Hydraulics At Work

# Hydraulic Control Response: How Fast Is 'Fast'?

At a seminar I attended on CAN-bus control systems (CAN is a protocol for the transmission and processing of control and sensor information over a 'bus' cable on automotive and mobile equipment) the presenter was asked:

"Because only one signal at a time can be sent over the bus, when used to control multiple, fast-response hydraulic valves, could the necessary prioritizing and queuing of the control signals cause a delay in the response of the system?"

To this the presenter replied: "How fast is fast?" And considering fast is a relative term, it needed clarification. Indeed, when it comes to the response time of hydraulic valves and variable-displacement pumps, which can also be controlled by a CAN-bus network, how fast is fast?

The typical cycle time for a two-solenoid valve, from one extreme to the other (ON-OFF-ON) for DC solenoid-operated valves is 35 to 70 milliseconds, and 25 to 60 milliseconds for AC solenoids. If we consider an average of say 50 milliseconds for wet-pin, DC solenoids, this means the spool could move from one extreme to the other 20 times per second (1000/50 = 20). This means the valve has a frequency response of 20 Hz.

This may seem fast, and it is--but not when you compare it to the frequency response of proportional and servo valves. The majority of proportional valves used in closed-loop control applications have a frequency response in the 50 to 80 Hz range. This means the spool can move from one extreme to the other 50 to 80 times per second without getting more than 90 degrees out of phase with its input signal (read pages 125-128 of Advanced Hydraulic Control for more detail on valve data).

And even that's not fast - relatively speaking of course. High-end proportional and servo valves used in applications requiring very fast response can climb into the 150 to 200 Hz range. That's up to 10 times faster than bang-bang and low-end proportional valves used in open-loop control (no feedback from the controlled parameter). Spool shift in five milliseconds IS fast!

This leaves variable pump controls. How fast are they? Well, with a typical frequency response of between 20 and 30 Hz, they are firmly at the slow end of the scale. And this relatively slow response explains why direct control of pump displacement isn't a practical alternative to a proportional or servo valve in the majority of demanding, closed-loop control applications.