Hot polymer expands actuator technology

Aug. 1, 2000
A new technology combines the small size of solenoids, the forces of hydraulic cylinders, and the proportional control of electric motors

Anew short-stroke linear actuator removes a design limitation often faced by engineers and offers precise control of actuation speed, force, and piston extension. “The technology opens barriers that mechanical engineers have been forced to design around for many years,” says Edward T. Schneider, president, TCAM Technologies Inc. “Engineers have always been looking at ways to stay under the 20-lb limit of traditional short-stroke actuators without buying custom solenoids.” By contrast, these new solid-state hydraulic actuators offer up to 500 lb of force, Figure 1.

Mr. Schneider, the inventor of this patented technology, founded the company 5 years ago.

Short-stroke linear actuators are used in applications from doorbells to jet aircraft. This solid-state actuator operates in these applications, plus, it can actuate hydraulic valves and brakes, replace a ball screw linear actuator system, actuate controls in car engines, and control robotic gripper manipulators.

In addition to supplying large forces, these new devices enable engineers to proportionally control actuation speed and stroke, as well as force. By varying electric power from 60 to 150 W, engineers can extend the piston anywhere in its range of travel.

How they operate

These capabilities are possible because the actuators use a technology the inventors call thermo-chemically activated motion (TCAM). Most short-stroke actuators are solenoids and consist of an electric coil, an iron shell or case to provide a magnetic circuit, and a movable plunger or piston to act as the working element and provide linear motion. They typically supply forces from 20 oz to about 18 lb. Strokes vary from thousandths of an inch to several inches.

These new solid-state actuators replace the magnetics with a thermally reactive polymer. Heat is supplied by an electric heating element that consists of an electrically and thermally conductive carbon-fiber or silicon-carbide grid. The heat expands the polymer that pushes against a piston, Figure 2. As the polymer cools, the piston retracts. Polymers are available to react at ambient temperatures from -50 to 625 F.

“Other actuator devices often must be designed around heat restrictions,” says Gene Miceli, vice president, Business Development & Marketing. “These polymerbased actuators, however, can thrive on the heat generated in some applications. For example, in an automobile engine, these thermo-chemically activated actuators can make use of the heat from the engine for their actuation. ” The electric energy can be ac or dc — any voltage, frequency, or waveform. You can vary the power by switching a set voltage on and off or by varying the voltage.

Polymer expansion is directly proportional to received electric power. As more power is applied, the polymer continues to expand. Uncontrolled, thermal stresses can cause the polymer to expand enough to deform its case or housing. The pressure vessel, though, depending on its material construction, can handle high pressure. The aluminum version of the P2, (Figure 3) for example, withstands pressures to 12,000 psi. Beryllium copper housings can reach 30,000 psi.

Depending on heat sink temperature and load weight , continuous power input is needed to maintain a specific temperature for the piston to hold position, Figure 4. Actuation speed depends on the heat sink temperature, applied power, and applied load, Figure 5.

Heat sink temperature. The heat sink can be a thick aluminum plate with unrestricted air flow, the housing of the actuator, or, in applications involving fluids, the fluid medium. For example, if the device is used to actuate a valve in an automobile engine block, the heat sink can be the coolant fluid.

Because of the constant heat loss from the heat sink, though, higher wattage (above 150 W) can provide faster actuation speed in some applications.

Applied power. During a fast actuation, there is less time for the heat sink to absorb heat. The higher the wattage to the device, the faster it actuates. “It’s cumulative power,” says Mr. Schneider. “Our device takes the heat we put in during the last second, plus the heat we put in this second and builds more force.”

There is a small difference in actuation time for equal ac and dc power. During ac actuation, the peak acceleration is about 10% faster than with equal power dc actuation.

Applied load. With increasing load, actuation speed slows. The device is actually generating a constant work output per unit time. Thus, for proper retraction on the Model P2, for example, there must be a minimum load of 25 lb, otherwise this actuator may develop a vacuum inside and fail to retract. Often, a return spring is fitted to ensure proper return.

The piston retracts when the polymer begins to cool. Cooling occurs when electrical power is reduced or removed. The retraction speed depends on the polymer’s cooling rate, heat sink temperature, and the weight of the load or other applied force such as a spring.

Engineers can use a simple solid state switch with a microprocessor-based control, such as a programmable logic control or personal computer with a data acquisition card, to monitor and control the electricity to the device. Connecting a microprocessor-based control to a linear potentiometer, LVDT, or load cell provides feedback for precise position and force control. Limiting total current or wiring a limit switch in series with the device will prevent it from traveling beyond its maximum piston extension.

“These actuators won’t fail catastrophically,” says Mr. Schneider. “They will pump down as they cool at a rate engineers can predict.” Indications of impending failure include drawing too much current, detectable with a computer control, or a slowing in stroke speed. When its life is over, simply replace. No maintenance is needed.

Other features

Because of the polymer’s thermal characteristics — thermal fields are inherently damped — these actuators do not have inertial dynamic characteristics nor do they oscillate. The load is in direct contact with the expanding polymer so there is no possibility of lost motion as is possible in gear lash. These actuators will not cause vibrations in a linear system, and they are not subject to them. The thermal fields also make these devices quiet, acoustically and electrically.

The polymer is stiff, with a bulk modulus, or hydraulic stiffness, of 1 million psi. Lastly, this device is a purely resistive load on system electronics.

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