Stepper motor
A stepper motor is an actuator that converts electrical pulses into angular displacement. Simply put, when a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (i.e., the step angle) in a set direction. You can control the amount of angular displacement by controlling the number of pulses, thus achieving accurate positioning; simultaneously, you can control the motor's speed and acceleration by controlling the pulse frequency, thus achieving speed regulation.
Stepper motors are classified into three types: permanent magnet (PM), reactive (VR), and hybrid (HB). Permanent magnet stepper motors are generally two-phase, with smaller torque and size, and a step angle typically of 7.5 or 15 degrees. Reactive stepper motors are generally three-phase, capable of high torque output, with a step angle typically of 1.5 degrees, but they generate significant noise and vibration. They were phased out in developed countries like Europe and America in the 1980s. Hybrid stepper motors combine the advantages of both permanent magnet and reactive motors. They are further divided into two-phase and five-phase: two-phase typically has a step angle of 1.8 degrees, while five-phase typically has a step angle of 0.72 degrees. Hybrid stepper motors are the most widely used. (200 Stepper Motors and Servo Motors 333332 First time following 51black electronics forum and "Single-chip Microcomputer Tutorial Network" official WeChat account to receive forum black coins reward.)
Basic parameters of a stepper motor:
1. Motor's inherent step angle
It represents the angle the motor rotates for each step pulse signal sent by the control system. The motor is given a step angle value at the factory; for example, the 86BYG250A motor is given a value of 0.9°/1.8° (meaning 0.9° for half-step operation and 1.8° for full-step operation). This step angle can be called the 'motor's inherent step angle,' but it is not necessarily the actual step angle during motor operation. The actual step angle depends on the driver.
The step angle β of a stepper motor is usually calculated using the following formula.
β = 360° / (Z·m·K)
In the formula, β is the step angle of the stepper motor;
Z – Number of rotor teeth;
m – the number of phases of the stepper motor;
K – Control coefficient, which is the ratio of the number of beats to the number of phases.
2. Number of phases of a stepper motor
This refers to the number of coil groups inside the stepper motor. Commonly used stepper motors include two-phase, three-phase, four-phase, and five-phase motors. Different numbers of phases result in different step angles. Generally, two-phase motors have step angles of 0.9°/1.8°, three-phase motors 0.75°/1.5°, and five-phase motors 0.36°/0.72°. Without microstepping drivers, users primarily rely on selecting stepper motors with different numbers of phases to meet their step angle requirements. However, with microstepping drivers, the 'number of phases' becomes meaningless; users can simply change the microstepping setting on the driver to change the step angle.
3. Holding Torque
Holding torque refers to the torque with which the stator holds the rotor in place when the stepper motor is energized but not rotating. It is one of the most important parameters of a stepper motor, and typically, the holding torque at low speeds is close to the holding torque. Because the output torque of a stepper motor decreases with increasing speed, and the output power also varies with speed, holding torque becomes one of the most crucial parameters for evaluating a stepper motor. For example, when people say a 2 N·m stepper motor, unless otherwise specified, they mean a stepper motor with a holding torque of 2 N·m.
DETENTTORQUE: This refers to the torque with which the stator locks the rotor in a stepper motor when it is not powered on. There is no standardized translation for DETENTTORQUE in China, which can easily lead to misunderstandings. Since the rotor of a reactive stepper motor is not made of permanent magnet material, it does not have a DETENTTORQUE.
Main characteristics of stepper motors
1. Stepper motors require a driver to operate. The driver signal must be a pulse signal. Without pulses, the stepper motor remains stationary. If an appropriate pulse signal is applied, it will rotate at a certain angle (called the step angle). The rotation speed is directly proportional to the pulse frequency. For example, a stepper motor with a step angle of 7.5 degrees and a full rotation of 360 degrees requires 48 pulses.
2. Stepper motors have the superior characteristics of instantaneous start and rapid stop.
3. By changing the sequence of pulses, the direction of rotation can be easily changed.
Therefore, printers, plotters, robots, and other devices currently use stepper motors as their core power source.
Features of stepper motor drivers
It is a dedicated integrated circuit that constitutes a stepper motor driver system:
A. Pulse distributor integrated circuits: such as Sanyo's PMM8713, PMM8723, PMM8714, etc.
B. Controller integrated circuits that include pulse distributors and current choppers: such as SGS's L297 and L6506.
C. Driver integrated circuits that only contain power drive (or include current control and protection circuits): such as MTD1110 (four-phase chopper driver) and MTD2001 (two-phase, H-bridge, chopper driver) from Shindengen Kogyo Co., Ltd.
D. Drive controller integrated circuits that include pulse distributors, power drives, current control and protection circuits, such as Toshiba's TB6560AHQ, Motorola's SAA1042 (four-phase) and Allegro's UCN5804 (four-phase).
Overview of Microstepping: Microstepping is a driving method that subdivides the inherent step angle of a motor into several small steps. Microstepping is achieved by precisely controlling the phase current of the stepper motor through a driver, and is independent of the motor itself. The principle is that the stator current is not raised to its full value all at once when energized, and the current is not immediately reduced to zero when de-energized (the winding current waveform is no longer an approximate square wave, but rather an N-level approximate stepped wave). The resultant magnetic force generated by the stator winding current will cause the rotor to have N new equilibrium positions (forming N step angles).
Servo motor
Servo motor internal structure
The servo motor internally includes a small DC motor, a set of speed-reducing gears, a feedback adjustable potentiometer, and an electronic control board. The high-speed rotating DC motor provides the initial power, driving the speed-reducing gear set to produce high torque output. The larger the gear ratio, the greater the output torque of the servo motor, meaning it can withstand greater weight, but the rotational speed will be lower. Its structural diagram is as follows:
Working principle of servo motor
A servo motor is a typical closed-loop feedback system, and its principle can be illustrated by the following diagram:
The reduction gear set is driven by a motor, and its terminal (output) drives a linear proportional potentiometer for position detection. The potentiometer converts the angular coordinates into a proportional voltage and feeds it back to the control circuit board. The control circuit board compares this voltage with the input control pulse signal, generates a correction pulse, and drives the motor to rotate in the forward or reverse direction, so that the output position of the gear set matches the desired value, causing the correction pulse to tend to 0, thereby achieving the purpose of precise positioning of the servo motor.
How to control a servo motor
A standard miniature servo motor has three control lines: power, ground, and control. The power and ground lines provide the energy required by the internal DC motor and control circuitry, with a voltage typically between 4V and 6V. This power supply should be isolated from the power supply of the processing system as much as possible (because servo motors generate noise). Even small servo motors can pull down the amplifier voltage under heavy loads, so the overall power supply ratio of the system must be reasonable.
Input a periodic positive pulse signal. The high-level time of this periodic pulse signal is typically between 1ms and 2ms, while the low-level time should be between 5ms and 20ms, which is not very strict. The table below shows the relationship between the positive pulse width of a typical 20ms periodic pulse and the position of the output arm of a micro servo motor:
servo motor power leads
There are three power leads, as shown in the diagram. The red wire among the three servo motor leads is the control wire, connected to the control chip. The middle wire is the SERVO operating power line, typically operating at 5V. The third wire is the ground wire.
servo motor speed
The instantaneous speed of a servo motor is determined by the interaction of its internal DC motor and gear set, and its value is unique under constant voltage drive. However, its average speed can be changed through segmented pause control. For example, a 90° rotation can be subdivided into 128 pause points, and the average speed varying from 0° to 90° can be achieved by controlling the duration of each pause point. For most servo motors, the unit of speed is "degrees per second".
Precautions for using servo motors
Unless you are using a digital servo motor, the above servo motor output arm position is only an inaccurate approximation. Ordinary analog micro servo motors are not precise positioning devices. Even micro servo motors of the same brand and model can vary greatly. It is normal for different servo motors to have a deviation of ±10° when driven by the same pulse.
For the reasons mentioned above, pulses shorter than 1ms and longer than 2ms are not recommended as drive signals. In fact, the initial design specifications for servo motors are only within a range of ±45°. Moreover, outside this range, the linear relationship between the pulse width and the rotation angle will deteriorate. It is especially important to note that pulse signals that cause the servo motor output position to exceed ±90° must never be applied, otherwise it will damage the servo motor's output limit mechanism or gear set and other mechanical components.
