FEATURE DRIVES, CONTROLS & MOTORS
as much as 10dB. This proves advantageous when step motors run near operators or in environments where audible noise is problematic.
Closing the
loop in demanding applications
There are many benefits to specifying closed-loop step motors in high-throughput industrial
applications, as Eric Rice, marketing director at Applied Motion Products, explains
F
or digital motion control in applications where positioning requirements are
predictable and repetitive, step motors offer a low-cost, simple and precise solution. But when used in more demanding industrial applications, such as those that require high throughput, these may fail due to problems inherent with open-loop control. Without feedback to close position and velocity control loops, the step motor’s actual position and speed are not monitored against commanded values, so processes cannot automatically detect and react to positioning errors. In situations where high accuracy and
consistent performance are imperative to the ongoing success of a process, closed-loop step motors provide such benefits as higher torque, lower power consumption, and quieter operation. Closed-loop (servo) control of the step motor’s position, velocity and torque results in higher acceleration rates and faster machine cycles when addressing the requirements of highly dynamic applications. Step motors are brushless DC
motors with a high number of poles, as visible in the familiar mechanical ‘teeth’ of the stator and rotor. The design allows it to position in precise angular movements (1.8 and 0.9˚ are common) simply by switching the DC winding currents in a pre-determined sequence. This is traditionally done open-loop, without any feedback signal from the motor shaft to the drive, making step motors a very cost-effective solution for many simple positioning and velocity- control applications. Being simple to install and low cost,
open-loop step motors are often used for positioning equipment such as
20 MARCH 2020 | DESIGN SOLUTIONS
linear actuators and rotary stages in automation applications. However, high- throughput applications that require rapid starting, stopping and changes in direction can pose challenges with torque and accurate positioning. Without a feedback device and closed- loop control software in the drive, the motion of an open-loop step motor is not automatically controlled to ensure accurate positioning in every movement. Adding a high-resolution encoder to the
step motor and a servo-control algorithm to the drive create a closed-loop step motor system. With these changes to the system, step motors operate faster, more accurately, cooler and quieter than open-loop step motors. As many of today’s applications such as pick-and- place systems and industrial 3D printers require fast and accurate positioning, closed-loop step motors offer several benefits to machine builders.
LESS HEAT AND NOISE Open-loop step motors are continuously driven by full current, regardless of the required torque. This results in heat generation and noise as the open-loop system continues to work at full-rated current regardless of load conditions. Closed-loop step motor systems
consume up to one third the power of open-loop systems when performing the same tasks because the step motor only receives enough current to deliver the required torque. The closed-loop system automatically reduces the current when less torque is demanded by a load. So, if the load gets lighter, the motor draws less current and runs cooler. As a result, the closed-loop step motor operates cooler and reduces noise by
/ DESIGNSOLUTIONS
LESS POWER CONSUMPTION Operating at the minimal amount of current to maintain commanded position and velocity, a closed-loop step motor consumes less power than its open-loop counterpart, contributing to it being more efficient to operate. In applications where multiple axes of step motors are used, this can have a significant impact on the total energy requirements for the machine or process.
ACCURATE POSITIONING A torque demand greater than the capabilities of an open-loop motor will result in missed steps and even stalling, which can go undetected because of the lack of feedback. If the open-loop motor loses steps, it does not automatically correct its position. In a closed-loop step motor system,
encoder feedback constantly informs the drive of the position and velocity of the motor. Based on this feedback the closed-loop system automatically corrects motion profiles on-the-fly and enables the step motor to reach position correctly every time. These improvements in positioning make a more accurate system overall, which is important for applications such as pick-and-place, labelling and scanning that require precision motion and high throughput.
HIGHER TORQUE A closed-loop step motor can generate peak torques up to 50% higher than an open-loop step motor of the same size. Greater torque enables the step motor to operate at faster acceleration rates, even while running at cooler temperatures. Faster acceleration rates lead to shorter overall move times and an increased number of cycles per minute.
Applied Motion Products
www.applied-motion.com
Left: Exploded view of a closed-loop step motor. These can generate peak torques up to 50% higher than an open-loop step motor of the same size. Greater torque enables the step motor to operate at faster acceleration rates, even while running at cooler temperatures
Closed-loop step motor systems consume up to one third the power of open-loop systems when performing the same tasks because the step motor only receives enough current to deliver the required torque
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