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Linear actuators get a servo look

Jan. 24, 2008
Edited by: Leland Teschler New actuators borrow ideas from linear motors to provide controlled displacement on a budget.

George Proctor
Vice President Copley Controls Corp. Canton, Mass.

Historically, industry has depended on ball screws, belt drives, and pneumatic mechanisms for automation’s positioning tasks. But the rising demand for millions of operating cycles, speedier throughput, and programming agility has exposed shortcomings in these mechanisms. There are often trade-offs when it comes to obtaining precise, controllable load positioning.

Recently, pneumatics makers have upgraded “bang-bang” pneumatic cylinders with external control valves, position encoders, and servoelectronics in an effort to address some of these difficulties. But it is daunting to close a servoloop around a high-friction piston and squashy compressed air. Control is problematic owing to the elasticity of air, finite air-travel time, air friction, and piston friction.

Direct-drive linear-actuator technology can provide a better approach. Direct drive refers to motion created by the direct application of electromagnetic drive force, rather than through a belt, ball screw, or some other intermediate drive. Of course, servomotors have been used in direct-drive configurations for many years. A more-recent development in this area is that of direct-drive linear servoactuators. They produce linear motion directly rather than convert rotary motion to a linear displacement. As is the case with rotary servomotors, the shedding of ball-screw mass, friction, wear, cogging, resonance, and so forth gives direct-drive linear actuators agility and lets them be easily controlled.

Operating Principles
The most recent advance in direct- drive linear actuators is called a tubular linear actuator. It evolved from linear-motor technology. A conventional way to describe a linear motor is to envision a rotary servomotor sliced lengthwise and then flattened. Motion between stator and what was formerly the rotor will now be linear, akin to a maglev train.

Tubular linear actuators have a form factor resembling that of a solenoid. The position of the “plunger” (called a thrust rod for a linear actuator) is controlled by the electromagnetic field developed in the coils surrounding it.

As in all significant engineering advances, ingenuity in applying basic principles is key to the invention. For a start, the tubular form factor is an engineering advance with fundamental benefits. Additional refinements, and a critical patented magnetic design, make these devices barely recognizable as rotary-motor descendents.

In the basic linear actuator, powerful rare-earth (neodymium) disk magnets encased within the thrust-rod setup alternate north and south magnetic fields that radiate out like spokes of a wheel. The stainless-steel (nonmagnetic) thrust-rod case protects against impact. (Mostly, this is protection against inadvertent dropping during shipping or assembly.)

The thrust rod is a key component. It embodies a patented magnetic design that produces a precise sinusoidal magnetic field pattern along its length. This magnetic design lets the actuator use integral Hall position feedback for a position loop (the sensors are built into the forcer) instead of a traditional external encoder.

This is important because linear encoders can cost almost as much as the actuator itself. Linear encoders are themselves precision instruments, requiring protected environments, precise alignment, and maintenance. Freedom from use of an external encoder simplifies actuator integration into automation equipment — there’s no finicky alignment concern, while harsh factory-floor environments are no problem.

The actuator’s electrically energized drive coils encircle the thrust rod and its enclosed magnets. Propulsion force is created between the thrust rod’s magnetic field and drive coils, and acts at right angles to both field and current directions. That is, the force is oriented along the thrust-rod axis.

More precisely, coils are grouped into three sets that create current vectors traveling in the direction of thrust-rod motion. Interaction between traveling current vectors and the thrust-rod permanent-magnet field produces actuator drive force. To move the thrust rod, the servodrive controls the three sets of forcer coils with three interrelated sets of drive currents. Amplitudes of the three drive currents, A, B, and C conform to the relationship A sin , B sin ( + 120°), and C sin ( + 240).

The basic mechanism may be configured as either a linear motor or a linear actuator. In a stage positioned by a linear motor, the thrust rod is supported at both ends, and the load-carrying forcer is supported by a low-friction slide bearing. On the other hand, linear actuators reverse the roles of thrust rod and forcer. The forcer becomes the stationary member and bolts to the machine frame. The thrust rod is now the component that positions the load. It glides on lubrication- free bearings mounted within the forcer.

The tubular form-factor brings striking advantages. Most fundamental, it ensures a mathematically ideal orientation between forcer drive coils and thrust-rod radial magnetic fields. All magnetic lines intersect current-carrying conductors at right angles. This orientation creates maximum force and force-per-amp efficiency. The high efficiency also produces minimal heat which maximizes duty cycles.

A subtle advantage of tubular symmetry is elimination of magnetic attraction between the forcer magnets and machine frame. This attraction undermines the performance of regular (flat) linear motors and actuators and may require special bearings to support the extra load.

Another point to note is that the heat-generating element of the actuator, the forcer coils, are surrounded on three sides by cooling air. The machine frame on the fourth side works as a heat sink. In contrast, conventional linear motors and actuators reverse this arrangement. Their forcer coils usually mount inside the permanentmagnet assembly, which inhibits heat removal.

Drive Signals
The actuators may be driven by any three-phase dc servodrives that respond to sine and cosine position signals. However, the best dynamic performance comes from using a drive specifically optimized for use with the actuator.

It should also be noted that the Hall-sensor outputs used for position feedback are analog signals. So the minimum sensed thrustrod distance depends on how well external circuitry can detect changes in the analog sin/cosine voltages. However, the digital servodrives normally powering the actuator typically need digital values for the sin and cosine position signals. Modern digital drives incorporate analog-to-digital converters that process the sine and cosine feedback signals into digital equivalents. Minimum measurable travel distance is then a function of the converter resolution. Standard servodrives use 12-bit converters, which give the actuator a 12-micron repeatability. For higher positioning sensitivity, you’d use a drive with higher-resolution converters.

The actuator described here is also the first to be designed for use in multiple-actuator assemblies. There are several direct-drive tubular linear motors on the market. Nippon Pulse Motor Co. recently announced a new linear-motor family, for instance. Lin Mot Inc. produces industrial linear actuators that lack the high-linearity magnetic design described here, hence they offer relatively coarse repeatability. They also can only work with Lin Mot’s proprietary two-phase servodrives.

Absence of gears and leadscrews make linear actuators low-noise devices. This has become important as OSHA increasingly follows close on the heels of European industrial codes which stringently limit workplace noise. Lack of wear-prone parts also gives these actuators a long operating life. The tubular linear actuator has a much longer life expectancy than a servodriven ball screw if there is a need for great positional precision and the load is light, the move is short, and the process speeds are high.

For similar reasons, the tubular linear actuators experience no inertia, hysteresis, backlash, or cogging. The upshot is superior dynamic stiffness and fast settling. For example, the Model STA11 actuator provides at least twice the bandwidth of conventional positioners. It can develop 92-N peak force, capable of accelerating light loads of 20 gm to 6 m/sec. The actuator’s exceptional dynamic stiffness lets the servo be tuned for high loop-gain operation,

The STA11 actuators are optimized for instrumentation applications and give a maximum stroke distance from 14 to 232 mm. Minimum repeatable stroke, using standard drives, is 12 microns. It is possible to get a longer travel through use of a different and bigger actuator that has more permanent magnets, larger forcer coils, and so forth. Actuator thrust rod diameter — in conjunction with forcer design — is set principally by drive-force requirements. The upper size limit depends on cost rather than fundamental physics.

Finally, the tubular linear actuators of today are not at their physical limits. Future applications will create a need for ever-smaller and more-sensitive actuators. It is also likely that future devices will be bus compatible with interfacing built into the package.

Application examples

It is useful to compare the performance of tubular linear actuators to ball-screw-driven equivalents in real applications. One example comes from a muscle probe in a Northwestern University biomedical lab. The situation calls for moving a probe into a muscle over a distance of 4 mm with a +150/–0-micron accuracy. A Model STA1108 tubular actuator handles this task and cycles the probe at a rate of between 80 to 112 Hz. It has been operating nearly continuously over about two years. In contrast, a ball-screw-driven actuator would be limited to about 5 Hz.

A second example comes from a printing applications where a Model STA1116 replaced servodriven ground ball-screw assemblies. The tubular actuator holds positional following error as defined at the load (print head) versus the requested trajectory to +300/–0 microns at 2.2 Hz. The trajectory is that of a nearly sinusoidal cam over 125 mm with encoder updates every 1 mm. The carriage speed is 442 mm/sec and the motor vertical movement is 0 to 17 mm. Running this profile continuously for 65 days and over 8 million cycles has not changed the measurable following error deviation.

For comparison, a ball screw for the same profile held 800 microns at 0.1 Hz and 8.9 mm at 2 Hz. The mechanical settling time for the intermediary moves of the cam limited the speed of the master travel to get even this larger following error.

An example of a tubular linear actuator is the Model STA 11, which provides a 14-to-232-mm stroke. Thrust-rod diameter is 11 mm. Peak force is 92 N, and maximum velocity is 4.7 m/sec. The actuator’s integral solid-state position sensor delivers 1-V peak-peak sine/cosine position signals for ± 20-micron repeatability. Forcer width and height are 26 and 61 mm, respectively. The actuators can be ganged together so a 10-actuator assembly is less than 12-in. wide. Thrust-rod diameter and forcer design depend principally on drive-force needs. The upper size limit depends on cost rather than on fundamental physics.

Tubular linear actuators in the STA Series consist of two parts: a thrust rod with internal permanent magnets and a forcer that encapsulates drive coils. The coils are driven by sinusoidal currents that set up magnetic fields that move the thrust rod back and forth. Hall sensors sit radially around the thrust rod in the forcer near the main support bearing. They provide position feedback so the actuator needs no separate position encoder.

A scope trace shows the kind of motion possible with tubular linear actuators. Depicted here is a pair of 80-mm down/ up moves with a 30-msec dwell in the down position after settling within a ±0.001-in. window. The test actuator is an STA 1116 with a 91-mm stroke. The green trace (limited position) is the output of the trajectory generator producing a trapezoidal motion profile. The actuator drive current is in yellow (actual current), and the white trace shows the velocity (actual actuator velocity). The following error, which is the difference between the limited position and actual actuator position, is in violet. The digital trace in orange is high whenever the following error exceeds ± 4 counts. At the setting of 4,096 counts/electrical cycle (25.6 mm), four counts represent 0.001 in. When this trace goes low, the motor is within ±0.001 in. of the target position. The dwell time to the next move is calculated to give a 350-msec cycle time needed to keep the RMS current within the motor’s rating. The mov

The basic thrust-rod/ forcer mechanism can be configured as either a linear motor or linear actuator. For example, in the X-Y stage illustrated here, the thrust rod is supported at both ends, and the load-carrying forcer is supported by a low-friction slide bearing. The positioning stage shown uses an STB 11 linear motor and has the same form-factor as those that use ball-screw actuators.

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