Servovalves for accurate control of position and force
Figure 21.9 shows a schematic diagram with a servovalve controlling the force of an actuator. The vertical cylinder in this circuit has position control, while the horizontal cylinder has force control. All the information about hydraulic power unit type, valve location, and filters, applies to this circuit or any other servo application.
The vertical cylinder in this circuit has accurate positioning like the cylinder in Figure 21.7, but this cylinder has controlled speed as well. An application might be a milling operation that requires accurate speed control but may need depth control as well. When fast, accurate positioning at multiple locations is important, use a servovalve.
When the PLC sends a signal to stroke the cylinder, it smoothly ramps up to any speed desired. A servovalve allows for accurate velocity change anywhere along the stroke when the controller calls for it. At the end of stroke, the cylinder decelerates smoothly, rapidly, and accurately to the commanded stopping position without shock. Again, the servovalve does the 4-way function while the electronic controls change speed and position. The servovalve must respond quickly enough to follow the controller's output signals or cylinder position and/or speed will not match the machine requirements.
The horizontal cylinder in Figure 21.9 must hold a constant force against a part, regardless of the load or other changes such as pressure drop or fluid viscosity. Even with a constant pressure source, fluctuations in cylinder friction, machine friction, or rod-end backpressure continuously affect cylinder force. To produce consistent cylinder force, use a servovalve to operate the cylinder and load-cell feedback to continuously modify the valve's spool position. Force stays exactly as set, regardless of system changes -- up to relief valve pressure.
As before, the electronics handle all the input and modifications to set and maintain the desired force. A servovalve controls oil flow as a 4-way directional valve, but has the ability to change flow as needed. It is the response of the servovalve to the electronic controller's changes that is most important. Less-expensive, more dirt-tolerant servovalves offer less-accurate control.
With the circuit in Figure 21.9, the vertical cylinder accurately reaches and maintains any position at any speed. The horizontal cylinder holds any force desired up to maximum pressure.
Stepper-motor-driven servovalves for cylinders
Figure 21.10 shows a simplified cutaway view of a stepper-motor-driven servovalve controlling a hydraulic cylinder. As it receives current pulses, the stepper motor turns in increments of a revolution. Stepper motors may require anywhere from 100 to 500 pulses per revolution. A stepper-motor drive is reliable and repeatable, and produces high torque.
This type servovalve is more dirt-tolerant than other designs. It does not require specific electronics, does not need feedback transducers, and is easy to troubleshoot. This valve may be a stand-alone unit for acceleration and/or deceleration circuits, or for controlling flow -- with or without feedback. Like other servovalves, it has little or no land overlap and a precisely fitted spool to reduce leakage. There are no control orifices to plug, so fluid cleanliness is not as important as with a standard servovalve.
Feedback to a stepper-motor-driven servovalve is mechanical and internal -- similar to the rudder control in Figures 21.4, 21.5, and 21.6. This means that when the cylinder meets resistance it cannot overcome, it will stall. When the cylinder stalls, there is no external feedback to show it has not made its complete stroke. Adding a limit switch or another external signal source helps this problem, but now the circuit resembles a standard on/off solenoid-valve setup.
The response of a stepper motor drive is a little better than the best proportional valves, but not equal to top-of-the-line servovalves.
In the cutaway view, a stepper motor drives a threaded shaft in a threaded spool. The spool can move in and out, but it cannot rotate unless the feedback ball screw in the piston rod turns. Electric pulses to the stepper motor turn the screw in the spool, making the spool shift. Spool shift ports fluid to the cylinder's cap end, making the cylinder extend. When the non-rotating piston and rod start forward, the internal ball screw turns the spool. The ball screw's mechanical linkage turns the spool in the reverse direction of the stepper motor, shifting the spool to stop cylinder movement. When the stepper motor turns, the cylinder extends. The faster the stepper motor receives pulses, the faster the cylinder travels. When the stepper motor stops turning and shifting the spool, the cylinder continues until the ball screw brings the spool back to center. Reversing rotation of the stepper motor reverses all the actions above, including cylinder direction.
From the above explanation, it is obvious that pulsing the stepper motor a certain number of times at a given rate strokes the cylinder to a certain position at a preset speed. If external forces try to move the cylinder out of its position, spool shift — caused by rotation of the ball screw in the piston rod — ports oil to offset these forces.