How many times have you dialed in your Proportional Valve Driver to perfection only to find everything out of whack the next time you use the machine? If this sounds familiar, then you have probably experienced the effects of temperature-induced coil variations coupled with the use of a Voltage Controller that lacks current feedback. Although there are other factors that can cause performance variations, using the wrong driver in particular will cause major issues (if you need a little background on this subject click here). So, if you are tired of experiencing headaches over inconsistent or unrepeatable proportional function control, consider the following.
Photo courtesy of Tesla Society of N.Y.
The first thing to keep in mind is that you should ALWAYS use a PWM (Pulse Width Modulation) device to drive your Proportional Cartridge Valves. There are two types of PWM coil drivers: Voltage Control and Current Control (or current feedback) drivers. Both types will control the output voltage applied to the coil proportionally to the input or control signal by varying the PWM duty cycle of the output. The voltage applied across the coil with its respective resistance controls the current that generates the magnetic force needed to move the valve actuator.
A voltage-only driver is an open-loop device that outputs one specific voltage for each specific input signal. Knowing that coil resistance is temperature dependant, we can conclude that the valve output with this type of driver will also be temperature dependant.
Current Drivers on the other hand have the added feature of current feedback.
By closing the loop on current, the controller in essence monitors coil resistance and adjusts the voltage to compensate for any changes in resistance. So, after you have run your machine hard for four hours, you may see some hydraulic performance changes but the proportional performance should remain constant. That means no more jerky fine-feathering control when the machine is cold, and no more having all your functions slow down, or dealing with a deadband increase when it warms up.
The bottom line is this: unless you would be happy with the control of your machine changing with temperature, always specify valve drivers or machine controllers with current feedback on the outputs. HydraForce uses current feedback on all our valve drivers and machine controllers, but no matter who or what you use, save yourself some aspirin and make sure you use current feedback.