Selection of Stepping Motor and Driver Chip

Selection of Stepping Motor and Driver Chip

1. Performance of stepping motor drive system

1) General knowledge of the system:

The stepping motor and the stepping motor driver constitute the stepping motor drive system. The performance of the stepping motor drive system depends not only on the performance of the stepping motor itself, but also on the quality of the stepping motor driver. The research on stepping motor driver is almost synchronized with that of stepping motor.

2) Overview of the system:

Stepping motor is an executive element that converts electrical pulse into angular displacement. When the stepper motor driver receives a pulse signal (from the controller), it drives the stepper motor to rotate at a fixed angle (called the "step angle") in the set direction, and its rotation runs step by step at a fixed angle.

3) System control:

Stepping motor can not work directly on DC or AC power supply, it must use dedicated driving power (stepping motor driver). The controller (pulse signal generator) can control the angular displacement by controlling the number of pulses to achieve the purpose of accurate positioning. At the same time, the speed and speed of the motor can be controlled by controlling the pulse frequency to achieve the purpose of speed control.

4) Purpose:

Stepping motor is a special motor for control. As an executing element, it is one of the key products of Mechatronics integration. With the development of microelectronics and computer technology (stepping motor driver performance improvement), the demand of stepping motor is increasing. Stepping motor has no accumulative error in operation, which makes it widely used in various automatic control systems, especially open-loop control systems.

2. Structure Classification of Stepping Motor

Stepping motor is also called pulse motor, including reactive stepping motor (VR), permanent magnet stepping motor (PM), hybrid stepping motor (HB), and so on.

1) Reactive step motor:

Also called induction, hysteresis or reluctance stepping motor. The stator and the rotator are made of soft magnetic material. There are multi-phase excitation windings on the large uniformly distributed poles of the stator. The small teeth and slots are evenly distributed around the stator and the rotator. The torque is generated by the change of magnetic conductivity after power is switched on. Generally three, four, five and six phases; It can achieve large torque output (consuming power, current up to 20A, driving voltage higher); Step angle is small (up to one-sixth of a degree); No positioning torque when power is cut off; The damp inside the motor is small and the oscillation time of single step operation (when the pulse frequency is very low) is long. Startup and run frequently.

2) Permanent magnet stepping motor:

Normally, the motor rotors are made of permanent magnet material. The stators made of soft magnetic material have multi-phase excitation windings. There are no small teeth and slots around the stators and rotors. After power-on, the interaction between the permanent magnet and the stator current magnetic field is used to generate the torque. Generally two or four phases; Low output torque (low power consumption, current generally less than 2A, drive voltage 12V); Large step angle (e.g. 7.5 degrees, 15 degrees, 22.5 degrees, etc.); When power is cut off, there is a certain retaining moment. Startup and run frequency is low.

3) Hybrid stepping motor:

Also known as Permanent Magnet Reactive, Permanent Magnet Induction Step Motor, it combines the advantages of Permanent Magnet and Reactive Magnet. There is no difference between the stator and the four-phase reactive stepping motor (but the two poles of the same phase are relative, and the N and S polarities produced by winding on the two poles must be the same). The structure of the rotor is more complex (the inner part of the rotor is a cylindrical permanent magnet, the soft magnetic material on both ends is coated with small teeth and slots around it). Generally two or four phases; Negative and positive pulse signals must be supplied; The output torque is larger than that of the permanent magnet (the power consumption is relatively small); Step angle is smaller than permanent magnet (generally 1.8 degrees); No positioning torque when power is cut off; Startup and operation frequency is high; At present, it is a kind of stepping motor with fast development.

3. Basic parameters of stepping motor

1) The inherent step angle of the motor:

It represents the angle at which the motor rotates for each step-by-step pulse signal sent by the control system. When the motor is out of the factory, a value of step angle is given. For example, the value given by STP-59D5026 motor is 0.9 degrees/1.8 degrees (0.9 degrees for half-step operation and 1.8 degrees for full-step operation). This step angle can be called "inherent step angle of the motor". It is not necessarily the actual step angle when the motor is working. The actual step angle is related to the driver.

2) Number of stepping motors:

Refers to the number of coil groups inside the motor. At present, there are two-phase, three-phase, four-phase, five-phase step motors commonly used. The step angle of two-phase motor is 0.9 degrees/1.8 degrees, that of three-phase motor is 0.75 degrees/1.5 degrees, and that of five-phase motor is 0.36 degrees/0.72 degrees. Increasing the phase number of a stepping motor can improve performance, but the structure and driving power supply of the stepping motor will be more complex and the cost will increase.

3) Hold Torque (HOLDING TORQUE):

Also known as static torque, it is a torque applied to a powered stepping motor shaft under a rated static current without generating continuous rotation. It is one of the important parameters of the stepping motor, usually the moment of the stepping motor at low speed is close to the hold moment. Because the output moment of the stepping motor decreases with the increase of speed and the output power changes with the increase of speed, keeping the torque becomes one of the important parameters to measure the stepping motor. For example, when people say that a 2N.m stepping motor means a stepping motor with a torque of 2N.m without special instructions.

4) Step Precision:

It can be expressed as positioning error or step angle error.

5) Moment angle characteristics:

The recovery torque of the stepping motor when it leaves the balance position changes with the offset of the rotation angle. The relationship between the static torque and the misalignment angle of the stepping motor is called the moment angle characteristic.

6) Static temperature rise:

Refers to the temperature rise when the motor is stationary and the rated static current is used for many phases in the specified operation mode to reach a stable thermal balance.

7) Dynamic temperature rise:

The motor runs without load at a certain frequency and operates according to the specified operating time. The temperature rise reached by the motor after the operating time is called dynamic temperature rise.

8) Torque characteristics:

It represents the relationship between the motor torque and the excitation current when the single-phase power is connected.

9) Startup moment-frequency characteristics:

The relationship between starting frequency and load torque is called starting moment frequency characteristic.

10) Operating Moment Frequency/Inertial Frequency Characteristics:

11) Up-down time:

Refers to the time required for the motor to rise from the starting frequency to the operating frequency or decrease from the operating frequency to the starting frequency.

12) DETENT TORQUE:

The moment at which the stator locks the rotor when the stepping motor is not powered on. DETENT TORQUE has no uniform translation method in China and is prone to misunderstanding. The rotator of the reactive stepping motor is not a permanent magnet material, so it has no DETENT TORQUE.

4. Stepping motor characteristics

(1) There is no accumulative error in the stepping motor: generally, the precision of the stepping motor is 3 to 5 percent of the actual step angle, and it does not accumulate.

(2) When the stepping motor is working, the pulse signal is added to each phase winding in turn in a certain order (the way the winding is switched on and off is controlled by a ring distributor in the driver).

(3) Even if the same stepping motor is used with different driving schemes, its moment-frequency characteristics are very different.

(4) Unlike other motors, the nominal rated voltage and current of the stepping motor are only reference values; Also because the stepping motor is powered by pulse mode, the power supply voltage is its voltage, not the average voltage, so the stepping motor can work beyond its rated range. However, the selection should not deviate too far from the rated value.

(5) The permissible temperature for the appearance of the stepping motor: excessive temperature of the stepping motor will first demagnetize the magnetic material of the motor, which will cause the moment to drop or even lose synchronization. Therefore, the permissible temperature for the appearance of the motor should depend on the demagnetizing point of the magnetic material of different motors; Generally speaking, the demagnetization points of magnetic materials are above 130 degrees Celsius, some even above 200 degrees Celsius, so the external temperature of stepping motor is completely normal at 80-90 degrees Celsius.

(6) The torque of the stepping motor decreases with the increase of speed: When the stepping motor rotates, the inductance of the windings of the motor will form a reverse electromotive force; The higher the frequency, the greater the reverse electromotive force. Under its action, the phase current of the motor decreases with the increase of frequency (or speed), resulting in a decrease in the torque.

(7) The stepping motor can operate normally at low speed, but it cannot start if it is higher than a certain frequency, accompanied by a howling sound.

Stepping motor has one technical parameter: no-load startup frequency, that is, the pulse frequency that stepping motor can start normally under no-load condition. If the pulse frequency is higher than this value, the motor cannot start normally, step loss or blocking may occur. Startup frequency should be lower under load. If the motor is to rotate at a high speed, there should be an acceleration process for the pulse frequency, i.e. the starting frequency is low, and then the speed of the motor rises to the desired high frequency with a certain acceleration (from low speed to high speed).

(8) Four-phase hybrid stepping motors are generally driven by two-phase drivers, so the four-phase motor can be connected into two phases by series connection or parallel connection. Series connection is generally used in low speed situations, where the output current of the driver is 0.7 times that of the phase current of the motor, thus the heat of the motor is small. Parallel connection is generally used in the case of high speed motor (also known as high speed connection), which requires 1.4 times the output current of the driver as the phase current of the motor, thus the motor is heated up.

(9) The power supply voltage of hybrid stepping motor driver is generally a wide range (for example, the power supply voltage of IM483 is 12-48VDC). The power supply voltage is usually selected according to the working speed and response requirements of the motor. If the motor has a high working speed or a fast response requirement, the voltage value is also high, but note that the ripple of the power voltage cannot exceed the input voltage of the driver, otherwise the driver may be damaged.

(10) The power supply current is generally determined by the output phase current I of the driver. If a linear power supply is used, the power supply current can generally be 1.1-1.3 times of that of I. If switching power supply is used, the power supply current can generally be 1.5-2.0 times of I.

(11) When the offline signal FREE is low, the current output from the driver to the motor is cut off, and the motor rotator is in a free state (offline state). In some automation devices, if direct rotation of the motor shaft (manual mode) is required when the driver is powered off, the FREE signal can be set low, the motor can be offline for manual operation or adjustment. After the manual completion, set the FREE signal high to continue the automatic control.

(12) Use a simple method to adjust the direction of rotation of two-phase stepping motor after power-on. Simply adjust A+and A-(or B+and B-) of the connection between the motor and the driver.

5. Principle of Stepping Motor Driver

Stepping motor operation requires an electronic device to drive, which is the stepping motor driver, which amplifies the pulse signal from the control system to drive the stepping motor. The speed of the stepping motor is proportional to the frequency of the pulse signal. By controlling the frequency of the stepping pulse signal, the speed of the motor can be adjusted. By controlling the number of step pulses, the motor can be positioned.

A typical stepping motor drive control system consists of three main parts:

1) Step controller, which is implemented by single-chip computer.

2) Driver, amplify the pulse output by single-chip computer to drive stepping motor.

3) Stepping motor.

The typical stepping motor drive control circuit diagram is as follows:

6. Selection of Stepping Motor

1) Select the type first, followed by the specific varieties and models.

2) The performance index, shape size, installation method, pulse power type and control circuit of the three stepping motors, reactive, permanent magnet and hybrid, are different. The price difference is also large, so the selection should be considered in a comprehensive way.

3) Step motor with control integrated circuit should be given priority.

7. The working modes of stepping motors can be divided into two types: power mode and servo mode.

1) Power type: The output torque is large, which can directly drive a larger load (generally using reactive, hybrid stepping motors).

2) Server type: The output torque is small and can only drive a small load (usually a permanent magnet, hybrid stepping motor).

8. The motor drive circuit of PWM speed control mainly has the following performance indicators

1) Output current and voltage range, which determine how much power the circuit can drive.

2) Efficiency, high efficiency not only means saving power, but also reducing the heating of the drive circuit. To improve the efficiency of the circuit, you can start by ensuring the switch working state of the power device and preventing the common state conduction (H bridge or push-pull circuit may have a problem, that is, two power devices are conducting at the same time to short-circuit the power supply).

3) Impact on the control input. The power circuit should have good signal isolation for its input, preventing high voltage and high current from entering the main control circuit, which can be achieved by using high input impedance or