Hydraulic Actuators

The hydraulic robot was popular in the 1970s and 1980s because of its ability to move very large payloads at a time when the size of electric motors for robotic servo systems was limited. For instance, the hydraulic robot was used to pick up engine blocks that weighed over 800 lb. from an assembly line and place them on a pallet. These types of robots were also used extensively for spot-welding applications where the spot-welder weighed over 50 lb. The hydraulic robot could move the spot-welder quickly to nearly any position required to make a weld inside and outside of an automotive body. Hydraulic robots also provided holding strength. Holding strength is very important in applications where a load is moved and then held in position for an application and then moved again. In the case of the hydraulic robot, in its day it was the only type of actuator that could move a large load and then hold it extended in the correct position until it was ready to move again. Today electric motor driven robots can provide the same function less expensively.

Hydraulic actuators are available as cylinders to provide linear motion, and as hydraulic motors to provide rotary motion. Hydraulic cylinders are designed to provide large mechanical advantage. That is, the hydraulic fluid pressure can be applied against the large area of a cylinder, and it can provide a multiplying force of over ten times the force of the fluid. This allows hydraulic robots to lift large loads. The only drawback to this advantage is when the larger cylinders are used, they take extra time to fill with fluid, which causes the robot to be slower.

Hydraulic motors allow for the same advantage to be provided for rotary motion. The proportional hydraulic servo valve is used to provide fast response and precise positioning. The servo valve also provides positive stop for fluid flow, which allows the robot to hold its location without sagging when large loads are moved. This is one advantage that may make the hydraulic robot more suitable than an electric or pneumatic robot.

The main problem with the hydraulic robot is that the control system of servo valves and actuators is highly complex and requires special training and tools for repair. These types of systems also tend to leak more often, which causes additional problems of extended downtime and excessive maintenance costs.