Technical Knowledge

Comparison of open-loop and closed-loop stepper motor systems
Release Date : 2020-06-17 17:03:19


Stepper motor systems are the foundation of the motion control industry.
 
Stepper motor systems have come a long way since the early days of voltage drive and full stepping. The first is the PWM driver and microstepping, then the digital signal processor (DSP) and anti-resonance algorithm. Now, the new closed-loop stepping technology ensures that the stepper motor will continue to be the cornerstone of the motion control industry in the next few years.
 



The above is a cross-sectional view of NiMotion's STM42 series of closed-loop stepper motors.
 
Whether it is linear or rotary motion, torque and efficiency are the two most important factors in determining the motor and drive system. Whether the application equipment is an automatic assembly system, a material handling machine, a 3D printer, a peristaltic pump, or other application equipment, a closed-loop stepper motor is the first choice and is suitable for the above application scenarios.
 
The advantages of closed-loop stepper motors are low cost, high-performance feedback system and advanced DSP to close the loop of the step motion control system. Such control can improve the performance of the closed-loop stepper motor driver, making it superior to open-loop systems.
As we have seen above, such a closed-loop system is implemented in an integrated motor design that includes a feedback device (encoder), driver and controller board, power supply, communication and I/O electronics, and the side of the motor and System connector on the back.
 
First, we discuss the comparison between high-performance closed-loop stepper motor systems and traditional open-loop stepper motor systems in terms of torque and efficiency.
 
The experimental test compares the acceleration (torque), efficiency (power consumption), position error (accuracy), heating value and noise level of the two systems. The results show that the performance of the closed-loop stepper motor system is better than the open-loop setting. Only the relationship between torque and acceleration is considered. The torque speed curve shows the peak and continuous torque range of the closed loop stepping system and the available torque range of the open loop stepping system. Usually, the actual torque will be converted into acceleration, therefore, the motor with more torque can accelerate the rated load faster.
 

1. Torque experiment comparison between open-loop and closed-loop stepper motor systems

 
In order to test this difference in torque performance in the experiment, open-loop and closed-loop stepper motor systems of the same size will get the same inertial load. The programming commands the two systems to execute the same motion curve, but the acceleration and maximum speed in each system will increase slowly until a positioning error occurs.

 



Assume that the maximum acceleration of the open-loop system is 1,000 r/s2 and the maximum speed is 10 r/s (600 rpm). The maximum speed of 10 rev/sec is related to the position where the flat part of the torque-speed curve ends. The closed-loop system (due to its higher torque-generating capacity) achieves a maximum acceleration rate of 2,000r/s2 and a maximum speed of 20r/s (1,200 rpm). This is twice the performance of an open-loop system and reduces the movement time by nearly half, from 110ms to 60ms.
 
For applications that require high throughput (such as indexing, edge guide positioning, and pick and place systems), closed-loop stepper motor systems have clear performance advantages.
 

2. The efficiency of open-loop and closed-loop stepper motor systems

 
To measure the relative efficiency of open-loop and closed-loop systems, assume that we repeat the same test for two identical motors of the same size. This time, we make the closed-loop and open-loop motors run side by side under the same inertial load, but run the programming to keep the motion curve constant and equal, so that the two systems perform the same workload.
 
 

When the two motors repeatedly index the same motion curve, the current drawn from the DC power supply that supplies the two systems will be measured and the power consumption will be calculated. It can be seen from the figure that the average power consumption of the open-loop stepping system is 43.8 watts, while the average power consumption of the closed-loop stepping system is only one-third of it-an average of 14.2 watts. The huge difference in power consumption indicates the higher efficiency of the closed-loop system. Now, users who want to improve the system efficiency of the open-loop stepping system can consider upgrading it to a closed-loop system, which can significantly reduce energy consumption.
 

3. Heating of open-loop and closed-loop stepper motor systems

 
The natural extension of power consumption testing is the study of motor heating. The open loop stepping system is a simple beast. Just set the drive to the rated current of the motor, regardless of whether the torque needs to be generated, the drive will provide the current to the motor as long as it can. This usually results in heat generation rather than energy toward the application function-which is why open-loop stepper systems usually operate at higher temperatures than closed-loop stepper systems. This also means that the machine designer must take additional measures to deal with this heat, usually by installing special protective devices near the stepper motor to protect the operator, the stepper motor will run near the human operator, or install Other cooling systems (such as fans).
 
Consider the results of motor heating tests conducted in the laboratory using the same open-loop and closed-loop systems as above. In this test, the two systems generate the same work again when driving the same inertial load, and allow them to run until they reach thermal equilibrium. The open-loop system reaches a case temperature of 76.0°C, while the closed-loop system reaches thermal equilibrium at a case temperature of only 36.9°C-less than half of the open-loop system. The significant reduction in motor heating may mean that machine manufacturers have reduced parts costs because they can omit additional protection and cooling subsystems.
 
 

4. Noise of open-loop and closed-loop stepper motor systems

 
Another common problem with open loop stepping systems is noise. As we all know, they make a lot of audible noise. In some environments, such as laboratories, hospitals and offices, this noise may cause interference problems for machine designers.
 
The noise emitted by the stepper motor is due to the high electrical frequency and rapid magnetic flux changes in the stator teeth, and is not affected by the load because the open-loop system operates at full load current. On the other hand, the closed-loop stepping system only provides enough current to the motor to control the load, thereby reducing audible noise.
 


In order to produce the test results shown in the noise figure attached to this article, the noise of each system was measured in a soundproof room. Compared to speeds from 0 to 20 rpm, the closed-loop system is much quieter than the open-loop option. This speed range coincides with the actual application speed range of the most commonly used stepper motor systems, which means that if you switch to a closed-loop system, most stepper motor applications can benefit from reduced motor noise.
 

5. Closed-loop stepper motor has higher precision, which can eliminate position error

 
The open-loop stepper motor system is highly praised for its ability to accurately locate the load without a feedback system, but the premise is that the open-loop system has sufficient torque so that no position error occurs during normal operation.
 


In order to improve accuracy and achieve a more powerful system design, closing the servo position loop around the feedback from the encoder allows the closed-loop system to automatically compensate for the increase in torque demand, otherwise the increase in torque demand will cause position errors in the open-loop system. This greatly improves the accuracy of the overall system, especially for high-precision dynamic applications (such as pick-and-place systems and 3D printers), which require rapid movement and frequent changes in direction.
 

6. upgrade the existing stepping system

 
From open-loop to closed-loop stepper motor systems, integrating components in stepper motor systems, motors, power amplifiers, and communications costs usually do not increase. The control equipment may require stronger integrated processing power or memory to servo control the motor, but these usually do not affect the price. Most of the cost difference between open-loop and closed-loop stepping systems is the addition of high-performance feedback devices, but as technology improves, the price of these devices becomes lower and lower. Therefore, now, the closed-loop stepper motor system maintains the cost advantage of the open-loop stepper system over other types of positioning systems (such as traditional servo systems), but it has greatly improved performance in almost all aspects. Generally, the energy saving and increased number of closed-loop systems quickly compensate for the small increase in the cost of feedback equipment.
 
In addition to the minimum cost increase, the greater torque that the closed-loop system can provide means that the shaft diameter of the closed-loop stepper motor may be larger, but usually this problem can be easily solved by simply replacing the coupling.
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