Because directional valves are either shut to open they offer no control and their operation can be very harsh. Proportional valves are used to control the speed of movement and provide smooth opening and closing.
In mobile excavators proportional control valves are widely used to move the arms and buckets smoothly. With mobile equipment the operator needs to be able to move the tools smoothly and have control of the speed they move them.
In industrial applications proportional control valves are used when different speeds are required and also when actuators need to stop in a specific position.
High performance proportional and servo valves are used when greater accuracy or precision of speed, pressure, or positional control is required, often in closed loop control rather than open loop control.
Hydraulic proportional control valves work in a similar way to directional valves except they do not switch from shut to open but gradually open a flow restriction orifice. The size of valve opening is proportional to spool movement, either controlled by the electrical solenoid or the hand lever. In this image you can see a directional spool switching (on top) and a proportional spool gradually opening (below). The proportional spool has a triangular notch that gradually opens a flow control orifice as it moves.
Flow is proportional to the valve opening but flow will vary as load or supply pressure changes.
Pressure drops are typically 30% of the load pressure. Proportional valves need extra pressure drop (p.d.) to control the flow. High performance servo valves may use 70bar p.d. to control the load. Think of hydraulic control valves as car brakes rather than engines. Valves, like brakes, are much smaller and control the vehicle's load by throttling or decelerating rather than accelerating like an engine.
With a manually operated proportional valve you simple control the position of the spool with the position of the operating handle.
Mobile control blocks often use proportional pilot pressure signal control signals on the ends of the spools. This allows remote pilot pressure control handles away from the valves.
Industrial proportional control valves more commonly use PWM (pulse width modulated) power control to the solenoids to control the force they exert against the spring and therefore the spool position.
You can usually find a proportional version of all types of hydraulic valves e.g. directional, pressure, flow, poppet or spool valves.
See our symbols sections for more details.
Quality and cost can be very different. Low end proportional valves may be appropriate for basic smoothing or speed control but for accurate or fast control in open or closed loop you will need a much better valve. There are a huge range of different quality and performance valves available these days and careful selections in very important.
Rough guides just to give you some ideas and basic checks are:
Don't try to control anything below 3-4 Hz, unless you really understand adaptive control.
3-10Hz might be manual control of some excavators or ramped opening of industrial plant to stop it juddering.
10-30Hz gives reasonable control in open loop.
30-80Hz good control in open and closed loop control.
Above 80Hz are the high performance systems with 200Hz higher than most and 400Hz being achieved by the highest quality test machinery.
We will add more details on this in some new training exercises coming soon. Follow us on twitter to hear when they are available.
You should always calculate the natural frequency of your overall mechanical-hydraulic system and then calculate the valve performance you need to achieve the accuracy and dynamic response you require. You can often measure the natural frequency by simply hitting the load with something heavy and then measuring the frequency of the pressure oscillations.
Make sure you have sufficient pressure available to control the load. A common error is to oversize a proportional valve so that you have plenty of pressure drop available but control is poor, rather than using a smaller rated valve that has higher losses and therefore poor control.
Consider the direction of your load and select the spool cut to suit. For example, if you are raising and lowering a load you can use a 2:1 ratio spool which has a bigger opening on the unloaded side of the cylinder than the loaded side. This means you have good control of the load when you lower it but don't waste energy on the unloaded side which has little influence on safety or control.
You may also want to specify a particular spool underlap or overlap to suit the type of control you will be using.
In industrial design it's common to build in higher p.d. losses to make sure of good control. In mobile design any inefficiency is extra fuel lost so additional development work is done to find acceptable control with minimum pressure loss.
Consider the valve and control system as one unit. Modern PLCs have hydraulic valve controls built in and hydraulic manufacturers have their own control cards of devices to ensure good control in a relatively simple way.
Keep your fluid clean. The finer the control the more a single piece of contaminant will affect its operation. You should run around 1-2 points cleaner on the ISO scale for proportional and 2-3 for servo control, probably 5 or 3 micron filters.
Unless your system is very simple or you've done it lots of times before then it's always worth running a proper dynamic simulation as static calculations of proportional control are not very effective.
Expect more wear on control lands and the tighter running tolerances needing to be kept in alignment. Look after your fluid well, its life may be reduced due to the higher valve pressure drops and running temperatures.
We will add some better proportional valve calculators soon.