Machine Design

May the force loop be with you

High-thrust linear motors make it possible to use force servoloops to suppress stiction.

By Mike Lindberg
California Linear Devices Inc.
Carlsbad, Calif.

Edited by Miles Budimir

Tubular linear motors from California Linear Devices are three-phase, direct-drive, brushless dc motors with NdFeB permanent magnets that can hit positioning resolutions down to 1 micron. Standard stroke lengths range from 2 to 20 in., with force capabilities from 50 to 1,200 lb and velocities to 100 ips.

The force signal from the strain gage in the load cell feeds back to the motor-drive amplifier, forming a closed-loop force-control system.

The plot of force versus command voltage shows that system stiction causes force hysteresis in open-loop control. The force hangs up after the thrust is commanded back down from the maximum.

Closed-loop control with a force feedback signal eliminates hysteresis. The graph shows the measured output force of a high-thrust linear motor in combination with a simple amplifier and strain gage in a closed-loop configuration.

Industrial applications such as welding, grinding, and material testing require controlled force. Controller force is usually applied by an actuator or thruster. There are several choices including hydraulic, pneumatic, rotary screw, or rotary belt thrusters.

Tubular linear motors are an attractive alternative. They use a shaft with integral bearings that slides into a stator assembly containing electromagnetic coils. Stator length and diameter sets the force level. Overall, the motors simplify the application of tightly controlled force and offer greater mechanical simplicity then the alternatives.

Traditional hydraulic and pneumatic thrusters often use open-loop force control. The force is created by applying pressure to a working gas or fluid in a hydraulic or pneumatic cylinder. A piston then converts the pressure to a force applied to the load.

Another open-loop method uses permanent-magnet motors. An electrical current is applied to the motor windings to deliver force or torque proportional to the applied current. A mechanical coupling transmits the resulting force to the load.

Though open-loop methods are simple to implement, mechanical stiction and drag inherent in these systems reduces the accuracy of applied force. Traditional thrusters also lack the controllability needed for a closedforce servoloop. As a result, open-loop systems have resulted in lower process yields and lower throughput.

A better approach utilizes closed-loop force control. With a sufficiently controllable thruster, closed-loop systems can have simpler electronics and mechanics than less accurate open-loop force-control systems.

A force sensor placed between the thruster and the load provides feedback for a correcting signal. The closed-force servoloop requires no more electrical hardware than the open-loop system. The strain-gage amplifier normally used to monitor force connects to the tachometer (velocity feedback) port of the drive amplifier.

One reason for closed-loop control is the persistent problem of stiction. It is not a stabilizing influence like friction, but sudden and stick-slip. Stiction causes position overshoot.

Stiction can be thought of as a high-frequency external force, typically between 40 and 100 lb in amplitude, which acts on the shaft acceleration. To compensate for this force, the servo response must be of equal or higher bandwidth. This leads to a smoother response and more accuracy with a higher loop gain. But it is possible to boost loop gain without inducing instability only if the bandwidth is sufficiently high. So a higher loop gain translates into higher precision. In general, however, the gain always depends on the system being controlled and its unique mechanical inertia and bandwidth.

Some thrusters are more controllable than others. Closed-loop force control is stable only if the thruster and its mechanical coupling to the load are tight enough to operate with a high loop gain. Ideally, the coupling should be extremely stiff exhibiting high dynamic response and little springiness, or backlash.

Good control of high-bandwidth motors requires solid mounting. A motor mount or equipment frame may be stiff enough to accommodate a traditional, low-bandwidth thruster. However, it may exhibit resonant frequencies with a higher-bandwidth linear motor. A much stiffer frame with higher mechanical resonant frequencies is needed to prevent instability.

Typically, closed-loop force-control applications involve tiny displacements. It doesn't take much change in displacement to effect a large change in force. For that reason, the coupling should be stiff with little compliance. Any backlash in the system coupling can cause a positional change that will effect a great change in the force.

It's also difficult to close a force loop if there is chatter. The forward-and-back movement of the thruster's mating surfaces make for rapid, instantaneous changes in the applied force.

Poor system controllability renders the loop unstable and often limits how high the loop gains can be set. Contributing causes include spongy assemblages such as air in a pneumatic cylinder, fluid inertia in a servohydraulic system, or qualities such as elastic coupling, backlash, and high rotor inertia in a more-complicated rotary-screw (mechanical) drive. Loops with relatively low gain may partially compensate for stiction, but significant force hysteresis remains.

On the other hand, a linear motor is sufficiently controllable and has a simple mechanical coupling tight enough to allow high loop gain. The result is excellent force resolution without the need for exotic mechanical bearings.

The tubular linear motor can realize closed-loop force control without using a separate motion controller. The force loop is closed using the velocity feedback port on the amplifier. The system is inherently simple and the motor easily controlled. These factors more than compensate for any perturbation caused by stiction.

Any motion-control card can then serve as a master for the system with the same level of simplicity available on an open-loop configuration. This approach provides an economical path for the upgrade of traditional controlledforce applications.

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