In a previous post, I discussed the similitude between electric and hydraulic motivation, and made it clear (I feel) they are actually quite similar in principle. In practice, electric and hydraulic systems differ slightly more, especially when “electronics” are factored in. However, on a larger scale, electric and hydraulic actuators aren’t terribly different either.
The job of either electric or hydraulic power systems is to convert their energy into mechanical force. This force is either rotation, as with motors, or linear, as with linear actuators. Hydraulics use the venerable hydraulic cylinder for linear actuation, and electrics uses either a linear induction motor or a rotating motor with a mechanical linear actuator, such as a ball screw. The motor/ball screw combination is not true electrically motivated linear actuation, so I’ll leave that out of the discussion.
A hydraulic cylinder is a simple capped tube, with a hole at one end of the tube. Through this tube fits the piston rod, which is attached to, you guessed it, a piston. The piston both separates the rod and cap volumes of the cylinder, and provide the surface area for which fluid pressure to create force against. I want to make it clear that force creates movement in a cylinder, not flow. Flow is merely the rate in which pressure can be created. Pressure applied to the piston allows flow to take place.
The linear induction motor has gained some popularity with the proliferation of computer controlled machinery. The linear motor uses a fixed plate magnet, typically as the base, with an electromagnet coil unit as the actuator. The actuator moves by varying the electrical power to the coils, creating the variable magnetic fields to push against the permanent magnet of the base plate. Linear induction motors are highly accurate with much more precision than a pneumatic actuator could achieve, but also very clean, unlike their hydraulic counterpart. Their downside is a relatively low force output and the requirement for air or liquid cooling in heavy duty applications.
Rotary actuators are a lot more similar, and other than different math used for the calculation of torque and power, they can be entirely interchanged in practice, in most cases. The divide between power density of hydraulic and electric linear actuators is reduced with electric motors, although hydraulic motors still win. Hydraulic motors cannot match the accuracy of electric motors, especially the synchronous ilk, and hydraulics are not even an option in most “clean” environments.
The hydraulic motor uses pistons, vanes, gears or other surfaces which pressurized fluid can push against, creating mechanical force in the form of torque. The size of the surface area, and the distance of that area from the point of rotation (i.e., displacement) multiplied by the pressure acting upon that displacement dictates the torque of the motor. Electric motor torque is more complex, requiring Lorentz force calculations, but it is essentially based on the strength of the opposed electromagnetic fields between the rotor and stator, the diameter of the rotor and the type of motor itself. There is little space enough to explain the mechanics here, and since I’m no expert in that field, I’ll just say bigger motors provide more torque, but usually spin more slowly, just as with hydraulic motors.
I’ll do another installment to compare the similarities of electrics and hydraulics, this time showing how similar the symbology is for drawing circuit diagrams. If any of you are still yet unconvinced of their kinship, you will have no doubt after the next installment.