What is in this article?:
- BOOK 2, CHAPTER 21: Servovalve circuits
- Servovalves for accurate positioning of actuators
- Servovalves for accurate control of position and force
- Stepper-motor-driven servovalves for motors
When a cylinder or fluid motor application needs precise control of position, speed, or force, an on/off solenoid or proportional solenoid valve will not do the job. Some rolling mills control metal thickness to a tolerance of ±0.0005 in. This is with metal passing through the rolls at 2500 to 3000 ft/min and more. To hold these kinds of tolerances requires more than a go, no-go hydraulic control valve.
Servo directional valves are the only hydraulic valves capable of controlling oil flow and/or pressure rapidly and precisely. Servo directional valves are 4-way, 3-position spool valves with all ports blocked in the center position. Usually, servovalve spools are controlled by high-pressure pilot oil. Many spools have feedback sensing to give repeatable positioning from a given input.
Servovalve spools differ from on/off or proportional valve spools because they have no overlap in center condition. Spool overlap makes proportional valves (and the actuators they control) respond slowly. With no overlap or underlap, any servovalve spool movement gives immediate flow and actuator response. The more closely the spool and body lands match at all four sealing areas, the more responsive the valve. This type of spool is difficult to manufacture, which makes the valve expensive.
Servo systems control actuators to very close tolerances in regard to position, speed, or force. Often a single circuit uses a combination of these functions. A cylinder may have to rapidly approach the work piece, then penetrate it to precise depth at a controlled rate.
While servovalves are very fast and precise, their electronic control is what really makes a servo system work so well. When a signal to move a cylinder starts an action, feedback from its movement modifies valve input to make it match control input. Regardless of pressure drop, fluid viscosity, load, or friction, feedback signals modify valve-spool position to make the cylinder perform exactly as the input signal commands. The only time the actuator falls behind is when it is underpowered.
Figures 21.1 and 21.2 show the schematic symbols for a typical servovalve as established by the American National Standards Institute and the International Standards Organization. Both symbols have parallel lines on both sides of the position envelopes. These parallel lines indicate a valve spool with infinite positions. The symbol shows a blocked center (P to A, B to T and P to B, A to T), but the spool seldom shifts all the way to either of these positions. Spools can shift any amount in either direction, producing increasing or decreasing flow to and from the actuator to move it in either direction.
Simple mechanical servocircuit
Figure 21.3 shows a simple mechanical servocircuit that controls rudder movement on tugboats. The rudder on a tugboat is big and directly in the prop wash, so the operator must have help in moving and controlling it. The lever-operated hydraulic valve in this circuit directs hydraulic power to move the rudder via a double-acting cylinder. If the valve is in the pilothouse, it does not show rudder position. Without knowing the rudder angle, engaging the propellers might be disastrous in some situations.
Figures 21.4 through 21.6 show an inexpensive manual rudder-control circuit. This circuit uses the same lever-operated control valve in Figure 21.3, but here it mounts on the rod of the double-acting cylinder. The operator controls the valve from the pilothouse with a lever called a “tiller.” A cable and pulley system connects the tiller to the valve. This all sounds a little crude but it works quite well on small boats.
The clevis-mounted double-acting cylinder attaches to the boat frame and the rudder lever. The lever-operated valve mounts directly to the cylinder rod so it moves with the rudder lever. When the operator moves the tiller to the right, as in Figure 21.5, the lever on the valve moves to the right. When the lever moves, it shifts the directional valve and ports oil from the pump to the cylinder's cap end and returns oil to tank from the rod end. The cylinder moves the rudder to the right as long as the operator keeps moving the tiller.
When the operator stops moving the tiller, as in Figure 21.6, the directional valve, moving with the cylinder rod, catches up and centers. When tiller movement ceases, the rudder stops and holds. The rudder and tiller stay in this position until the operator steers in a different direction. At all times the operator knows rudder position by looking at the tiller angle.
The mechanical servosystem is nothing more than a force multiplier. In this case, the formerly hard-to-move rudder now moves with slight manual force. At the same time, the tiller position indicates the rudder angle because of mechanical linkage feedback.
An automobile power-steering system uses similar circuitry. Steering wheel movement shifts a directional valve that powers a cylinder to move the steering mechanism. When the steering wheel moves, front wheel angle changes. When steering wheel motion ceases the front wheels stop and hold.
The rudder control circuit shown here might be adapted to control a pressing action where cylinder movement follows the motion of the operator's hand. This gives accurate position with a great amount of force from the operator's intuitive feel.