Some of the most complex motion control problems arise from hydrodynamic control applications because the media involved (air or hydraulic fluid) do not respond linearly to control inputs. The choice between open-loop and closed-loop control revolves around the concept of feedback and the importance of incorporating feedback into a particular motion system.
Using feedback in closed-loop control provides smooth and precise motion, but this requires a motion controller that can utilize the feedback information. Open-loop control may not require the setup and programming of a closed-loop motion system, but it lacks the flexibility and precision offered by a closed-loop system. The decision depends on the application and its requirements; both open-loop and closed-loop control have their own advantages and disadvantages.
01 When to choose open-loop control
Many applications use open-loop control, including those where precise position or speed control of an actuator (the "operational endpoint" of a control system) is not critical. With open-loop control, there's no need to strive to match the actual speed or the pressure or force applied by a motion system to a calculated target value. A target is set, but how the system achieves it is less important.
Open-loop control is typically used when speed is critical but precision is less important, such as retracting the tool after a machining step or pre-positioning the tool before contacting the workpiece. Actuators can adjust their speed based on load variations, or, in the case of hydraulic systems, on changes in oil pressure and temperature.
Open-loop control is not entirely "feedback-free." It uses independent limit switches, photodetectors, or pressure switches to determine when motion should stop or when a pressure limit is reached. Some specialized motion controllers do not require start/stop motion control; for example, a general-purpose computer with a programmable logic controller ( PLC ) is sufficient. Using physical limit devices that must be installed in specific locations can cause problems because, if the machine is used to process materials of different sizes, the physical position of the limit devices needs to move during product changeovers.
Open-loop control can be used during the setup of a fluid dynamics system, for example, when checking the polarity of valve connections and wiring, finding positive and negative limits, checking valve linearity, and checking smooth motion.
This diagram illustrates a high-pressure hydraulic system with position feedback from a magnetostrictive displacement sensor (MDT) located within the hydraulic cylinder and differential pressure signals from pressure sensors mounted at both ends of the cylinder. Control of the hydraulic fluid flow is achieved via a proportional valve. The motion controller can be programmed to smoothly transition from open-loop to closed-loop motion when the cylinder and die contact the workpiece, and then back to open-loop motion as the press head retracts.
02 When to choose closed-loop control
Closed-loop control is essential for applications requiring contour following, synchronization, or the movement of one axis to drive the movement of another, or for applications demanding high precision, high-speed operation, flexibility, or speed. Other applications requiring closed-loop control include those that need to maintain accuracy under varying loads or environmental conditions.
Depending on the application requirements, closed-loop control of varying complexity can be achieved. Some simple analog controllers simply operate on proportional control, meaning the controller adjusts the output as a function of the difference between the actual temperature, flow rate, position, velocity, or pressure value and the target value. In a proportional-integral-derivative (PID) control loop diagram, "P" refers to proportional control.
For some motion systems, if sufficient mechanical friction provides damping to prevent oscillations, proportional control alone is sufficient. However, many hydraulic systems tend to be underdamped (moving like a block on a spring). In such cases, attempting to control an oscillating system by increasing the proportional gain may actually worsen the oscillations.
Since a control system relying solely on P-gain requires an error to move the system at a specific speed, the system's response to new inputs will lag if there is a demand for varying speeds. For tighter closed-loop control, other gain methods will play a specific role.
To enable a motion axis to move quickly and reliably to a target position, the use of proportional gain is often necessary. Even a small error between the actual and target states can cause an actuator using only a proportional control system to move to the target setpoint.
Mechanical realities of the system, such as variations in the zero-point characteristics of a hydraulic valve or friction between moving parts (static or dynamic), can prevent the system from hitting the target. The integrator portion of the control equation accumulates errors over time, eventually increasing the necessary output to move the actuator.
The role of differential gain is to provide electronic damping as proportional gain increases, helping to prevent actuator oscillations. The effectiveness of differential gain depends on several key factors, such as the resolution of the feedback device's output value and whether the known sampling time is strictly adhered to. Since differential gain is a factor that multiplies the speed error, accurate confirmation of the motion axis's speed is crucial.
03 Feedforward in Closed-Loop Control
The effectiveness of a closed-loop control system depends on its response to the error between the actual system measurement and the target value. However, a constraint for using PID-based control is that there must be at least some error; otherwise, there will be no motion. In many applications, this is not a problem, but having an estimate of the required output before the error occurs can still enhance the accuracy and smoothness of motion tracking. This is where feedforward gain comes into play.
Unlike the PID gain used for feedback error, the feedforward gain is multiplied by the target velocity and acceleration, and then summed to obtain its contribution to the output.
Feedforward is simply the open-loop gain used as a predictor. They are particularly useful in hydraulic systems, partly due to the properties of the fluid and the physical differences in how the hydraulic fluid operates on the rod side of a cylinder versus the open side of the piston. Different gains are typically required to obtain the desired piston speed and acceleration in each direction of motion.
In theory, if the predicted gain is calculated correctly, there should be no error when the system moves. In the real world, systems don't work perfectly. Always keep system stability in mind; the goal is to use predictive methods to make the system operate within 90% to 95% of the desired motion profile. Therefore, the error correction capability of the PID algorithm only needs to handle 5% to 10%.
While ensuring precise system operation, using a programmable motion controller offers the added advantage of allowing control parameters to be changed quickly and easily to meet ever-changing production needs. New setpoints can be downloaded from a monitoring PLC or computer via Ethernet to enable new part types, and there is no limit to how often parameters can be changed.
Using electronic motion controllers for more precise control of motion profiles offers additional benefits, including smoother motion, reduced machine shock and vibration, lower maintenance costs, and extended machine lifespan. These benefits also improve the quality and consistency of manufactured products.
In hydraulic applications, when controlling pressure or force, it's important to note that if the force or load in the opposite direction of motion suddenly disappears, the actuator may abruptly start moving. To address this, using open-loop control with pressure or force limiting capabilities is a good solution, minimizing the impact of unwanted control responses.
04 Combination of open-loop and closed-loop control
On the same machine, open-loop control is often combined with closed-loop control, using one control method at different parts of the machine's work cycle to achieve the best advantages. For example, open-loop motion can be used in the retraction direction to quickly open a punch press, so that the processed parts can be released.
Adjusting the machine's operation can be completely simplified because only the extension direction (the part of the work cycle that completes the stamping operation) needs to be adjusted for precise operation; the open-loop portion of the work cycle does not need to be adjusted.
Ultimately, the choice between closed-loop and open-loop control for fluid dynamics applications depends on the specific application requirements. By using a programmable motion controller that can switch between different modes, system integrators can gain the advantages of both approaches.
However, it should be noted that even the best motion controller cannot compensate for poor overall system design or poor selection of components elsewhere in the system, such as motors, valves, or sensors required for closed-loop control loops.
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