Overview:
1. Introduction to the characteristics, advantages and disadvantages of stepper motors and servo motors
1.1 Introduction to the two types of motors in point-to-point control or speed regulation applications
Stepper motors and servo motors are mainly used for precision positioning and can also be used for speed control applications. Stepper motors are generally not used as power sources due to their low efficiency; they are also not recommended for torque control due to torque ripple. Servo systems, on the other hand, can be used for torque control and can even be considered as a replacement for frequency converters as a power source.
When stepper motors are used for speed control, control commands are typically pulse commands, and speed is adjusted by changing the pulse frequency. Compared to frequency converter speed control, it has advantages such as high torque at low speeds, easy start-stop control, and short acceleration/deceleration times (reaching the target speed in milliseconds under suitable voltage and load conditions). Furthermore, it has a wider speed range, and under conditions of reasonable load-inertia ratio matching, a separate reduction gear mechanism is usually unnecessary. The disadvantage is that the operating noise is relatively higher.
Compared to frequency converters, servo motors offer advantages in speed control applications due to their shorter acceleration and deceleration times, typically reaching the desired speed within tens of milliseconds, and providing a wider speed range. For speed control and torque control applications, analog voltage signals are recommended as the control signal.
1.2 Performance characteristics and comparison of stepper motors and servo motors:
2. Motor selection and application experience
2.1 Motor Drive Selection Method
Equipment manufacturers can refer to the following methods when selecting motors:
1) Operating environment, required protection level, operating noise level, temperature rise level, etc.;
2) Determine the mechanical specifications, load, rigidity, and other parameters;
3) Confirm the motion parameters: speed, stroke, acceleration/deceleration time, cycle, accuracy, etc.
4) Calculate load inertia and select motor inertia;
5) Calculate the required torque for the motor;
6) Select a motor whose maximum speed can meet the application requirements.
Personnel in relevant positions at the equipment manufacturer can obtain this information through the following division of labor.
Mechanical designers must first calculate the moment of inertia of moving parts and then calculate the required torque.
2.2 Application Experience
1) Proper assembly and connection between the motor and the load.
2) Attention should be paid to the heat dissipation of the drive and motor.
3) Select appropriate drivers and power supplies, and set the current and microstepping appropriately.
4) Correct electrical connections and reasonable electrical assembly process.
5) Design a reasonable motion curve.
The table below provides some references for personnel in relevant positions in equipment manufacturers, such as mechanical engineers, electrical engineers, and software engineers.
Figure 1
Driver control signal wiring diagram notes:
1) Wiring diagram for pulse and direction signal terminals; firstly, the signal voltage amplitude condition of the driver must be met. Then consider the signal output type of the host system, which can be basically divided into: differential type, NPN type (drain, current-source type), and PNP type (source, current-push type).
2) Taking the more common NPN type output signal as an example, it's crucial to grasp the concept of a loop: Current flows from the positive terminal of the signal power supply to PUL+, passes through the internal circuit, flows out from the PUL- terminal, then through the positive terminal of the upper controller's pulse output port to the negative terminal (the positive and negative terminals of both NPN and PNP type pulse output ports can be understood as unidirectional conduction switches), and then flows back from the negative terminal of the pulse output port to the negative terminal of the signal power supply, forming a complete loop as shown in Figure 1. Differential outputs are more special; usually, either terminal can generate both push and pull current, so the "same-name terminals" of several differential signals cannot be connected in parallel to form a common anode or common cathode connection. Regardless of the type of upper-level output or the type of driver signal interface, as long as a complete, switchable loop can be formed, it's acceptable.
3) Please also note that when the signal voltage is not 5V and a resistor is needed to limit the current, the direction and pulse circuits should each have their own current-limiting resistors instead of sharing a single resistor.
4) To ensure the normal operation of the driver and motor, it is recommended that the power supply for the control command be completely isolated from the power supply of other inductive loads. If isolation is not possible, be sure to install a freewheeling diode for the inductive load.
Motion curves and parameters should be rationally planned and set:
The software engineer's tasks include: planning the motion control curve for each axis, understanding the time and travel of each action, and rationally configuring the initial velocity, acceleration time, maximum speed, and reversal time to achieve the highest efficiency and best results. Using a trapezoidal acceleration/deceleration mechanism as an example, this section explains how to plan and design a motion curve, as shown in Figure 2 and its accompanying explanation.
If your host controller is not a general-purpose control card or PLC, please note the following:
1. Do the high and low level times of the control signals meet the requirements in the driver's manual? Drivers have upper limits on their signal input frequencies. Stepper motors typically have a limit of 200kHz, and servo motors typically have a limit of 500kHz. The terms 200kHz and 500kHz have a prerequisite: a duty cycle of 50%. In practice, 200kHz means that both the high and low level times are no less than 2.5 microseconds; 500kHz means that both the high and low level times are no less than 1 microsecond.
2. Control signal amplitude: High level must be higher than 3.5V; low level must be lower than 0.5V. The MCU output port usually cannot directly drive the driver; an amplifier circuit is needed to amplify the current driving capability so that the pulse direction output signal can reach the typical application current of 10 mA.
3. The timing of the control signals must meet the requirements of the instruction manual. Typically, the driver requires the direction signal to precede the pulse's effective edge by at least 1-2 microseconds.
Figure 2
illustrate:
V1: Takeoff speed
V2: Top Speed
t1: Acceleration time
t3-t2: Deceleration time, usually designed to be the same as the acceleration time t1.
For stepper motors with a frame size of 60mm or less, it is recommended to set the start-up speed below 1.5 rpm. The acceleration/deceleration time to reach the recommended maximum operating speed of 20 rpm is 30-150 milliseconds. For motors with an end cap size of 86mm or larger, it is recommended to set the initial speed below 1 rpm. The acceleration/deceleration time to reach the recommended maximum operating speed of 10 rpm is 80-200 milliseconds. For servo motors of 400W and below, it is recommended to set the initial speed below 3 rpm. The acceleration/deceleration time to reach the recommended maximum operating speed of 50 rpm is 15-200 milliseconds. For servo motors of 750W-2000W, it is recommended to set the initial speed below 2 rpm. The acceleration/deceleration time to reach 30 rpm is 40-300 milliseconds.
Knowing the total journey S and the allowed time T,
V1, V2, and t1 can be calculated in this way.
S = (V1 + V2) * t1 + V2 * (T - 2t1); For ease of calculation, let V1 = 0; then V2 * (T - t1) = S; Knowing the values of S and T and the empirical range of V2 and t1, if you determine either V2 or t1, the rest will be determined.
Figure 3
