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Understanding and Preventing Stepper Motor Miss-Steps and Over-Runs

Home > Understanding and Preventing Stepper Motor Miss-Steps and Over-Runs

Understanding and Preventing Stepper Motor Miss-Steps and Over-Runs

2024-01-26 15:37:49

Stepper motors are open-loop controlled motors that convert electrical pulse signals into angular or linear displacements. They are the main components of modern digital program control systems and have a wide range of applications. Under non-overload conditions, the motor's speed and stopping position only depend on the frequency and number of pulse signals, and are not affected by load changes. When a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle, called the "step angle," in the set direction. By controlling the number of pulses, the angular displacement can be controlled, and precise positioning can be achieved. By controlling the pulse frequency, the motor's speed and acceleration can be controlled, achieving the purpose of speed regulation.

Stepper motors are a type of feedback motor, and their working principle is to use electronic circuits to convert DC into intermittent power supply, multi-phase sequence control current, which is used to power the stepper motor. The driver is a multi-phase sequence controller that powers the stepper motor intermittently. Although stepper motors have been widely used, they cannot be used like ordinary DC or AC motors under normal conditions. They require a control system composed of dual loop pulse signals, power drive circuits, etc. Therefore, using stepper motors properly is not an easy task, involving mechanical, motor, electronic, and computer knowledge. As an execution component, stepper motors are one of the key products of mechanization, widely used in various automated control systems. With the development of microelectronics and computer technology, the demand for stepper motors is increasing day by day, and they have applications in all areas of the national economy.

Classification of Stepper Motors

Stepper motors can be classified into the following types based on their structure: variable reluctance stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HS), single-phase stepper motors, and planar stepper motors.

The running performance and control methods of stepper motors are closely related. The control systems of stepper motors can be divided into three categories based on their control methods: open-loop control systems, closed-loop control systems, and semi-closed-loop control systems. Semi-closed-loop control systems are generally classified as open-loop or closed-loop systems in practical applications.

Variable reluctance: The stator has windings, and the rotor is made of soft magnetic material. The structure is simple, the cost is low, the step angle is small, up to 1.2°, but the dynamic performance is poor, the efficiency is low, and the heat generation is large, making it difficult to ensure reliability.

Permanent magnet: The rotor of a permanent magnet stepper motor is made of permanent magnetic material, and the number of poles on the rotor is the same as the number of poles on the stator. Its characteristics are good dynamic performance, large output torque, but low precision, and large step torque angle (generally 7.5° or 15°).

Hybrid: The hybrid stepper motor combines the advantages of variable reluctance and permanent magnet stepper motors. The stator has multiple windings, the rotor is made of permanent magnetic material, and both the stator and rotor have multiple small teeth to increase the precision of the step torque. Its characteristics are large output torque, good dynamic performance, small step angle, but complex structure, and relatively high cost.

According to the windings on the stator, there are two-phase, three-phase, and five-phase series. The most popular is the two-phase hybrid stepper motor, accounting for more than 97% of the market share. The reason is its high cost-performance ratio, and it has good effects when paired with a micro-division driver. The basic step angle of this type of motor is 1.8° per step, and when paired with a half-step driver, the step angle is reduced to 0.9°. When paired with a micro-division driver, its step angle can be divided up to 256 times (0.007° per micro-step). Due to friction and manufacturing accuracy, the actual control accuracy is slightly lower. The same stepper motor can be paired with different micro-division drivers to change the precision and effect.

Selection Method for Stepper Motors and Drivers

  1. Determine the required torque: Static torque is one of the main parameters for selecting a stepper motor. A large torque motor is required for large loads. The larger the torque index, the larger the motor's external size.
  2. Determine the motor's running speed: For high speed requirements, choose a motor with a larger phase current and smaller inductance to increase power input. Also, choose a higher power supply voltage when selecting the driver.
  3. Choose the motor's installation specification: Such as 57, 86, 110, etc., mainly related to torque requirements.
  4. Determine the required precision and vibration: Determine whether micro-division is required and how many divisions are needed.
  5. Choose the driver based on the motor's current, micro-division, and power supply voltage.

Working Principle

The rotor of a motor is usually a permanent magnet, and when current flows through the windings on the stator, a vector magnetic field is generated. This magnetic field will rotate the rotor by a certain angle, causing the magnetic field direction of one pair of poles on the rotor to coincide with the magnetic field direction of the stator. When the stator's vector magnetic field rotates by a certain angle, the rotor will also rotate by that angle. Each input pulse drives the motor to rotate by a certain angle, and its angular displacement is proportional to the number of input pulses. Its speed is proportional to the pulse frequency, and the direction of rotation can be controlled by changing the order in which the windings on each phase are powered.

Heat Generation Principle

In general, various motors have iron cores and windings. The windings have resistance, and when current flows through them, there will be losses, which are proportional to the resistance and the square of the current. This is what we often call copper loss. If the current is not standard DC or sine wave, there will also be harmonic losses. Iron cores have magnetic hysteresis and eddy current effects, and when they are in an alternating magnetic field, losses will also occur. The size of the loss depends on the material, current, frequency, and voltage. These losses will be expressed in the form of heat, affecting the motor's efficiency. Stepper motors generally seek high precision and output torque, and their efficiency is relatively low. The current is generally large, and the harmonic component is high. The frequency of current changes with the speed, making the overall heat generation situation of stepper motors more serious than that of ordinary AC motors.

Main Structures

A stepper motor, also called a stepper, uses electromagnetic principles to convert electrical energy into mechanical energy. People have been using this type of motor since the 1920s. With the increasing popularity of embedded systems (such as printers, hard drives, toys, windshield wipers, pagers, robotic arms, and video recorders), the use of stepper motors has also increased. Stepper motors come in various shapes and sizes, but regardless of their shape and size, they can be classified into two categories: variable reluctance stepper motors and permanent magnet stepper motors.

A stepper motor is driven by a group of coils wound on the teeth of the stator, which is the stationary part of the motor. In most cases, a coil wound into a circular shape is called a solenoid, while in a motor, the coil wound on the teeth is called a winding, coil, or phase.

Stepper Motor Speed Control Technology

Due to the widespread use of stepper motors, research on their control has become increasingly common. During startup or acceleration, if the step pulse changes too quickly, the rotor cannot follow the changes in the electrical signals due to inertia, resulting in stalling or missed steps during startup, and over-running during stopping. To prevent stalling, missed steps, and over-running, and to increase the working frequency, stepper motors need to be controlled during acceleration and deceleration.

The speed of a stepper motor depends on the pulse frequency, the number of teeth on the rotor, and the number of steps per revolution. Its angular velocity is proportional to the pulse frequency and synchronized in time. Therefore, under the condition that the number of teeth on the rotor and the number of steps per revolution are constant, the required speed can be obtained by controlling the pulse frequency. Due to the synchronous torque of the stepper motor, the starting frequency is not high. Especially as the power increases and the diameter of the rotor increases, the starting frequency and the maximum running frequency may differ by as much as ten times. Therefore, a stepper motor cannot directly reach its running frequency during startup, and there must be a startup process, gradually increasing the speed from a low speed to the running speed. Similarly, during stopping, the running frequency cannot be immediately reduced to zero, and there must be a high-speed deceleration process gradually decreasing to zero.

The output torque of a stepper motor decreases with the increase in pulse frequency. The higher the starting frequency, the smaller the starting torque, the worse the load-driving ability, and the more likely it is to cause missed steps during startup. During stopping, the motor may also experience over-running. To quickly achieve the required speed without missing steps or over-running, it is necessary to ensure that the torque required for acceleration during the acceleration process fully utilizes the torque provided by the stepper motor at each running frequency without exceeding it. Therefore, a stepper motor's operation usually requires acceleration, uniform speed, and deceleration stages. The shorter the acceleration and deceleration times and the longer the uniform speed time, the better. Especially in applications requiring fast response, the time from the starting point to the end point should be as short as possible, which requires the shortest possible acceleration and deceleration times and the highest possible uniform speed.

Exponential and linear acceleration and deceleration methods are commonly used in stepper motor speed control. Exponential acceleration and deceleration consider the intrinsic torque-frequency characteristics of stepper motors, ensuring that the motor does not miss steps during operation while fully utilizing the motor's inherent characteristics, shortening acceleration and deceleration times. However, it is difficult to implement due to changes in the motor load. Linear acceleration and deceleration only consider the relationship between the angular velocity and pulse of the motor within the load capacity range, which does not change with changes in the power supply voltage or load environment. The acceleration is constant in this method, but it does not fully consider the characteristics of the output torque of the stepper motor changing with the speed, causing the motor to easily miss steps at high speeds.

Stepper Motor Micro-Stepping Control

Due to the limitations of manufacturing technology, the step angle of a stepper motor is generally large and fixed, making the resolution of stepping low and inflexible, causing vibration and noise during low-frequency operation, and easily damaging or fatiguing the physical device. These shortcomings limit the effective use of stepper motors as excellent open-loop control components. Micro-stepping drive technology can effectively overcome these shortcomings to a certain extent.

Micro-stepping drive technology is a driving technology developed in the mid-1970s that can significantly improve the comprehensive performance of stepper motors. In 1975, the American scholar T.R.Fredriksen first proposed the control method of micro-stepping the stepper motor step angle at the American Incremental Motion Control System and Device Conference. In the following twenty years, the micro-stepping drive of the stepper motor has developed rapidly and gradually matured by the end of the last century. Research on micro-stepping drive technology in China started around the same time as in other countries and developed significantly in the mid-1990s. It is mainly applied in the fields of industry, aerospace, robots, and precision measurement, such as tracking satellite optical latitude and longitude instruments, military instruments, communication and radar equipment, and micro-stepping drive technology has been widely used, making the number of motor phases independent of the step angle, providing convenience for product design. Currently, the micro-stepping drive technology for stepper motors mainly uses chopper constant current drive, pulse width modulation drive, and current vector constant amplitude uniform rotation drive control to greatly improve the running accuracy of stepper motors and promote the development of stepper motors in medium and low power applications toward high speed and precision.

Micro-stepping drive technology can effectively improve the output torque and running smoothness of stepper motors, reduce vibration and noise, and improve the control accuracy and stability of the system. The basic principle of micro-stepping drive technology is to control the current in each phase winding of the stepper motor by changing the duty cycle of the power switch in the power drive circuit, thereby changing the magnetic field distribution of the stator, and generating a smooth rotating magnetic field to drive the stepper motor to rotate smoothly.

Understanding and Preventing Miss-Steps and Over-Running in Stepper Motors

Miss-steps in stepper motors refer to the situation where the motor does not respond to each pulse and fails to move to the specified position. Over-running, on the other hand, refers to the situation where the motor moves beyond the specified position.

In simple or low-cost motion control systems, stepper motors are often used. The main advantage is that position and speed can be controlled using an open-loop method. However, since it is an open-loop control, there is no feedback of the load position to the control loop. The stepper motor must correctly respond to each magnetization change. If the magnetization frequency is not appropriate, the stepper motor cannot move to the new position, resulting in permanent errors between the actual position of the load and the position expected by the controller, leading to miss-steps or over-running. Therefore, preventing miss-steps and over-running is crucial for the normal operation of stepper motor open-loop control systems.

Miss-steps and over-running generally occur during the startup and stopping of the stepper motor. Under normal circumstances, the limit startup frequency is relatively low, while the required running speed is often high. If the system starts at the required running speed directly, it will exceed the limit, causing missed steps or stalling during startup. If the system stops sending pulses immediately when it reaches the endpoint, causing it to stop immediately, the stepper motor will rotate beyond the expected balance position due to inertia.

To overcome the miss-step and over-running phenomenon, appropriate acceleration and deceleration control should be added during startup and stopping. This can be achieved using motion control cards, PLCs with control functions, or microcontrollers as the upper control unit to control the acceleration and deceleration of the motor, thereby avoiding miss-steps and over-running.

In simple terms, when a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (step angle) in the set direction. The angular displacement can be controlled by controlling the number of pulses, achieving precise positioning. The motor's speed and acceleration can be controlled by controlling the pulse frequency and reaching the purpose of speed regulation. The technical parameter of a stepper motor is the no-load starting frequency, which is the pulse frequency at which the stepper motor can start normally under no-load conditions. If the pulse frequency is higher than the no-load starting frequency, the stepper motor cannot start normally, possibly causing missed steps or stalling. Under loaded conditions, the starting frequency should be lower. If the motor needs to run at high speed, the pulse frequency should follow a reasonable acceleration process, starting at a low frequency and gradually increasing to the desired high frequency (the motor's speed increases from low to high).

The startup frequency is equal to the startup speed multiplied by the number of steps per revolution. The no-load startup speed is the speed at which the stepper motor starts rotating without using a speed reducer or load. When the stepper motor is rotating, the inductance of each phase winding will form a counter-electromotive force. The higher the frequency, the greater the counter-electromotive force, and the smaller the current, resulting in a decrease in torque.

Assuming that the total output torque of the reducer is T1, the output speed is N1, the reduction ratio is 5:1, and the step angle of the stepper motor is A, then the speed of the motor is 5*(N1), and the output torque of the motor should be (T1)/5. The motor's working frequency should be 5*(N1)360/A. Therefore, you should look at the torque-speed characteristic curve: the coordinate point [(T1)/5, 5(N1)*360/A] should be below the torque-speed characteristic curve (starting torque-frequency curve). If it is below the torque-speed curve, you can choose this motor. If it is above the torque-speed curve, then you cannot choose this motor because it will miss steps or may not even start rotating.

In addition, you need to determine the working state and the required maximum speed. If you have determined these, you can calculate using the formula provided above. Based on the maximum speed and the size of the load, you can determine whether the stepper motor you are currently using is suitable. If it is not suitable, you should know what kind of stepper motor to choose.

Also, after the stepper motor has started, it can increase the frequency under the condition that the load does not change. The reason is that the stepper motor actually has two torque-speed curves. One is the starting torque curve, and the other is the pull-out torque curve. The pull-out torque curve represents the state where the motor can start at the starting frequency, increase the load after starting, and not miss steps, or the state where the motor can start at the starting frequency, increase the speed under constant load, and not miss steps.

Regarding the step angle, for example, if you are using A-B-C-D single four-phase control, the step angle is the angle that A passes through. Regarding the maximum pull-in frequency, it refers to the frequency interval between A and B. The values given in the manual are usually greater than a certain value, but in actual applications, it is better to give the maximum value. For example, if it is greater than 250PPS, then the delay should satisfy 1/delay <=250, delay>=4ms. If you give 3ms, it will not work.

In conclusion, there are indeed people researching ways to detect missed steps and stalls without using encoders. However, these methods are still far from mature and cannot compete with encoders. There is still a long way to go.

In fact, using encoders is the current trend in stepper motor development. If closed-loop control is required, a sensor or encoder is necessary to feed back the current rotational state of the stepper motor to the controller, allowing the controller to make appropriate adjustments (acceleration or deceleration). This is the current state of technology.

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