SLAS

Actuators

From LabAutopedia

Jump to: navigation, search

An actuator is a mechanical or electromechanical device for moving or controlling a mechanism or system.  Any laboratory automation that involves physical movement involves some type of actuator. 


Contents

Rotary actuators

Rotary actuators create motion and force along a rotary axis utilizing an externally applied energy source. Types include:

Motors

Electric motors use electrical energy to produce mechanical energy.  They can be broadly classified into those operating on Alternating Current (AC) and Direct Current (DC), although there are also Universal Motors that can use either power source.  Most motors involved in laboratory automation devices are DC motors of the type described below.

Brushless DC motor:

Brushless DC motors (BLDC) are commonly used where precise speed control is necessary.  They have no chance of sparking, making them better suited to environments with volatile chemicals.  They are very efficient, running much cooler than the equivalent AC motors, and so are often used in applications integrated with electronics where heat must be controlled.  Brushless DC motors are actually 3 phase AC motors driven by a DC source. The DC direct-current electricity is converted to three phase Alternating-current by an electronic device often refered to as an ESC, or Electronic Speed Controller.  The controller performs the same power distribution found in a brushed DC motor, but using a solid-state circuit rather than a commutator/brush system.  BLDC motors offer several advantages over brushed DC motors, including higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI). With no windings on the rotor, they are not subjected to centrifugal forces, and because the electromagnets are located around the perimeter, the electromagnets can be cooled by conduction to the motor casing, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter. The maximum power that can be applied to a BLDC motor is exceptionally high, limited almost exclusively by heat, which can damage the magnets. BLDC's main disadvantage is higher cost.  BLDC motors require complex electronic speed controllers to run. Brushed DC motors can be regulated by a comparatively simple controller, such as a rheostat.

Stepper motor:

A stepper motor is a brushless, synchronous DC electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely, without any feedback mechanism (see open loop control). Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system  Stepper motors operate much differently from normal DC motors, which rotate when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple "toothed" electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, such as a microcontroller. To make the motor shaft turn, first one electromagnet is given power, which makes the gear's teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a "step." In that way, the motor can be turned to a precise angle. Sophisticated drivers can proportionally control the power to the field electromagnets, allowing the rotors to position between the step points and thereby rotate extremely smoothly.  Stepper motors can also "lock" into position with a high degree of torque. 

Stepper motors are generally operated open loop, i.e. the driver has no feedback on where the rotor actually is. Stepper motor systems must thus generally be over engineered, especially if the load inertia is high, or there is widely varying load, so that there is no possibility that the motor will lose steps. There is often a tradeoff between a small sized but expensive servomechanism system and an oversized but relatively cheap stepper.

A new development in stepper control is to incorporate a rotor position feedback (e.g. an encoder or resolver), so that the commutation can be made optimal for torque generation according to actual rotor position. This turns the stepper motor into a high pole count brushless servo motor, with exceptional low speed torque and position resolution. An advance on this technique is to normally run the motor in open loop mode, and only enter closed loop mode if the rotor position error becomes too large -- this will allow the system to avoid hunting or oscillating, a common servo problem.

Servo motor:

A servo motor is a DC motor with built in gearing and feedback control loop encoder circuitry (closed loop control), which may be analog or digital.  They are normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops are made quickly and accurately. Servo motors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes.  A servo motor is generally much smoother and faster in motion than a comparable stepper, and will have a much higher resolution for position control.  Servo motors can exert a higher torque during movement than can a stepper motor, but less torque than a stepper in a fixed position.  Servo motors are more complex and expensive than stepper motors. 

Analog encoders often take the form of rotary potentiometers (commonly referred to as a "pot").  These are mechanical devices, consisting of a three-terminal resistor with a sliding contact that forms an adjustable voltage divider.  Their primary disadvantage for motor position control is the wear that accompanies any mechanical device and the slow effect of such wear on the encoder feedback.  Their position feedback is absolute in nature and will not change as the result of a power cycle.  Analog encoders also come in optical form, which are constructed to produce a unique dual analog code that can be translated into an absolute angle of the shaft (by using a special algorithm). 

Digital encoders are devices that convert motion into a sequence of digital pulses.  By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements. Encoders have both linear and rotary configurations, but the most common type is rotary. Rotary encoders are manufactured in two basic forms: the absolute encoder where a unique digital word corresponds to each rotational position of the shaft, and the incremental encoder, which produces digital pulses as the shaft rotates, allowing measurement of relative position of shaft.  Encoders may utilize mechanical or optical position tracking.  Mechanical encoders utilize a metal disk containing conducting and non-conduction surfaces fixed to the shaft, which is physically swept by a series of stationary, sliding contacts.  Optical encoders are composed of a glass or plastic code disk with a photographically deposited radial pattern organized in tracks. As radial lines in each track interrupt the beam between a photoemitter-detector pair, digital pulses are produced.  Absolute encoders generate a unique signal for each position of the motor, and so are not subject to counting error or loss of count due to power failure, as is the case with incremental encoders.  Devices using incremental encoders must, after powercycle, go to a known position (e.g. "home") to reset the relative positional counting process.  Absolute encoders are more expensive and generally larger than incremental encoders.   

Linear Actuators

Linear actuators create motion and force along a straight line utilizing an externally applied energy source.  Types include:

Pneumatic

A pneumatic actuator converts energy (in the form of compressed air, typically) into motion. The motion can be rotary or linear, depending on the type of actuator. Some types of pneumatic actuators include:

  • Single acting tie rod cylinders that use the force imparted by air to move in one direction (usually out), and a spring to return to the "home" position.
  • Double acting tie rod cylinders that use the force of air to move in both extend and retract strokes. They have two ports to allow air in, one for outstroke and one for instroke.  More reliable than single acting cylinders, but requiring two inputs and more costly than single acting cylinders.
  • Rotary actuators that use the force imparted by air to drive create linear motion, which is mechanically converted to rotary motion.
  • Rodless actuators that use a mechanical or magnetic coupling to impart force, typically to a table or other body that moves along the length of the cylinder body, but does not extend beyond it.
  • Grippers that move in either parallel or angular motion of surfaces that will grip an object. The gripper can be used as part of a "pick and place" system that will allow a component to be picked up and placed somewhere else.

A Pneumatic actuator mainly consists of a piston, a cylinder, and valves or ports. Once actuated, compressed air enters into the cylinder at one end of the piston and imparts force on the piston. Consequently, the piston becomes displaced (moved) by the compressed air expanding in an attempt to reach atmospheric pressure. The larger the size of the piston, the larger the output pressure can be. Having a larger piston can also be good if air supply is low, allowing the same forces with less input. A typical input range is 20-100 kPa.  Pneumatic systems are often found in settings where simple movement is needed and where even rare and brief system failure is unacceptable.  Although the most common form is two-position, pneumatic devices can be designed with intermediate stopping points, to be adjusted so as to control the amount of extension and/or retraction of the piston once actuated

Hydraulic

Hydraulic acuators, also called a hydraulic cylinder, are a fluid pressure actuated linear motion piston within a enclosed cylinder. The two isolated sides of the hydraulic piston are either pressurized or de-pressurized to achieve linear movement of the piston to create the linear motion and force. These are similar to pneumatic cylinders, but are used when higher motive forces are needed and slower response is acceptible.  An example of a hydraulic actuator is a hydraulic cylinder utilized on an automotive repair lift utilized by mechanics to access the underside of a automobile for maintenance. The fluid pressure supplied to the hydraulic actuator is either supplied by a manual fluid pump or electric pump.  

Solenoid

Solenoids are devices that convert electrical energy into linear motion.  Electricity passes through an electric coil, creating a magnetic field, forcing a movable steel or iron shaft (armature) to move in or out.  Although typically weak over anything but very short distances, solenoids may be controlled directly by a controller circuit, and thus have very fast reaction times.  Solenoids can be actuated in milliseconds, or can be velocity controlled to provide smooth, noiseless actuation.  They can be designed for linear or rotatary movement, dual or variable position. 

Solenoid valves are an electromechanical valve for use with liquid or gas.  They are used as control elements in fluidics, to shut off, release, dose, distribute or mix fluids. They can also serve as the switch for routing air to any pneumatic device.  Solenoids offer fast and safe switching, high reliability, long service life, good medium compatibility of the materials used, low control power and compact design.

Piezoelectric actuators

The piezoelectric effect is a property of certain materials in which application of a voltage to the material causes it to expand. Very high voltages correspond to only tiny expansions. As a result, piezoelectric actuators can achieve extremely fine positioning resolution (sub-nanometer), but also have a very short range of motion (several hundred microns).  Peizo actuators can react in microseconds, with acceleration rates of more than 10,000g.  Peizo actuators do not generate magnetic fields, nor are they affected by them.  The piezo effect continues to occur at temperatures approaching 0 Kelvin, so piezo actuators are ideal for use in cryogenic conditions.  Piezoelectric materials exhibit hysterisis which makes it difficult to control their expansion in a repeatable manner. [1]

Piezoelectric linear drives can be used to create microscale positioning systems, forming the basis of linear stages without the use of classic electromechanical acutators, such as rotary motor/leadscrew assemblies

Electromechanical

Electromechanical actuators typically convert rotary motion (e.g. from an electric motor) into linear displacement and force via screws and/or gears to which the rotary driver is connected, often to manipulate the position of linear stages or rotary stages. Typically, a rotary driver is mechanically connected to a lead screw so that the rotation of the electric motor will make the lead screw rotate. A lead screw has a continuous helical thread machined on its circumference running along the length (similar to the thread on a bolt). Threaded onto the lead screw is a lead nut with corresponding helical threads. The nut is prevented from rotating with the lead screw (typically the nut interlocks with a non-rotating part of the actuator body). Therefore, when the lead screw is rotated, the nut will be driven along the threads. The direction of motion of the nut will depend on the direction of rotation of the lead screw. By connecting linkages to the nut, the motion can be converted to usable linear displacement. Most current actuators are built either for high speed, high force, or a compromise between the two. When considering an actuator for a particular application, the most important specifications are typically travel, speed, force, and lifetime.

Electroactive Polymers

EAPs are polymers whose shape is modified when a voltage is applied to them. They can be used as actuators or sensors. As actuators, they are characterized by being able to undergo a large amount of deformation while sustaining large forces. Due to the similarities with biological tissues in terms of achievable stress and force, they are often called artificial muscles, and have the potential for application in the field of robotics, where large linear movement is often needed.[2]

References

  1. PI - Piezo Nano Positioning: Tutorial: Piezoelectrics in Nanopositioning, Designing with Piezoelectric Actuators
  2. Flex that muscle SPIE
Click [+] for other articles on 
The Market Place for Lab Automation & Screening  Valves and Pumps