically by any amount means that more force must be exerted when traveling up than down. Thus, when starting movement upward, more torque is demanded than when moving down. When bringing things to a halt, the situation is reversed.

What's more, gravity acting on a load moving downward can transform a motor into a generator. "The motor will actually create electricity, which now has to be absorbed someplace in the system," Dieck says.

A final set of specs involves the environment. When servomotors are used in manufacturing, they have to be able to withstand whatever comes their way.

"Other important considerations when selecting servomotors include the drive supply voltage and number of phases, as well as environmental conditions such as ambient temperature, moisture content and washdown, if required," says Robert Swalley, motors and drives product specialist engineer at Beckhoff Automation. Washdowns are common in food processing, where equipment has to be cleaned regularly. Surviving a periodic washing can mean handling caustic chemicals, along with a torrent of water. In that case, motors most likely will be made of stainless steel, for example, to withstand the required cleaning.

If flammable or explosive vapors are present, says project manager Dave Beckstoffer of Portescap, then the inner construction of a motor becomes critical, with brushless servomotor designs offering the advantage that they don't produce anything that can set off combustion. "If you've got a dc motor, you've got some arcing across the brushes," Beckstoffer says of the alternative approach.

A final environmental factor is the power that a motor consumes. Here, a servomotor uses only as much current as needed to control the load, and thus can save energy, says Bryan Knight, automation solutions team leader at Mitsubishi Electric Automation. "Further energy savings can be achieved by combining two or three servo drives into one package, and allowing one servomotor to use braking energy from another servomotor, instead of wasting it as heat."