Actuators - An Overview

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Actuators come in many forms. Two primary actuator types are rotary, and linear. An example of a rotary actuator is an electric motor, which is an actuator that converts an electrical signal into a rotating motion of its shaft. Linear actuators, on the other hand, produce a non-rotary motion, such as a sliding motion, or piston motion in response to an input signal. The distinction is somewhat abstract, however, as a linear actuator can be built using a normal rotary motor as its basis, and vice verse. In each case, the initial motion produced is mechanically converted to the other motion-type. Also, there are overlapping semantics, such that a well known linear actuator, such as the voice-coil in a loud-speaker, would seldom be referred to as an actuator.

Interestingly, for some applications, the mechanical conversion is from a natural linear motion, into a rotational motion. The internal combustion engine in your automobile is a good example of this type of conversion, which converts the up-and-down motion generated by the engine's pistons, into tire rotation.





This entry will deal, primarily, with linear actuators. Human muscles are a good example of a linear actuator. Linear actuators are often built by converting the rotary motion produced by a traditional rotary motor into a linear (back-and-forth) motion. These will often use a worm- or screw-gear, or some form of pulley system, to convert the motor's rotational motion into linear motion.

There are also many actuator types which convert signals directly to linear motion. Perhaps the simplest, and one of the earliest forms of such an actuator is the solenoid. A solenoid employs a ferrous, or magnetic rod, which moves in and out of a coil of wire. As a current is passed through the coil, a magnetic force proportional to the current is produced in the coil. This draws the rod inward toward the coil, or pushes it outward, away from the coil (depending on the direction of the current and magnetization of the rod).

A voice coil in a loud-speaker is another example of a naturally linear actuator, which works on the same principal as a solenoid (though, often, the rod is stationary, and the coil moves based on current).

A similar principal is employed in magneto-electric linear motors. These employ many coils (or magnets) along a track in order to facilitate very long linear displacements. This type of linear actuator is, in fact, the principal behind (or should I say "underlying") the forward propulsion system in mag-lev trains. In this case, the displacements can be measured in miles (or kilometers).





A piezo electric element can be considered a linear actuator, because it converts a signal into a motion (a displacement). It is a very small displacement, but it is nonetheless a linear motion. The pros of this kind of actuator include reliability, power-efficiency, and repeatability. The down side, of course, is their very short displacement. Even the best displacements are measured in less than 10% of total length.

A new kind of piezo-actuator uses many piezo elements to move a rod back and forth by essentially walking the rod under them. This allows much greater displacements, but adds complexity, which reduces reliability.





Actuators can operate by moving pistons with gas, such as air pressure (pneumatics), or fluid pressure (hydraulics). These are sometimes referred to as fluid-based actuators. This is true, even for the pneumatic (gas based) systems.

Air-Muscles - A Special Pneumatic Category - Though not new, air-muscles are coming back into favor among many who design with pneumatic actuator systems. Part of the reason for this is the relatively recent availability of huge amounts of cheap processing power, which can be used to process all types of feedback from many different modalities. This reduces the need for rigidity and repeatability at the physical (dead reckoning) level. Air muscles work by inflating a tube (essentially acting as a balloon) inside a braided steal, or plastic sleeve. This causes the sleeve to contract in the perpendicular direction of the balloon expansion, producing a linear back and forth motion.

The down-side to pneumatics and hydraulics - Whether air or fluid, one large down-side to this type of actuator is the need to have another actuator in the control path. Specifically, in order to make a pneumatic or hydraulic cylinder move, you need to first be able to open and close a valve that supplies air or liquid to it. The opening and closing of that valve is where your electrical signal will first be converted to a motion. Because air and fluid don't move anywhere near as quickly as electrons, there will also be a non-trivial delay added in the path (on the order of a few tens of milliseconds or more). Not at all good for dexterity.





Don't be misdirected - The magic is happening at the valve actuators. The art and science of converting fluid pressure into motion has been around for many centuries, and it is fairly stable. New materials are, of course, continuing to improve reliability of the components used, but when it comes to designing a better robotic system, the place to keep your focus, is squarely on the capabilities of the valves. The valves provide air or liquid to the actuator in response to your electrical command-signal.

Solenoid valves have been the norm. They are either entirely opened or entirely closed. They have been useful for many automation tasks, and are well known and easy to come by. The problem with these valves is that their all-or-nothing character translates into quick, jerky movements. Some control over a range of motion can be achieved by pulsing the valve, opening and closing it quickly. This is not that good for a whole host of reasons.

Proportional Control Valves - Or simply "proportional valves" are used to adjust the amount of pressure to the actuator over a continuous range, from very little to a lot. These can be the chattering pulsed variety alluded to above, or they can be modern servo-controlled valves that include positional feedback for accurately opening the valve by a specified amount. Those that open and close the valve in a truly (physically) proportional way, can be controlled by an analog voltage-, or current-level, or they can be controlled by a digital value (where more bits equals more discrete steps of openness).





The capabilities of artificial muscles are often compared to human muscles, and that's not a bad metric, as long as you don't consider it the ultimate metric.

Large hydraulic systems used in earth-moving equipment for example, outstrip human muscles in strength, but may not be the best choice for fast dexterous responses on, say, finer-sized manipulators. Part of this is certainly in the added few-millisecond delay that will be needed to operate a valve with your signal, which in turn, controls the supply of air or fluid pressure to the actual actuator.

Human muscles (at least the skeletal muscles used to manipulate the outside world) have employed a single mechanism for both the large, approximated movements (e.g., the arm muscles), as well as for the small, intricate motions (e.g., the fingers at the end of the arms). The artificial mechanism that looks most like the actin-filament system used by human muscles is the new long-displacement (actually, long-travel), high-resolution piezo system (see resources section below). This operates by employing a bunch of tiny piezoelectric elements that are positioned against a moving bar. The piezoelectric elements act like tiny feet that "walk" the bar back and forth underneath them.

I'll just start to throw out some of the actuator capabilities that may be important in a world where feedback is increasingly employed in robotic, and adaptive-robotic systems. That is, in a world where the old standard of repeatability is becoming increasingly less important. Can you think of more?

• Strength and range of strength (obvious, included for completeness)
• Place-holding characteristics (if any).
• Displacement (percentage difference between full contract, and extend)
• Dexterity (whatever that is)
• Speed off the mark (initial response delay)
• Range of speed (slow to fast)
• Accuracy over a range of speeds and loads
• Smoothness of motion over range of speed and range of load
• Smoothness of the range itself (how many steps of continuous change).
• Energy efficiency and heat dissipation
• Operating conditions: temperature range, humidity, gas- or water-proof, etc.

I wonder if anybody knows of a workable (i.e., "implementable") definition of dexterity?





When you combine a positional sensor with an actuator you have the peanut butter and jelly of the robotics world. Now you have an actuator that will not simply convert your signal into motion/force. The addition of the positional sensor within a servo gives you an actuator that will convert your signal representing a position, into a position of the actuator over its displacement.

In practice, the word "Servo" has come to mean a fairly specific, and semi-standardized electronic component, which provides an actuator and sensor combination. Within this more narrow, practical perspective there are only a few manufacturers of these units. The electrical signaling for them is fairly standardized, and they would be interchangeable, except that there may be small differences in connector configurations between them. These differences are not insurmountable though, and there are people on the web who can show you how to use an exacto-knife to modify the connectors, and perhaps alter the pin-outs, in order to make them plug-compatible across different manufacturers. For more information on servos, and these techniques, see the link to the Robotics Society Tutorials listed in the Sources and Resources section below.





Just for completeness, and because we often get our best ideas from nature, it is probably a good idea to review how animal muscles perform their function. They are, after all, still the best linear actuator out there for their purpose.

• Piezoelectrict PZT:
• Tech-Notes Page at PI:


• Nextline - High Force, High Resolution, Long Travel Linear Actuators (Overview and how they work)
  To my understanding, this Nexline technology works more like biological muscles than any other artificial actuator currently on the market. To see what I mean, compare the working principle on page 8 of this presentation to the second video above.


• Designing With Piezoelectric Transducers: Fundamentals


• Linear motors as a viable alternative to conventional technologies
  A nice article, with a lot of useful and practical information.


• A Novel PID Controller for Pneumatic Proportional Valves with Hysteresis


• Society of Robots Tutorials (rotary and linear)
  • DC Motors
  • Robot Dynamics
  • Servos
  • SMAs (bad choice: included for completeness)
  • Solenoids
  • Waterproofing a servo
  
  
  
• Air Muscles:
  • Air Muscles (intro)
  
  
  • How to make air muscles
    for all the true makers out there.
  
  
  • Example of Air Muscles in Use
    [From this blog]
  
  
  
  
• Proportional Pneumatic Valve (example). . . Also, see: drawings showing inner-workings