Pressure control is often used in pressing, grinding, and testing systems. These applications often require control of pressure increases that are much more complex and precise than can be achieved with ordinary relief valves or pressure regulators. Modern applications can take full advantage of the full power. This advantage is achieved through the use of differential control, where the force is obtained by multiplying the pressure on both sides of the piston by their respective areas, and the net force is obtained by subtracting the force from the rodless cavity.
The commonly used closed-loop control method is to realize the precise control of pressure by compensating for the viscosity change of the medium. The use of proven PID algorithms is much more sophisticated than the limited proportional control that mechanical devices with springs can provide. This special performance allows us to correct and reduce overshoot while controlling pressure.
What is stress? Why is it so important for precise control?
Pressure is the force received per unit area, that is, the total force ratio by the total stressed area. The pressure in the hydraulic system is generated by applying force to the oil in the constant volume system or by adding oil to the constant volume system. In this article, we will ignore the effect of thermal expansion.
The basic formula of pressure change:
This formula tells us that the pressure changes with the volume and the volume elastic modulus β, which reflects the compressibility of the liquid. For example, the bulk elastic modulus of oil is about 200,000 psi, while that of water is about 312,000 psi. This means that if the capacity is reduced by 0.1%, the oil pressure will increase by 200 psi, and the water pressure will increase by 312psi. In other words, water is harder to compress than oil.
The following formula defines the effect of time on force and pressure:
In other words, assuming that the volume and the bulk elastic modulus are constant, we can calculate the rate of change of the force and pressure at any time given the speed, flow, and current capacity.
Next, we will focus on formulas (2) and (3).
Application of the formula
The pressure can be controlled in a variety of ways, usually using a servo valve or a proportional valve with a servo spool. Equation 2 shows that to raise a certain pressure, liquid must be injected into the chamber. The rate of liquid addition should be proportional to the desired pressure increase. When the pressure reaches the ideal value, the filling is stopped, and the system keeps the pressure constant all the time. In practical applications, a small amount of leakage is always present, and the controller must add liquid to the system at the rate at which the liquid leaks. Therefore, the net flow Q is always zero, and the pressure change rate is also zero. To reduce the pressure, the slide valve must be adjusted to allow the liquid to flow through the body. When the ideal pressure value is reached, the valve body closes again to keep the pressure constant. The key here is that the rate of pressure change depends on the flow rate, not the pressure itself.
If the system is properly designed, typical proportional valves can be used for pressure control. The key is to have A special gap or pore between the A and B ports of the valve body. Equations 2 and 3 can be used to calculate the ideal leakage when the maximum ideal rate of pressure drop is known. The motion controller must control the valve body to provide fluid to the system at a rate equal to the fluid loss through the pores at all times. If the flow rate is too low, the pressure will decrease as the liquid flows out. If the flow rate is too high, the pressure will increase. The advantage of this method is that when the system is moving, it is damped and will not produce violent vibration.
Meaning of control
In a hydraulic drive system, the pressure may increase rapidly within a millisecond. But machinery doesn't respond so quickly. Mechanical systems are governed by pressure, not by the rate of pressure change. Therefore, if the pressure change rate is not controlled, it is easy to make the mechanical equipment jump to the set value. Imagine you are driving, and when you see a red light, you gradually slow down to a gentle stop instead of a crash stop. The pressure controller should do the same job.
When performing pressure control, at least the following four points should be considered:
1. The pressure sensor reaction time must be fast enough. In cases where the material is not easily compressed, it is common for the oil pressure to increase at a rate of 200psi per millisecond. Pressure sensors with millisecond time constants do not respond quickly enough to pressure changes in use. The pressure response time constant must be in the order of 100μs to work properly. If the empirical method alone uses the sensor to measure, the speed is at least ten times faster than expected.
2. The sampling time of the pressure sensor must be fast and have a fixed interval. In practical applications, such as the metal stamping mentioned above, the pressure can vary by hundreds of psi in as little as 10 milliseconds.
3. Sampling time must be constant. If the scan is supposed to be 10 milliseconds per scan, but it is actually 9 milliseconds per 11 milliseconds, the pressure ratio calculation will differ by at least 20%. Therefore, a constant sampling time is very important for the accurate calculation of the pressure change rate.
4. PLC is not necessarily the best choice. The function of PID in PLCS was originally designed to control temperature or air pressure, not for fluids that are difficult to compress. When they are in the millisecond range, the time constant of the PID in the PLC is in the minute level. For pressure control, a specially designed PID controller is required.
Control option
The first thing to understand is that while we can control position, force, or pressure, we cannot do all three at the same time. At any position, if there is an obstruction, the actuator will be subjected to a resistance equal to and opposite to the driving force. By doing sinusoidal motion tests with actuators, we can monitor the pressure, but cannot control the pressure or force at the same time as controlling its position. Because the pressure or force is determined by the force of the test sample on the actuator. The actuator may provide a sinusoidal force or pressure, but its position is determined by the linearity of the elastic coefficient of the test material.
Stress
There may be no interference during the movement, so there is no point in trying to control the pressure or force. The advantage of controlling only the pressure or force is that there is no need to consider the position. The operator can simply change the set point, but must pay attention to the situation of empty load. In a similar situation, the piston of the hydraulic cylinder will accelerate until the pressure matches the set value. In tests where only pressure or force is controlled, the speed of the actuator should be monitored to avoid a sudden drop in load. When the load is detected to disappear, the actuator may switch to position or speed mode.
Position and pressure constraints
Another option is to limit position or speed through pressure or force constraints. It works by running two control loops at the same time, and only the minimum value of both control outputs is passed to the hydraulic valve. Whether the system will reach the position set point or the pressure set point depends on which one is reached first. Therefore, if the load of the hydraulic cylinder suddenly disappears when the pressure is set, the pressure will drop and trigger the pressure PID controller to produce a large control signal. However, as the system accelerates, the speed error decreases, and the control signal from the PID speed controller decreases as the error decreases. The smaller value of the two signals comes from the speed PID controller. So the actuator is now controlled by the position loop option and does not exceed the set value of the speed.
Initiation and regulation of pressure or force control
Regulation is the process of selecting the best increment for optimal control of position, speed, pressure, or force. Regulating pressure or pressure control systems are different from regulating position systems because the hydraulic cylinder does not move as much when the pressure changes. The easiest way to adjust the pressure or force is to fully extend the actuator until the system pressure is reached, while setting all gains to 0. Then, enter the set value or gain only with a small proportional gain in the PID.
This "small" proportional gain can be estimated by:
This formula enables the worker to determine the position of the starting regulation that cannot be determined during the pressure regulation process. You can start by finding a control signal with a full output of 10V or 40mA, or 100% of the valve, and using the VCCM equation to calculate the maximum speed (see Jack L. Johnson, P.E., "Basic Electronics for Cylinder Motion Control"), or by looking at a table to determine. The dynamic equation must be divided by twice the area. This is because the increase in pressure on one side of the piston is accompanied by a decrease in pressure on the other side.
Note: The estimated proportional gains are not accurate, but they will be very close to ideal initial values.
When the proportional gain is activated, the pressure or force of the system will reach the set point or approach it. The vast majority of errors are due to leaks. The next step is to make the controller compensate or increase the flow rate by slowly increasing the integral gain until the error is zero.
Now, the system is controllable, and it can be adjusted through dynamic responses. It is achieved by adjusting the slope between the two pressures of the operating band. A slope of pressure or force should grow slowly at first, as it is easier to maintain control. This PID gain should be adjustable for better control. Trying to adjust the pressure or force PID in response to step changes in the system is very difficult, and it is difficult to get satisfactory results because pressure shocks can cause leaks. A smooth pressure transition is a good solution. When the pressure or force is changed, the action of the system will depend on the increase and decrease of the pressure between the two setpoints.
