Stepper motor systems are the cornerstone of the motion control industry. We will examine the differences between open-loop and closed-loop systems and learn about the latest developments in stepper motors, which are faster, quieter, and more energy-efficient than ever before.
Stepper motor systems have come a long way, starting with voltage-driven and fully stepping early stages. First came PWM drives and microstepping, then digital signal processors (DSPs) and anti-resonance algorithms. Now, new closed-loop stepping technology ensures that stepper motors will continue to be a cornerstone of the motion control industry for years to come.
This is a cross-sectional view of the StepSERVO closed-loop integrated stepper motor from AppliedMotionProducts.
Whether the motion is linear or rotary, the two primary considerations in determining which motor and drive system is most suitable are torque and efficiency. This applies to end applications such as automated assembly systems, material handling machines, 3D printers, Cartesian positioners, peristaltic pumps, and countless others, where stepper motors are the preferred technology.
The latest development in stepper systems involves using low-cost, high-resolution feedback devices and advanced DSPs to create a closed-loop stepper motion. This control improves closed-loop stepper performance, making it superior to open-loop systems. As we have seen, such a closed-loop system is implemented in an integrated motor design, including the feedback device, driver and controller boards, power supply, communication and I/O electronics, and system connectors on the side and back of the motor.
Comparison of open-loop and closed-loop stepper systems
Let's first discuss the comparison between high-performance closed-loop stepper systems and traditional open-loop stepper systems in terms of torque and efficiency.
Closed-loop stepper systems outperform open-loop systems, as demonstrated by laboratory test results comparing acceleration (torque), efficiency (power consumption), positional error (accuracy), heat generation, and noise levels. Only the relationship between torque and acceleration is considered. The torque-speed curves show that the peak and continuous torque range of the closed-loop stepper system is significantly better than the available torque range of the open-loop stepper system. Typically, torque in the real world translates into acceleration—therefore, a motor with greater torque can accelerate a given load more quickly.
To test this difference in torque performance in the laboratory, open-loop and closed-loop stepper motor systems of the same size were subjected to the same inertial load. Both systems were programmed to execute the same movement profile, except that the acceleration rate and maximum speed were increased slowly in each system until they produced positioning errors.
Here we have a motion profile comparison between open-loop and closed-loop systems. This is a comparison between the StepSERVO closed-loop system and the open-loop system. The closed-loop system (due to its higher torque generation capability) achieves a maximum acceleration of 2,000 rpm² and a top speed of 20 rpm (1,200 rpm), as shown here.
Assume the open-loop system has a maximum acceleration of 1,000 rpm² and a top speed of 10 rpm (600 rpm). This maximum speed of 10 rpm corresponds to the end of the flat portion of the torque-speed curve. The closed-loop system (due to its higher torque generation capability) achieves a maximum acceleration of 2,000 rpm² and a top speed of 20 rpm (1,200 rpm). This is twice the performance of the open-loop system and reduces the travel time by nearly half—from 110 milliseconds to 60 milliseconds.
For applications requiring high responsiveness (such as indexing, edge-guided positioning, and pick-and-place systems), closed-loop systems offer significant performance advantages.
Comparison of open-loop and closed-loop efficiency
To measure the relative efficiency of open-loop and closed-loop systems, we assume we repeat the same test using two identical motors of the same size. This time, we run the closed-loop and open-loop motors side-by-side with the same inertial load, but program them to keep the motion curves constant and equal, so that both systems can perform the same workload.
This is a relative comparison chart of the StepSERVO closed-loop system and the open-loop system. Note the superior performance of the closed-loop stepper system, as shown in the laboratory test results. Compare the acceleration (torque), efficiency (power consumption), position error (accuracy), heat generation, and noise levels of the two systems.
When both motors repeatedly point to the same movement curve, the current consumption from the DC power supply to both systems is measured, and the power consumption is calculated. The value graph shows that the average power consumption of the open-loop stepper system is 43.8 watts, while the average power consumption of the closed-loop system is only one-third—14.2 watts. This significant difference in power consumption clearly demonstrates the higher efficiency of the closed-loop system. Any user wishing to improve the efficiency of their open-loop stepper system can now consider a simple upgrade to a closed-loop system and expect a significant reduction in power consumption.
The power consumption comparison between this StepSERVO closed-loop system and the open-loop system demonstrates the higher efficiency of StepSERVO.
How to solve the problem of motor overheating
A natural extension of power consumption testing is the study of motor heat generation. Open-loop stepper systems are simple beasts. Simply set the driver to the motor's rated current, and the driver will do its best to supply current to the motor, regardless of whether torque is needed. This typically results in heat being generated rather than energy being produced for the application's functionality—which is why open-loop stepper systems generally operate hotter than their closed-loop counterparts. This also means that machine designers must take extra measures to deal with this heat, often including special protection around the stepper motor running near a human operator, or installing additional cooling systems such as fans.
Consider the results of a motor heating test conducted in a laboratory using the same open-loop and closed-loop systems as described above. In this test, both systems again generated the same workload to drive the same inertial load and were allowed to run until they reached thermal equilibrium. The open-loop system reached an enclosure temperature of 76.0°C, while the closed-loop system reached thermal equilibrium at only 36.9°C—less than half the temperature of the open-loop system. This significant reduction in motor heating translates to lower component costs for machine manufacturers, as they can omit the additional protection and cooling subsystems.
This is a comparison of laboratory test temperatures for the StepSERVO closed-loop and open-loop systems. It shows the results of motor heating tests performed using the same open-loop and closed-loop systems as the other tests detailed in this article.
Noise test comparison
Another common complaint about open-loop stepper systems is that they generate a considerable amount of audible noise. In certain environments, such as laboratories, hospitals, and offices, this noise can pose a real problem for machine designers.
The noise emitted by a stepper motor originates from the high electrical frequency and rapid flux changes in the stator teeth, and because the open-loop system operates at full rated current regardless of the load. On the other hand, a closed-loop stepper system provides the motor with sufficient current to control the load, thus producing less audible noise.
The noise level shown here corresponds to the speeds of the open-loop and StepSERVO closed-loop systems. The closed-loop stepper system provides sufficient current to the motor to control the load, resulting in significantly lower audible noise compared to the open-loop setup.
To produce the test results shown in the acoustic noise graphs accompanying this article, the acoustic noise of each system was measured in a soundproof chamber. The closed-loop system was significantly quieter than the open-loop option, with speeds ranging from 0 to 20 rpm. This speed range corresponds to the actual speed range of applications most commonly using stepper motor systems, meaning that the vast majority of stepper motor applications would benefit from reduced motor noise if switched to a closed-loop system.
Better accuracy can eliminate positional errors
Open-loop stepper motor systems are valued for their ability to accurately position loads without feedback mechanisms or closed-loop control systems, provided that the open-loop system has sufficient torque margin to prevent positional errors during normal operation. To improve accuracy and for a more robust system design, disabling the feedback from the high-resolution encoder in the servo position loop allows the closed-loop system to automatically compensate for increases in torque demand that would otherwise lead to positional errors in the open-loop system. This significantly improves overall system accuracy, particularly for highly dynamic applications such as pick-and-place systems and 3D printers requiring short, rapid movements and frequent changes of orientation.
A dynamic torque comparison between StepSERVO closed-loop (peak torque), StepSERVO closed-loop (continuous torque), and open-loop (available torque). These graphs show the peak and continuous torque ranges of the closed-loop stepper system and the available torque range of the open-loop stepper system. The maximum acceleration output of the open-loop system is 1,000 rpm², and the maximum speed is 10 rpm (600 rpm). This maximum speed of 10 rpm corresponds to the position where the flat portion of the torque-speed curve ends.
Upgrade the existing stepper system
In integrated stepper motor systems, the cost of the motor, power amplifier, and communication typically doesn't increase when transitioning from open-loop to closed-loop. Control electronics may require more central processing power or memory to servo-control the motor, but these usually don't impact the list price. The cost difference between open-loop and closed-loop stepper systems primarily lies in the addition of high-resolution feedback devices, but manufacturing improvements have made these devices increasingly cheaper. Therefore, closed-loop stepper systems now retain the cost advantage of open-loop stepper systems relative to other types of positioning systems (such as traditional servo systems) but offer significantly improved performance in almost every aspect.
Aside from minimal cost increase, NEMA frame-size products simplify the upgrade from open-loop to closed-loop stepper systems. Closed-loop NEMA23 stepper motors have the same frame dimensions, guide diameter, bolt hole circle, and bolt hole diameter as open-loop NEMA23 stepper motors, thus the mounting bracket remains unchanged. The greater torque achievable in a closed-loop system means that the shaft diameter of the closed-loop stepper motor may be larger, but this can usually be easily addressed by simply changing the coupling.
