Proportional valves are fantastic—they can provide infinitely variable hydraulic flow or pressure control to facilitate accurate control of actuators. From Wal-Mart to Bloomingdales, so is the range of proportional valves available. Last month, I discussed the basics of what is available. This month, I discuss how to apply them.
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Josh Cosford, CFPHS, The Fluid Power House (Cambridge) Inc.
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I won't get crazy-advanced and discuss system curves and frequency response; I'll save that for Jack Johnson. This piece won't be about motion control so much as it will be about how to apply three-quarters of proportional valve applications.
Most proportional valves would be spring-centered or offset spool or poppet. By providing variable current to their solenoid coils, magnetic force applied to a pin will move the poppet or spool, proportional to the current applied. For example, a 24-V valve coil might require anywhere from 0.4 to 1.6 A to shift the valve throughout its range.
You could control a proportional valve with a simple variable resistor, but this method is inefficient in its use of electrical energy, and provides poor accuracy of electronic control functions. The preferred method of prop valve control is with Pulse Width Modulation, or PWM. Because a variable current is required for proportional control, a PWM controller fires voltage stable (in this case) pulses of current, rather than continuous amperage via resistors.
The length of time (also known as width or duty cycle) in which the controller fires a signal will vary. Small pulses will yield low current average over a given period of time. As you increase the duty cycle—or frequency—of the pulses, you increase the average current over the same given period of time. Every pulse is a full blast of electrical energy, with no resistor limiting the current. The pulses are extremely fast, as to be unnoticeable in the operation of the coil and valve, almost as if steady current was applied.
The astute amongst you will have observed my mention that a valve coil might required between 0.4 and 1.6 A to shift the full range of the valve. Why would 0.4 be the starting point of valve shifting? There are two reasons for starting well above zero current. One reason is related to the design of the valve spool within the body of the valve and its zero lap, or off-center response. The spool may have to travel slightly before opening a flow path.
The other reason for an i-min signal to be more than zero is static friction. It takes a body within the hold of friction more energy to start moving than to keep moving (up to a point, of course). The i-min signal gives that extra punch to break free from static friction predictably and repeatedly.
Static friction is a problem with proportional valves both before initial open and any other time the spool position changes. Small changes in current may not result in any spool movement at all, but as current is increased to move the spool, it may overshoot its desired position as it overcomes friction. One way to prevent the stick from stiction is not give the spool a chance to come to a rest at all. By vibrating the spool ever-so-slightly, we can prevent issues from static friction while doing little to disturb valve function.
The effect used to vibrate a proportional valve spool is called dither. The dither frequency is specific to each valve and application, but can be anywhere from 60 Hz to 300 Hz, give or take. The valve driver, or controller, will have an adjustable dither frequency—as well as adjustable i-min and i-max settings—amongst other adjustments (like ramp rates) to maximize performance.
The body of knowledge required to apply high performance proportional and servo valves is nearly as vast as the entire works of the rest of the world of hydraulics, especially as it relates to motion control. It would be impossible to cover everything you need to know to work with a prop valve system, but I hope I've given you a taste of what's involved.
Josh Cosford is a certified fluid power hydraulic specialist with The Fluid Power House (Cambridge) Inc. Contact him at joshc@fluidpowerhouse.com or call (519)-624-7109.