Linear Actuator Glossary

Linear motor A linear motor is essentially a rotary electric motor laid down on flat surface.

Mechanical actuators A mechanical linear actuator with digital readout. Mechanical actuators typically convert rotary motion of a control knob or handle into linear displacement via screws and/or gears to which the knob or handle is attached. A jackscrew or car jack is a familiar mechanical actuator. Rotation of the jack handle is converted mechanically into the linear motion of the jack head. Mechanical actuators are also frequently used in the field of lasers and optics to manipulate the position of linear stages, rotary stages, mirror mounts, goniometers and other positioning instruments. For accurate and repeatable positioning, index marks may be used on control knobs. Some actuators even include an encoder and digital position readout. These are similar to the adjustment knobs used on micrometers except that their purpose is position adjustment rather than position measurement.

Hydraulic actuators Hydraulic actuators or hydraulic cylinders typically involve a hollow cylinder having a piston inserted in it. The two sides of the piston are alternately pressurized/de-pressurized to achieve controlled precise linear displacement of the piston and in turn the entity connected to the piston. The physical linear displacement is only along the axis of the piston/cylinder. This design is based off the principles of hydraulics. A familiar example of a manually operated hydraulic actuator is a hydraulic car jack. Typically though, the term "hydraulic actuator" refers to a device controlled by a hydraulic pump.

Direct drive linear actuator Direct drive linear actuators are a new technology based on the linear motor that has been pioneered by British company called Copley Motion Systems. Their ServoTube linear actuator uses tubular linear motor technology but is the first linear motor-based actuator to gain IP67 certification for use in wash down and dirty industrial environments.

These new linear actuators differ from linear motors in 2 ways:
- The rod moves rather than the motor body
- The position sensing is integrated into the actuator body to protect it from dirt, damage.

Advantages
- Electrically operated designs can be self-contained, requiring only electrical power and a control
interface (which can be as simple as a button or toggle switch).
- They can be operated in parallel or series, using multiple actuators to perform a single task.
- They can be very precise, some designs use servo motors and/or specialized sensors to accurately
measure displacement.
- They can be made very fast compared with alternative technologies, (with direct drive type typically
10 times faster than ball screw drives).
- They are relatively cheap and easy to work with compared to other methods of providing linear
motion.

Disadvantages
- Hydraulic actuators can start leaking and are difficult to control.
- Electro-mechanical actuators are prone to mechanical wear and tear.
- While high capacity is possible, the material and/or motor limitations on most designs are surpassed relatively quickly. Increasing structural support and using more powerful motors would be required (increasing the size, weight, and cost) and the advantages of using a linear actuator are lost. (Current linear motors have a particularly low load capacity, which is too small for most normal applications).
- Most types of current actuators are fairly weak in tension, with some having little or no load bearing capability when "pulled" or during retraction. There are certainly exceptions to this, but many are simply not designed to handle type of stress (loading profiles for common actuator uses typically do not require high tensile strength).