Pressure environment: the use of motion controller for pressure control

Pressure control is often used in presses, grinding and testing systems. These applications often require the control of pressure increases, the complexity of the control and precision is much higher than the ordinary relief valve or pressure regulator can be achieved. Modern applications can take full advantage of the net force. This advantage is achieved by the use of differential control, that is, the pressure on both sides of the piston  steel plate punching machine and their corresponding area multiplied by the force, and then rodless force minus the rod cavity force to obtain a net force.
Now commonly used closed-loop control method is to compensate for the viscosity of the media to achieve the precise control of pressure. The use of proven PID algorithms is much more precise than the limited proportional control that can be provided by mechanical devices with springs. This special feature allows us to correct and reduce the overshoot while controlling the pressure.

boring machine

What is stress and why is it so important to precise control?
Pressure is the force per unit area, that is, the total force than the total force area. The pressure in the hydraulic system is generated by applying force to the oil in the constant volume system or adding oil to the constant volume system. In this paper, we will ignore the effect of thermal expansion.
The basic formula of pressure change:
This formula tells us that the pressure with the capacity and bulk modulus of elasticity β change, while the volume elastic modulus β reflects the liquid compression. For example, the oil has a bulk modulus of elasticity of about 200,000 psi and a bulk modulus of elasticity of about 312,000 psi. This means  punch machine for Head that if the capacity is reduced by 0.1%, the oil pressure will increase by 200 psi and the water pressure will increase by 312 psi. In other words, water is more difficult to compress than oil.
The following formula defines the effect of time on force and pressure:
That is, assuming that the volume and bulk modulus of elasticity are constant, we can compute the force and the rate of change of pressure at any time, given the known velocity, flow rate, and current capacity.
Next we will focus on the formula (2), (3).
Application of Formulas
Pressure can be controlled through a variety of ways, commonly used method is the use of servo valve or servo valve with proportional valve. Equation 2 shows that in order to increase the pressure, liquid must be injected into the chamber. The rate of liquid addition should be proportional to the expected increase in pressure. When the pressure reaches the desired value, the filling is stopped and the system remains constant. In practice, a small amount of leakage is always present and the controller must add liquid to the system at a rate of liquid leakage. Therefore, the net flow rate Q is always zero and the pressure change rate is also zero. To reduce the pressure, the spool must be adjusted to allow fluid to flow through the valve body. When the desired pressure is reached, the valve is closed again to keep the pressure constant. The key here is that the pressure change rate is determined by the flow rate rather than the pressure itself.
If the system is designed properly, a typical proportional valve can be used for pressure control. The key lies in the body of the A port and B between the mouth to set up a special gap or pore. Equation 2 and Equation 3 can be used to calculate the desired leak rate, given the maximum ideal pressure drop rate. The motion controller must control the valve body so that it supplies liquid to the system at a rate equivalent to the rate at which the liquid is lost through the pores. If the flow rate is too low, the pressure will be reduced with the flow of liquid. If the flow is too high, the pressure will increase. The advantage of this method is that when the system is damped by the role of movement does not produce severe vibration.
The significance of control
In a hydraulic drive system, the pressure may increase rapidly within one millisecond. But the machinery does not respond so quickly. The mechanical system is subject to pressure, not pressure. Therefore, if the rate of change in pressure is not controlled, it is easy to jump to the set value of mechanical  CNC movable drilling machineequipment. Imagine you are driving, when you see a red light when you will gradually slow down to a gentle stop, rather than emergency stop. The pressure controller should do the same.
At least the following four points should be considered when performing pressure control:
1, the pressure sensor response time must be fast enough. In the case where the material is not easily compressed, it is common for the oil pressure to increase at a rate of 200 psi per millisecond. Pressure sensors with millisecond time constants are not fast enough to respond to pressure changes in use. The pressure response time constant must be 100 μs for proper operation. If a sensor is used for empirical measurement, it is at least ten times faster than expected.
2, the pressure sensor sampling time must be fast and have a fixed interval. In practical applications, such as the above-mentioned metal stamping, the pressure can change in just 10 milliseconds hundreds of psi.
3, the sampling time must be constant. If the scan time is 10 milliseconds at a time, and actually 9 milliseconds at a time, 11 milliseconds, the calculation of the pressure ratio will differ by at least 20 percent. Therefore, a constant sampling time is very important for accurate calculation of pressure change rate.
4, PLC is not necessarily the best choice. The function of the PID in the PLC is originally designed to control the temperature or pressure, not the fluid that is difficult to compress. When they are in the millisecond range, the time constant of the PID in the PLC is minute level. For pressure control, a specially designed PID controller is required.
Control options
First of all, to understand, although we can control the position, force or pressure, but at the same time can not do all three. In any position, if there is an obstruction, the actuator will be subject to a force of the same size, in the opposite direction of resistance. By performing the sinusoidal motion test with the actuator horizontal boring machine, we can monitor the pressure but can not control the pressure or force while controlling its position. Because the pressure or force is determined by the force exerted by the test sample on the actuator. The actuator can provide sinusoidal force or pressure, but its position is determined by the linearity of the elasticity of the test material.

horizontal boring machine

pressure
There may not be interference during motion, so trying to control pressure or force is meaningless. The advantage of controlling only pressure or force is that no position is considered. The operator can simply change the setpoint, but it must be noted that no-load condition. In a similar situation, the piston of the hydraulic cylinder is accelerated until the pressure matches the set value. In tests where only pressure or force is to be 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 be switched to position or speed mode.
Position and pressure limits
Another option is to limit the position or speed by pressure or force limitations. By operating both control loops simultaneously, and passing only the minimum of the two control outputs to the hydraulic valve. Whether the system will reach the position setpoint or the pressure setpoint depends on which one is reached first. Therefore, if the load suddenly disappears when the cylinder is at the pressure setpoint, the pressure will drop Laser cutting robots and cause the pressure PID controller to generate 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 of the two signals comes from the speed PID controller. So the actuator is now controlled by the position loop option and will not exceed the speed setpoint.
Pressure or force control of the start and regulation
Regulation is the process of selecting the best increment for optimal control of position, velocity, pressure or force. Adjusting the pressure or pressure control system is not the same as adjusting the position system because the hydraulic cylinder does not move too 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, with all gains set to zero. Then, enter the setpoint or use only a small proportional gain in the PID. This “small” proportional gain can be estimated from:
This formula allows the operator to determine the position of the start-up adjustment that can not be determined during the pressure adjustment process. It is possible to start by finding a control signal with a full output of 10V or 40mA or 100% of the valve, and use the VCCM equation to calculate the maximum speed (see Jack L. Johnson PE, “Cylinder Motion Control Fundamental Electronics”). Table to determine. The dynamic equation must be divided by twice the area. This is because the piston side of the pressure increase along with the other side of the pressure drop.
Note: The estimated proportional gains are not accurate, but they are very close to the ideal initial values.
When the proportional gain is activated, the system pressure or force will reach the setpoint or approach it. The vast majority of the error is due to leakage caused. The next step is to slowly increase the integral gain so that the controller compensates or increases the flow until the error is zero.
The system is now controllable and can be tuned by dynamic response. Is achieved by adjusting the ramp between the two pressure bands of the operating frequency band. The pressure or force ramps should begin to grow slowly, because it is easier to maintain control. The PID gain should be adjustable to achieve better control. Attempting to adjust the pressure or force PID to respond to a step change in the system is very difficult, and it is difficult to obtain satisfactory results because a pressure shock can cause a leak. Smooth pressure transfer is a good solution. When changing pressure or force, the system action will depend on the pressure between the two set values ​​of growth and decline.
From position control to pressure or force control
Many applications require a combination of pressure or force control and position control, as shown in Figure 1:
The red line is the position of the cylinder; the horizontal axis is time; the vertical axis is the amplitude; the blue line is the pressure.
The establishment of such a system requires knowledge of mechanics and control. If the material is easily compressed, the transition from position control to pressure control is relatively easy because the material can absorb a lot of energy during deformation. Conversely, for metal and metal between the extrusion block and stop block, from the displacement control to pressure or force control is more difficult. Because the stop block can not absorb too much energy. But we can adjust the contact speed so that the total kinetic energy of the actuator and the die or the machine tool matches the energy absorbed and released on the workpiece. Simply put, because the hydraulic Drilling Machine for ring system  can not quickly change the direction or release the excess energy, if the implementing agencies and die movement speed fast enough, will produce transient pressure. If the actuator is too slow to approach the workpiece, the pressure will be delayed when it reaches the workpiece. This requires that the actuator be sufficiently pressurized at the beginning, and that this process is usually slow and does not reach the desired value.
There are some special considerations for adjusting the coordinate system for position control and pressure control. Often, it is helpful for the controller manufacturer to provide a paint adjustment tool. For example, using Delta’s RMCTool software, you can use the command buttons in the Adjust tool dialog to control the axis or ramp to the specified pressure within a specified time after selecting some initial constant PID and feedforward gain. This adjustment is done in both directions, and the target and actual values ​​can be compared at any time to determine if the gain parameters in the pressure control loop need to be changed.
Figure 2 is a very representative of the metal pressure map, which reflects the process of metal pressure control from the position to control a series of changes. At 0.1 second, the pressure ramps to a desired pressure value and is held for a short period of time during the metal stamping process; then the cycle is entered at approximately 0.4 seconds and the controller completes the stamping cycle by means of the transfer force control and the start of the reverse mode.

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