Proportional and Servo Valves

As we already know, directional valves are either open or shut, they offer no control and their operation can be very harsh. Proportional valves, on the other hand, are used to control the acceleration and/or speed of a load, therefore, providing smooth and controlled 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 implements smoothly and have control over the speed they move at.

In industrial applications proportional control valves are used when different speeds are required or used when actuators need to stop in a specific position.

High performance proportional and servo valves are used when greater accuracy or a high-precision of speed, pressure, or positional control is required, often with electrical closed-loop control rather than open loop control.

Hydraulic proportional control valves work in a similar way too directional valves except they do not switch from shut to open but gradually open a restricted flow 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 although the actual flow will vary as the load or supply pressure changes.

Proportional valve pressure drops are typically 30% of the load pressure. Proportional valves need this 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, to be located away from the valves.

Industrial proportional control valves more commonly use electrical actuation with PWM (pulse width modulated) power control to the solenoids. This controls the force the coil exerts against the spring and therefore the spool position.

You can usually find a proportional version for all types of hydraulic valve 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 is a huge range of different quality and performance valves available 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 the 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 up to 400Hz being achieved by the highest quality test machinery.

We will add more details on this in later training exercises. Follow us on twitter to hear when they are available.

You should always start by calculating 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 for the valve to adequately brake and control the load. A common error is to oversize a proportional valve so that you have plenty of pressure drop available for the load, but control is poor. You should try to use a smaller valve that has a high pressure loss and therefore gives good 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 might want to use a 2:1 ratio spool. This 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 when you raise it. Therefore better efficiency with no influence on the 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. We will not discuss this further here because it is normally only of concern for high-performance control systems and if you are working at this level then you should already be discussing performance simulations with your valve suppliers.

In industrial design, it's common to build in higher p.d. losses to make sure of good control. While in mobile design any inefficiency is extra fuel used so additional development work is done to find acceptable control with minimum pressure loss. What this means for maintenance is that on industrial systems there may some margin for adjusting the control to give better performance or efficiency. However, on mobile equipment, you may only have a narrow margin to set the valves so that you still have confidence they will perform across the full range of operating conditions.

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 or 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 with beta 200 ratings.

Expect more wear on the valve's control lands the higher the performance and therefore tolerances of the valve. Because the valve spools have tighter tolerances they will also be more sensitive to contamination and wear.

Look after your fluid well, its life may be reduced due to the higher valve pressure drops and high local fluid temperatures. The extra shear forces within the fluid, due to the high load braking forces, put extra strain on the fluid, which can result in changes in the fluid's properties.

TBC

Unless your system is very simple or you've made them lots of times before, then it's always worth running a proper dynamic simulation as static calculations of proportional control are not completely effective.