Hydraulic vs Servo Motor
gbradley said:
Maybe you can write an article about the tool changing system that you worked on in
Control Design Magazine?
I would sure like to read it.
I have a couple articles in Control Design. Can you tell me which month? I think I know the application. I think there was a Parker HSE ( hydraulic systems specialist ) involved that did the real work. If so, his name is Bruce Besch. I will check
I can find out more from him. I didn't get involved in why the old application didn't work. I just gave some tech support on getting the hydraulic design to work.
gbradley said:
I don't know nuthin from nuthin, but I do find it surprising that Hydraulics are replacing servos, and not the other way around.
You aren't alone. Servo motors and hydraulic systems have different strengths and weakness. Servo motors are better for smaller applications and some of the bigger applications that have a continuous duty cycle. Hydraulic systems are better for large mass applications, low duty cycle applications and those where one needs to apply force or torque as in a press.
Read this
http://www.controldesign.com/Media/MediaManager/wp_010_delta_electrohydrailics.pdf
Cost wise it is easy to see that servo motors and especialy stepper motors are much cheaper than a small hydraulic system. Servo motors are also very efficient on applications like conveyors were there are few mechanical loses and the motor is running continuosly. Servo motors also have an advantage in that I bet most of you would try to make a servo motor to run before trying to get a hydraulic system to running. There is a built in prejudice toward servo motors over hydraulics.
Now for the hydraulic advantages.
1.
ONE motor/pump can supply pressurized oil ( read energy ) to many actuators or hydraulic motors. The motor/pump only needs to supply enough energy during cover the energy needs during one cycle. When an actuator isn't moving, energy can be stored in accumulators for use during peak usage. Servo motors must each be sized for the peak loads and each axis must have its own motor.
2. Hydraulic systems are ideal for applying a force or torque as in a press. When the hydraulic system applies force, there is very little energy consumed. There is just a little leakage. A motor must apply torque which will cause heat and possibly destroy the servo motor.
3. On big systems the motor/pump can be mounted remotely so the power unit is not part of the system being moved. Hydraulic motors and cylinders are small relative to the force that can be applied. The ability to use hose to transport energy from one point to another is a big advantage in some applications. Some ships have hydraulic drives. This isn't as efficient as a direct drive to propeller but then the shaft doesn't need to be perfectly aligned, in fact the propeller can be rotated if need be. That can be handy when docking.
4. Hydraulic works well with either motors ( rotary ) or cylinders ( linear ).
Hydraulic disadvantages.
1. Hydraulics are non-linear. This can be minimized by proper design but too many hydraulic designers don't know how to properly design a system.
2. Hydraulics with servo valves can be in-efficient. A lot of energy can be lost as the oil flow through a servo valve.
3. It can leak and be a environmental mess. One of the things I stess is using a servo controller to make 'smooth moves'. Acceleration and deceleration ramps using S curves reduces the hydraulic shock. This puts less stress on the system which means fewer break downs and leaks.
Think of hydraulics as just a way to get energy from one point to another much like a drive shaft gets energy to the rear wheels of a car. The hydraulic guys may take offense to this. They like to talk about does a pump pump oil or pressure. It is a religious issue. I think of oil as just a way to move energy from one point to another. It just happens to be very flexible.
Finally, I really don't know right know why that customer, mentioned in Control Designed, switched to hydraulics. My guess is that it is an application that should have been a hydraulic application in the first place but someone decided that servo motors were the latest craze without evaluating the relative merit of hydraulic vs servo motor.
I am done for the night.