In the last 20 years or so, researchers have developed several classes of electroactive polymers — plastics that can change shape when one applies a charge to them. Because this mechanism resembles an animal's muscle, that's what actuators based on this technology are often called. Electroactive polymer artificial muscles (EPAMs) consist of a thin layer of polymer film between two compliant electrodes. When a voltage potential is applied across the electrodes (by Maxwellian forces) the two electrodes attract each other, forcing the incompressible film to contract in thickness and expand in area. When mechanical constraints and output linkages are fixed to the film, the expansion and contraction of the film can be harnessed for useful work. The universal muscle actuator configuration — generally the building block for more complicated arrangements — is constructed of two independent stacks of film, attached at their centers and separated by a lightweight spacer. Each stack of film is then attached to a frame, which gives structure to the film, and is usually used for mounting. Also attached to each stack is an output disc, by which force and stroke are transmitted from the expanding film to a load.
Designers can also layer EPAMs in multiple planar configurations or in linear rolls for additional displacement, stroke, or force — to 20 to 30% displacement in some cases. In contrast, shape memory alloys and piezoelectric technologies might get only 1% direct displacement. And compared to conventional electromagnetic motors, EPAM has a significant advantage in power density.
For economic and technical reasons, robotic applications are not on the immediate horizon for EPAMs just yet. Even so, other valve, small actuator, and gripper applications are. Automatic focus and zoom motion on lens positioners already can be accomplished with silent EPAMs. As actuator position is a function of voltage, actuator resolution is in the microns.
In off-road industries, EPAMs replace conventional hydraulic valves for drive-by-wire applications. The conventional design includes a motor, gearhead, belt drive, and ball screw — over 100 parts. In contrast, the entire EPAM system is comprised of an expanding and contracting linear-roll actuator, controllable with variable-voltage input. It has significantly lower cost and weighs less than a tenth of the conventional electric motor actuator.
Actuator rundown
On low-speed linear motion applications, instead of converting high-speed rotary motion (commonly required from conventional rotary motors) universal muscle actuators can directly drive linear positioning devices at frequencies from dc power to several hundred Hz. This allows designers to remove complicated, fatigue-prone gear reduction and motion conversion power trains.
EPAM technology can also be used for “infinite stroke” applications, such as rotary motors and pumps. These applications convert the reciprocating motion of universal muscle actuators to single-direction motion by way of clutches or check-valves, respectively.
As rotary motors, units consist of a clutch mounted to a universal muscle actuator configuration, converting its motion from linear to rotational.
As a closed-loop pneumatic system, a universal muscle actuator (connected to valves and pressure sensors) serves as an inflatable bladder system. Reading inputs from user-controlled switches, an integrated microcontroller processes pressure levels from the sensors, directing the EPAM valves to release air from or add air to the bladders (in conjunction with the operation of the EPAM pump) until the bladders fill to the desired level.
Efficiency
Particularly in applications where high-speed rotary motion is converted to low speed linear motion, direct-drive EPAMs provide low-loss power transmission. No gears and bearings mean their associated frictional losses are also eliminated.
EPAM devices are variable capacitors — so as far as efficiency goes, EPAMs consume little power when actively holding a position, and return their stored electrical energy when discharged. Thus, an EPAM actuator can be significantly more efficient than conventional electromagnetics, particularly in dc or low frequency applications.
Controls
EPAM actuator controls are dependent upon design requirements. For example, a pump may include frequency and voltage control for flow rate, pressure, and fluid power control. For an EPAM-based motor, because EPAM movement follows the electrical input signal, designers can vary voltage, frequency, and waveform — in addition to PWM control. This leads to more design flexibility when a particular behavior is desired — because these additional “axes” of actuator control allow EPAM actuators to operate in sophisticated ways.
Because EPAM is an analog technology, its motion accuracy depends mainly on the quality of the feedback sensor.
FAQs about EPAMs
What input voltage is required for an EPAM actuator?
EPAMs accept voltages of 1 to 24 Vdc from batteries, or 100 to 240 Vac at 50 or 60 Hz input. Because the EPAM is a capacitive load, power draw primarily occurs when the device is charging. For example, to move a lens 0.3 mm for an auto-focus function requires less than 100 mW.
EPAM actuators have a broad range of frequency operation. Some run at less than 1 Hz to maximize displacement, while other actuators, such as EPAM speakers, run as high as 17 kHz. Frequency is one of the controllable parameters that can be used to optimize EPAM performance.
What percent strain can EPAM achieve?
Typical operating strains of EPAM devices in the no-load state (where the device does not move any force or weight through its stroke) are 5 to 15% over the active length of the device. (Active length is the length of EPAM film parallel to the direction of motion. The total device length is equal to the active length plus the length of the support structure.) Maximum strains of up to 380% have been demonstrated in laboratories, but there is a tradeoff between strain and life cycles when loaded. For applications requiring longer life, the actuator must be designed to operate at strain levels below 15%.
Strain is also dependent upon frequency. Typically, as frequency increases, strain decreases. The strain frequency response is dependent upon material properties, configuration design, and control electronics. Development is ongoing to improve the maximum strain at which devices can be reliably operated. Diaphragm devices typically achieve strain that is 10 to 30% of the device's height.
What is the maximum force an EPAM actuator can exert?
There is no theoretical limit, but there are limits for a given volume of EPAM. A linear relationship exists between force and the number of layers. For example, one-layer diaphragm devices have a force of 0.5 N and 20-layer devices of the diaphragm configuration have demonstrated a blocked force of 10 N.
There is a displacement tradeoff for this force. An EPAM actuator starts with its maximum force level (blocking force) and then decreases as it expands outward with voltage, until it reaches zero force at maximum displacement. As the number of EPAM layers increases, force increases as well — though the EPAM device must be physically larger.
Do EPAM components exhibit hysteresis?
In certain applications and configurations, EPAM actuators exhibit hysteresis. Actuators can be customized to minimize its impact.
For more information, e-mail the editor at [email protected] or visit artificialmuscle.com.
A bit of history
Machine vision allows robots to see in three dimensions. Software in artificial intelligence helps robots make rudimentary decisions once vision information is captured. Force sensing, CAD, and simulation improve the economics of implementing robots in automated assembly and other manufacturing applications. But robots are still limited by two things: cost and energy inefficiencies.
Since the industrial market transitioned from early hydraulic-driven arms to electric motors in the 1980s, electromagnetic actuation has remained the convention not only for robots, but also for motion devices in general. So in the early 1990s, U.S. agencies spurred some groups to address the inefficiency issues of electromagnetically driven robots and funded development of technology that would drive with more efficient, high power density actuation. This funding led to EPAM technology — which is now being commercialized.