The field of automation refers to today's hot spots, and servo motors play an important role in it. They are often used for more accurate speed or position control components in projects. Designers of automation equipment often need to face a variety of different needs of motor selection. This article first introduced the selection procedure and principle of the servo motor, then explained how to select the servo motor for the automation equipment design, and finally introduced the selection considerations for the servo motor.
Servo motor selection procedure(1) Determine the requirements of the movement conditions of the load mechanism, that is, the acceleration/deceleration speed, the movement speed, the weight of the mechanism, and the movement mode of the mechanism.
(2) Calculate the load inertia of the mechanism by selecting the appropriate load inertia calculation formula according to the operating conditions.
(3) Select the appropriate spurious servo motor specification based on load inertia and servo motor inertia.
(4) Calculate the acceleration torque and deceleration torque by combining the servo motor inertia and load inertia of the primary selection.
(5) Calculate load torque based on load weight, configuration method, friction coefficient, and efficiency.
(6) The maximum output torque of the primary servo motor must be greater than the acceleration torque + load torque; if it does not meet the conditions, other models must be selected to verify and meet the requirements.
(7) The continuous instantaneous torque is calculated based on the load torque, acceleration torque, deceleration torque, and holding torque.
(8) The rated torque of the primary servo motor must be greater than the continuous momentary torque. For example, if it does not meet the conditions, it must be calculated and verified by other models until it meets the requirements.
(9) Complete the selection.
1. Select the load inertia calculation formula according to the operation requirements and calculate the load inertia of the mechanism.
2. Normally, the movement condition requirements of the load mechanism are clearly defined, that is, the acceleration/deceleration rate, the running speed, the weight, and the operation mode. This is necessary for selecting the servo specifications.
3, load weight, configuration, friction coefficient to calculate the load torque.
4. Load inertia and servo motor inertia are also necessary considerations when selecting the motor specification. The user can calculate the acceleration torque and deceleration torque based on the two, and select the false selection specifications for the line.
5, load torque, acceleration and deceleration torque, hold torque, calculate continuous instantaneous torque. The continuous instantaneous torque must be less than the rated torque of the primary servo motor. Otherwise, only other qualified specifications can be selected.
6. Pay attention to the acceleration torque + load torque <maximum output torque of the primary servo
1, application scenarios
Controlled motors in the field of automation can be divided into servo motors, stepper motors, and variable frequency motors. In parts that require more accurate speed or position control, servo motor drives are selected.
The inverter + inverter motor control method is a control method that changes the motor speed by changing the input motor power frequency. Generally only used for motor speed control.
Servo motor compared with stepper motor:
a) Servo motor uses closed-loop control, stepper motor is open-loop control;
b) Servo motor uses rotary encoder measurement accuracy, stepper motor uses step angle. The accuracy of the former at the level of ordinary products can reach the order of magnitude of the latter;
c) Control methods are similar (pulse or direction signals).
2, power supply
Different from the power supply, the servo motor can be divided into AC servo motor and DC servo motor.
The two are still better choices. For general automation equipment, Party A will provide a standard 380V industrial power supply or 220V power supply. At this time, the servo motor corresponding to the power supply can be selected, eliminating the need for power type conversion. However, there are some devices, such as the shuttle board in the warehouse, AGV cars, etc., due to their own nature of the movement, most use their own DC power supply, so the general use of DC servo motor.
3, brake
According to the design of the operating mechanism, whether or not the reverse rotation of the motor is caused in the power-off state or the stationary state is considered. If there is a reverse trend, it is necessary to select a servo motor with a brake.
4, selection calculation
Before the calculation of the type selection, the first thing to be determined is the position and speed requirements of the end of the mechanism, and in addition, the transmission mechanism is determined. The servo system and the corresponding gear unit can be selected at this time.
During the selection process, the following parameters are mainly considered:
4.1, power and speed
Calculate the required power and speed of the motor based on the structural form and final load speed and acceleration requirements. It is worth noting that, under normal circumstances, it is necessary to select the reduction ratio of the speed reducer in combination with the speed of the selected motor.
In the actual selection process, for example, if the load is a horizontal movement, the formula P=T*N/9549 cannot be clearly calculated because of the friction coefficient of each transmission mechanism and the uncertainty of the wind load coefficient (the torque cannot be accurately calculated). In practice, it has also been found that the maximum power required to use the servo motor is often the acceleration and deceleration phases. Therefore, the power of the required motor and the reduction ratio of the reducer can be quantitatively calculated by T=F*R=m*a*R (m: load mass; a: load acceleration; R: load rotation radius).
The following points need attention:
a) The power redundancy coefficient of the motor;
b) Consider the transmission efficiency of the mechanism;
c) whether the input and output torque of the reducer is up to the standard and has a certain safety factor;
d) Is there any possibility of speeding up in the later period?
It is worth mentioning that in the traditional industries, such as cranes and other industries, the use of ordinary induction motor drive, there is no clear requirement for acceleration, the calculation process uses an empirical formula.
Note: When the load is running vertically, pay attention to the gravitational acceleration.
4.2 Inertia matching
To achieve high-precision control of the load, you need to consider whether the motor and system inertia match.
For the question of why inertia matching is needed, there is no statement on the Internet. Personal understanding is limited, not explained here. Interested friends can verify and inform themselves. The principle of inertia matching is: considering that the inertia of the system is converted to the motor shaft and the inertia ratio of the motor is not greater than 10 (Siemens); the smaller the ratio is, the better the control stability is, but the larger motor is needed and the cost performance is lower. If you do not understand the specific calculation methods, please do your own university "Theory of Mechanics."
4.3, accuracy requirements
After calculating the change through the reducer and the transmission mechanism, the control accuracy of the motor can meet the requirements of the load. Reducer or some transmission mechanism has a certain return clearance, all need to be considered.
4.4, control matching
This aspect mainly communicates with the electrical designer, such as whether the servo controller's communication method matches the PLC, the encoder type, and whether or not the data needs to be extracted.
It is worth mentioning that to do automated design, we must learn to use external forces. Especially when it comes to non-standard automation, the selection and calculation of too many equipments are often overwhelmed, and overworked dogs are the norm. Now servo motor manufacturers will provide technical support, as long as you provide him with parameters such as load, speed, acceleration, etc. They have a set of their own software to automatically help you calculate and select the appropriate servo motor, very convenient.
1. Some systems such as conveyors, lifts, etc. require the servo motor to be stopped as quickly as possible. However, the servo does not have regenerative braking in the event of a fault, emergency stop, or power failure and cannot slow down the motor. At the same time, the mechanical inertia of the system is large. At this time, the dynamic brake must be selected based on the weight of the load and the working speed of the motor.
2. Some systems need to maintain the rest position of the mechanical device, which requires the motor to provide a larger output torque and a longer stop time. If the self-locking function of the servo is used, the motor may overheat or the amplifier may be overloaded. In this case, the motor with electromagnetic brake must be selected.
3. Some servo drives have a built-in regenerative braking unit, but when the regenerative braking is more frequent, it may cause the DC bus voltage to be too high. At this time, a regenerative braking resistor must be provided. Whether the regenerative braking resistor needs to be reprovisioned or not, please refer to the instruction of the corresponding sample for matching.
4. If a servo motor with electromagnetic brake is selected, the motor's moment of inertia will increase, and the torque must be considered when calculating the torque.
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