Motion System Design

Actuator solutions to linear problems

Over the last few years, the humble actuator has received a small make-over. University and business exploration of new materials in actuator designs is providing solutions to a range of problems from machine chatter to airplane wing configurations

Actuators are a common way to provide single- axis motion. Rack and pinion, chain, belt, cable, ball screw, and electric solenoid are a few of the more than 15 types of actuators that are available. Recently, the number of actuators increased with a new type of single-axis actuator. This type uses an expanding material to provide the linear motion. One version uses a polymer that expands under applied current. (See “Hot Polymer Expands Actuator Technology,” PTD, 8/95, p. 31). Another actuator version uses an alloy that expands when subjected to a magnetic field.

How the actuator works

This new version of a linear actuator has a drive rod made of Terfenol-D, a magnetostrictive metal alloy of terbium, dysprosium, and iron. An applied magnetic field causes the material to change its geometric dimensions: this is called magnetostriction.

In addition to the drive rod, the actuator is composed of copper wire coil, and permanent magnets, and housed in aluminum or stainless steel, Figure 1. The copper wire is wound around the drive rod. When current of 1.4 to 3.4 A from an external power supply is applied to the coil, it creates a magnetic north-south orientation at the molecular level. This new orientation causes the drive rod to lengthen as the diameter shrinks. When current is removed, the rod returns to its original shape.

The rod responds to the application or withdrawal of current almost instantaneously. The expansion is proportional and repeatable. The amount of force that the actuator supplies to linearly move an object depends on the size of the rod. A 12-mm diam rod can exert at least 200 lb of force. A 75-mm diam rod can exert at least 9,000 lb. Commercial versions of this actuator have available displacements in the thousandths of an inch. (A few research versions have displacements over 2 in. See the subhead, Actuator as linear motor).

Thus, these actuators are for applications that need high speed and high force, such as machining.

Countering chatter

When operators run a machine tool, they must often compromise between the quality of the cut or finish and the productivity rate. During cutting, vibration between the tool and workpiece, particularly in the radial direction, affects the dimensional accuracy and surface quality of the part. This vibration is commonly known as machine chatter. Thus, operators frequently can’t operate the machine tool at optimum speed because of the risk of damage to the part.

Various methods used to dampen chatter include hydraulic actuators, piezoelectric actuators, step motors, and manipulation of cutting speed through a control algorithm. But each method has its limits. The hydraulic actuators deliver the necessary force to manipulate the depth of cut but they tend to have low bandwidth frequency response. They are limited to machining applications where the depth of cut is done at slow speed. Piezoelectric actuators have a higher bandwidth but are limited to low force machining applications. Step motors also tend to be limited to applications where cutting is done at slow speeds. And with the solution that involves manipulation of the cutting speed during the process, the inertia of the spindle and drive system becomes the limiting factor.

The vibrations from chatter are measurable, opening up the possibility of countering them and thus cancelling their effects. Professors John Sutherland, Kee Moon, D. Liu, and A. R. Kashani from Michigan Technical University, and T. J. Sturos from Caterpillar Inc., have found a way to provide active, on-line manipulation of the cutting tool in a turning process. They created a tool holder that incorporates a magnetostrictive actuator, feedback sensors, and the cutting tool, Figure 2. The flexor in the tool holder is rigid in the cutting and feed directions (tangential and longitudinal) and flexible in the radial direction.

A rate-feedback control gets information from an accelerometer, which measures the actual tool vibration. The ratefeedback control then gives information to a PC, which uses these data to control the current it sends to the actuator. The current signals cause the actuator to expand and contract opposite the chatter to provide a counter vibration. Once the vibration is countered, operators can either increase machine speed and achieve higher productivitity, or run the machine at original speeds and get better surface finish.

In a test, an aluminum workpiece was turned on a Cincinnati Milicron CNC chucking center using the new tool holder. The actuator in the tool holder used a 12-mm diam rod. The workpiece’s dimensions were: diam of 82.55 mm, length of 342.9 mm, with 9.52-mm lengthwise slot. Test feed rate was 0.0762 mm/sec and the depth of cut was 0.635 mm. Based on the interactive control of the actuator and control system, surface finished improved by 40%.

Actuator as linear motor

Engineers at Northrup Grumman Corp. are using these Terfenol-D-based actuators to develop a “smart wing” concept for airplane wings. Smart wing refers to the ability to adjust the shape of the airfoil cross-section, in flight, to reduce the effects of drag, increase the amount of payload in an aircraft, or reduce fuel consumption. If initial test results hold, projections indicate that jet fuel consumption costs, currently about $7 billion annually, can be cut by 5%.

The actuators, which function as linear motors in this application, form trusstype ribs in a two-spar wing of a Gulfstream III aircraft, Figure 3. The linear motor-actuators expand and retract to vary the wing’s structural shape.

A drive rod of Terfenol-D is placed in a stator tube of non-magnetic material. The stator and rod are fabricated to close tolerances so that when no magnetic field is present, the rod has an interference fit. When a field is present, the rod shrinks in diameter, resulting in a clearance fit. As shown in Figure 4, several coils are used along the length of the motor rather than one full-length coil. Each coil is turned on sequentially in a wave motion. In the beginning of the cycle, the wave causes the left end of the rod to fit loosely and extend while the remaining rod material to the right fits tightly. As each coil segment is energized, the rod shrinks near the coil, but continues to extend to the left. When the last coil is energized, the rod completes its move to the left. The whole process creates an inchworm effect.

Extension speed depends on the frequency at which the coils are energized. The holding power to withstand structural loading is a function of the length and number of coil sections.

This experiemental motor is 9 in. long when contracted. It has a travel of 2.5 in., a maximum velocity of over 0.2 ips, and a force of 200 lb. The actual motor can be developed to supply 7,000 lb of force. The motor has few moving parts. With power off, the system is locked.

Based on initial tests of this smart wing, engineers found cruising speed could increase from Mach 0.78 to Mach 0.88 and for certain flight conditions, drag was reduced over 75%.

Information for this article was provided by Michigan Technological University, Houghton, Mich., Northrop Grumman Corp., and Etrema Products Inc., Ames, Iowa. For more information on the tool holder, contact Dr. Kee S. Moon, Michigan Technological University, Department of Mechanical Engineering and Engineering Mechanics, 1400 Townsend Drive, Houghton, Mich., 49931-1295. For information on the actuator as linear motor, contact Northrop Grumman Corp., Advanced Technology Development Center, Bethpage, N.Y. 11714.

For more information on the magnetostrictive actuator contact Etrema Products Inc.

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