Comparison of Switch Control and Closed Loop Control

There are a number of factors to consider when deciding whether to use switch control or closed-loop control. In fact, this decision is not as simple as Apple’s choice of Apple because the switch control  Laser cutting robots (sometimes referred to as “Bang Bang control”, because the actuator suddenly stops when the bang noise) is fixed output, and closed-loop servo Control means continuous linear feedback and continuous linear output control.

angle bending machine

In particular, an on-off valve without feedback may be used to provide coarse position information using discrete limit switches or photoelectric sensors, or continuous position or velocity feedback may also be used. Continuous feedback can come from analog feedback devices, encoders or magnetostrictive displacement transducers (MDTs). There are many ways that we can achieve the same functionality as using a servo or proportional valve by using only open circuits or on / off valves with the same feedback. In short, some high-end system with encoder boring machine or MDT to achieve feedback, servo valve or servo proportional valve to form a closed-loop control loop. These options are tabulated, in the form of feedback, and in the horizontal form of control (see Table 1).
Obviously, the cheapest feedback is simply not used. This feedback-free control is usually performed by the operator using their observations and reactions to operate the actuator for “loop closure.” For automation, the applicability of this approach is limited, only for the final position and the cylinder fully extended position consistent situation. The operating speed of the actuator varies with load, pressure and oil temperature. However, this approach is accurate enough for many devices.
The second way is discrete feedback control (using limit switches and photoelectric sensors). In this solution, an on-off valve or a proportional valve is used to control the hydraulic system. Although the limit switch and photoelectric sensor wire can be connected to the valve, or the use of mechanical relays, but the worst result will be the system is not flexible enough. Changing the wiring is very expensive, and the position of the limit switches and photoelectric sensors can vary depending on the manufacturing requirements. In addition, the accurate installation of limit switches and photoelectric sensors to the required location is also very difficult. Therefore, in order to provide the required flexibility angle shear, a computer or a programmable controller is usually provided between the detection device and the valve. Because of this, a lot of potential expenses must be considered, that is, the cost of programming.
The most flexible way of feedback is to provide position feedback continuously. The MDT is mounted inside or next to the cylinder to provide accurate position feedback signals. According to the continuous feedback signal, you can determine the position by determining the speed of the speed of change. This information is critical for devices that require precise and repeatable motion.
Control output selection
Open-loop control with on-off valves – the easiest to use on-off valves, but the most limited flow control. To achieve synchronous action control is very difficult. However, a multiple on-off valve can be used to achieve a rough control of the speed. In the past, it was common to have a large number of flow and small flow valves in the system. When the actuator is close to the set point, the flow rate of the fluid is reduced by closing the large flow valve to slow the actuator and only the small flow valve supplies oil. When the actuator is very close to the set point, shut down. This combination of high and low speed valves allows for relatively precise control without feedback. On request, the on-off valve can also be used with limit switches or continuous feedback devices. Back to the late 80s of last century, early 90s, large / small flow valve with the MDT feedback with the use of common in the sawing machine positioning system.
Open loop control with proportional valves – Replacing high and low speed on / off valves with proportional valves can improve accuracy and throughput. Even a simple system can use open-loop control until a “ramp region” is reached. When the actuator approaches the preset target point, the output signal decreases as a function of the set remaining distance. This is not strictly closed-loop control, because the error (the distance between the actual point and the target point) is not used as feedback. However, this is an effective control method when the load is relatively constant.

angle steel bender

Proportional and servo valves are available in open-loop or closed-loop systems.
Sometimes the open-loop system can meet the requirements. If a process is sufficiently repeatable, then the output signal required to achieve the desired speed can be determined. This is easy to implement because a relatively simple controller or a PLC with analog output can be used. The analog output voltage range of the PLC or computer is from -10 V to +10 V, and the spool can continuously change when the voltage acting on the valve High Speed Drilling Machine changes. So that flow and speed can be roughly controlled as long as some variables, such as pressure and load through the valve, remain constant.
A more accurate method is to use feedback in the form of a distance function to change the output, rather than closing the loop. Turning the control output into a distance function is usually done with a PLC or a simple motion controller. This form of control is more applicable in systems that require more stability than required for accuracy. In general, accurate motion adjustment is not required in these systems.
Closed-loop control
The use of closed-loop control is mainly due to its flexibility, accuracy, speed controllability and the ability to maintain fine adjustment under different operating conditions, such as load variations. As productivity requirements increase, more and more equipment and production processes require the use of more complex closed-loop controllers.
The most high-end control is the continuous feedback and closed-loop control. Closed-loop control compares the position feedback signal sent by the feedback device to the desired position. The error multiplies the proportional gain (the ratio of the position error unit to the output voltage or current signal) to the output. The larger the error, the greater the output signal that corrects this error. The high-side controller increases the proportional (P) gain with the integral (I), differential (D) gain and pre-feedback. The control system shown in Fig. 1 indicates that the gains are combined with each other to perform an accurate closed-loop control.
Feedforward is actually just an open-loop gain as a predictor of the combination of closed-loop control. For example, if you know that a voltage of 1V causes the actuator to run at 2in./sec., Then a voltage of 4V can achieve a speed of 8in./sec. Of course, this assumption is a prerequisite for a linear drill machine for flange hydraulic system response, but in fact not the case. However, if the feedforward gain block is used to provide an output close to the desired voltage, the PID gain can correct any unexpected changes in load nonlinearity. If there is no feedforward unit, the PID unit will need to compensate for larger errors, which will lead to system hysteresis and increased instability.
PID, feedforward, continuous feedback, and the combination of a servo valve or servo proportional valve allows the controller to move to the desired position at user-set speed or acceleration and deceleration. Controlled acceleration and deceleration reduce hydraulic and mechanical system fatigue and damage. Take the press, for example, when the tool is close to the workpiece, the speed can be reduced to provide only the kinetic energy required for the job. Figure 2 shows a high-end press hydraulic system, the position feedback provided by the MDT, differential pressure (force) feedback from the cylinder installed on the two pressure sensors. Control is achieved by a proportional servo valve.

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