What is a Linear Actuator?

Linear Actuators refer to the broad class of devices that convert one type of motion (usually rectilinear or electro-rotational) into linear motion. Specifically of interest to us are hydraulic or pneumatic devices that are inherently linear actuators as shown in the drawing. An unbalanced fluid pressure applied to the piston generates force that can move an external object. Since liquids are nearly incompressible, a hydraulic cylinder can provide controlled precise linear displacement of the piston. The displacement is only along the axis of the piston.

Linear Actuator diagram

For robust environmentally challenging linear actuator designs, a key consideration is an accurate, reliable mechanism to determine the position of the piston in the cylinder at all times.

Linear Actuator Design Considerations for Hydraulic or Pneumatic Cylinders

Creating “smart cylinders” or incorporating position feedback into hydraulic or pneumatic cylinders used to mean one thing: Gun drill the rod of the hydraulic cylinder (or specify tubular stock), and install a rod-type sensor. Cylinder designers also had to assume the sensor would exit the clevis end or make accommodations for an internal mount. There is now a new class of sensors which do not require gun drilling of the rod, and the sensor installs via the industry standard #8 SAE fluid port.

Linear Position Transducer Use in Long Stroke Sensing applications

One of the problems with long stroke rod-type sensors is general robustness. High shock/vibration environments present problems, and if the sensor is oriented horizontally, the sensor rods tend to sag causing wear issues, even catastrophic failure. SL Series position sensors enable specifiers to orient any stroke length actuator horizontally and still provide reliable position feedback, as there is no sensor rod – only a tensioned braided stainless steel cable. (Standard stroke lengths to 7 meters – contact CPI for longer stroke applications.) The SL Series sensor can also be used for telescoping hydraulic cylinders as well has piston accumulators.

Use of CPI’s SL Series Position Sensors

Harsh environment linear actuator applications requiring position feedback call for SL Series linear position sensors.  Highly shock/vibration immune, these units operate in temperatures up to 300F and are not affected by environmental factors such as humidity and salt spray.  In fact, they will run in hydraulic fluid, water, even salt water.  For a more detailed treatment of our sensor technology and how it is superior to existing Rod-Based and String-Pot type linear position sensors, see our page on LVDT Based Position Sensors.

Industries and installations include mining, oil & gas, hydroelectric generation, subsea and offshore, refinery, and steel (foundry).