Fluid power applications often present some of the most intricate motion control challenges due to the non-linear responses of the media involved, such as air or hydraulic oil. The decision to utilize open-loop versus closed-loop control hinges on the role of feedback and its significance for the specific motion system in question.
In closed-loop control, feedback can facilitate smooth and accurate motion, but it necessitates a motion controller that can effectively utilize this feedback. While open-loop systems may avoid the complexities of configuring a closed-loop system, they typically lack the precision and adaptability that closed-loop systems offer. The choice ultimately depends on the specific application and its requirements, as both control types come with their own advantages and disadvantages (see Table).
When to Choose Open-Loop Control
Automotive Lift Repair Orlando Open-loop control is suitable for many applications, especially where precise positioning or velocity control of the actuator (the motion system’s “business end”) is not critical. In these systems, there’s no attempt to match the actual velocity or pressure/force exerted by the motion system to predetermined target values. While there’s an overall goal, the specific method of achieving it is less significant. Open-loop control is often employed when speed matters more than precision, such as retracting a tool after machining or pre-positioning a tool before it contacts a workpiece. The actuator may operate at varying speeds due to changes in load or, in hydraulic systems, fluctuations in oil pressure or temperature.
That said, Automotive Lift Repair Orlando open-loop controls are not entirely devoid of feedback. They can incorporate discrete limit switches, photo eyes, or pressure switches to determine when motion should stop or when a pressure threshold is reached. Operating on/off motion controls typically doesn’t require a specialized motion controller; a general-purpose computer like a programmable logic controller (PLC) suffices. However, using physical limit devices positioned at specific locations can create challenges when processing materials of varying dimensions, as these devices need to be relocated during production changeovers. Regardless of whether open-loop control is used during operation, it should be employed during the setup of a fluid power system to check valve plumbing, wiring polarity, limit positions, valve linearity, and ensure smooth motion.
When to Use Automotive Lift Repair Orlando Closed-Loop Control
Applications that necessitate precise profile following, synchronization, or the gearing of one axis to another, or those demanding a high degree of operational flexibility and accuracy should implement closed-loop control. This includes scenarios that require maintaining precision amidst changing loads or environmental conditions.
Automotive Lift Repair Orlando Closed-loop control can vary in complexity based on application needs. Some simple analog controllers utilize just proportional control, where the output of the controller adjusts based on the difference between actual and target values for temperature, flow, position, velocity, or pressure. In this context, “P” in a proportional-integral-derivative (PID) control-loop diagram (see Figure 1) refers to proportional control.
Proportional-only control may suffice in certain motion systems if there’s sufficient mechanical friction for damping, which helps prevent oscillations. However, many hydraulic systems can be under-damped (like a mass on a spring), meaning that increasing proportional gain to control an oscillating system might exacerbate the issue.
Since a control system dependent solely on P-gain requires an error to prompt movement at a specific velocity, any need to change velocities can result in a delayed system response. For tighter closed-loop control, additional gain terms are necessary, each serving a distinct purpose.
An integral gain is often essential for enabling a motion axis to reach a target position quickly and reliably. Even a minor discrepancy between actual and target states can lead a proportional-control-only system to struggle in moving the actuator to the setpoint.
Mechanical realities, such as variations in Automotive Lift Repair Orlando hydraulic valve characteristics or friction among moving components (both static and dynamic), can hinder the system from achieving its target. The integrator component of the control equation accumulates error over time, resulting in an increased output that prompts actuator movement.
The derivative gain offers electronic damping to prevent oscillations as proportional gain is increased. Its effectiveness relies on key factors, including the resolution of feedback device output values and strict adherence to established sampling times. Given that derivative gain is a multiplicative factor applied to the velocity error, precise velocity determination of a motion axis is critical.
Feed Forwards in Automotive Lift Repair Orlando Closed-Loop Control
The efficiency of a closed-loop control system depends on its response to errors between actual and target measurements. However, a limitation of PID-based controls is that motion cannot occur without some error present. While this may not be problematic for many applications, precise and smooth motion tracking can be enhanced by estimating the necessary output before an error arises. This is where feed forward gains become valuable.
Feed forward gains (illustrated as velocity feed forward, Fv, and acceleration feed forward, Fa, in Figure 1) are multiplied by the target velocity and acceleration and combined to contribute to the output.
Feed forwards serve as open-loop gains used as predictive factors, particularly beneficial in Automotive Lift Repair Orlando hydraulic systems. This is partly due to fluid characteristics and the differences in how hydraulic fluid interacts with the rod and open sides of a hydraulic cylinder’s piston. Separate gains are generally required to achieve the desired piston velocity and acceleration for each motion direction.
Theoretically, if predictive gains are accurately calculated, there should be no error during system movement. However, real-world systems often deviate from perfection. With stability as a priority, the objective is to utilize predictive terms to drive the system within approximately 90 to 95% of the desired motion profile, allowing the PID algorithm to correct only the final 5 to 10%.
Beyond ensuring precise system operation, a programmable motion controller provides the added advantage of allowing for quick and easy changes to control parameters to adapt to evolving production needs. New setpoints can be downloaded via Ethernet from a supervisory PLC or computer to accommodate new part types, with no limit on the frequency of parameter updates.
More precise motion profile control using an electronic motion controller enhances smoothness, reduces shock and vibration in the machine, lowers maintenance costs, and prolongs the machine’s lifespan—all while improving the quality and consistency of production output.