Drives & Controls Magazine May 2023

38 n MOTORS May 2023 How and when to use closedloop control for DC motors For many applications, DC motors are driven in open loop. This means there’s no feedback from the motor’s output stage to its input. The speed of an open-loop motor is controlled by regulating the voltage. However, when a load is applied, such as the resistance of a drill bit against a dense material, the motor speed will decrease. The magnitude of this decrease depends on the motor’s speed-torque characteristic. A motor’s speed increases or decreases in a linear relationship with the input voltage. Depending on the voltage, the extremes of the motor’s limits are defined by its no-load speed (the maximum speed) and the stall torque, where the load prevents the motor from rotating. Between these limits, rotational speed decreases with the load, based on the motor’s speed-torque characteristic. Applications such as domestic drills or screwdrivers can usually tolerate a decrease in speed as the load fluctuates or increases. Using a more powerful motor or adding a gearbox can maintain speed and torque as the load increases. However, this will add to the size, mass and cost of the application. It also will not resolve the issue of speed fluctuations, which will still occur. Some professional applications – such as surgical power tools or peristatic pumps – demand accurate and stable speed control. To regulate speed in a DC motor continuously at a desired point, a controller with closed-loop speed control is needed. Closed-loop control When feedback from the motor’s output, including its speed, is returned to the input stage, this effectively “closes the loop”, allowing the output to be adjusted as required. To achieve this, the controller measures the speed of the motor continuously. Feedback is provided by devices such as Hall sensors, encoders, or any process that monitors the motor’s electromotive force (EMF). The controller compares the actual speed output to the input reference and adjusts the voltage or current to maintain a constant speed, depending on the input command. Whether open- or closed-loop control is used, the input voltage and current are limited as a result of the design of the motor and the wider system, or by the power supply. As a result, the motor’s speed and torque are also limited. Surgical tools Surgical hand tools can work either in open- or closed-loop. The choice depends on the application. With closed-loop control, the user presses a button to define the requested speed. When drilling or cutting commences, a load is applied, but the output speed of the tool remains stable and consistent, even if the torque load increases. This means that a surgeon doesn’t have to press harder on the button to maintain speed, reducing stress. Most significantly, this closed-loop approach can provide more control if a stable speed is required throughout. Alternatively, with surgical tools that control speed by regulating voltage, this is determined by how hard the surgeon squeezes the trigger. When a load is applied, the speed drops, and the surgeon controls the process by the pressure they exert. This might be preferable for applications where the surgeon wants more flexible control over a process, and a constant speed is not vital. Though DC motors are typically driven in open-loop, introducing a closed-loop can increase in speed control precision for critical applications. Because speed can affect, or depend on, wider attributes such as voltage or torque, talking to motion specialists early in the design phase can help to optimise the outcome of a project. n For some for miniature DC motor applications, such as surgical tools, closed-loop control can provide stable speed regulation under fluctuating loads. Knowing whether to choose open- or closed-loop control, and how to control speed, are vital design considerations. Samuel Klein, an application engineer with Portescap, explains how to achieve closed-loop DC motor control. The speed and torque of a DC motor vary with voltage Open-loop voltage control of a DC motor Closed-loop speed control of a DC motor