Servo motor selection precautions analysis

In order to meet the requirements of mechanical equipment for high precision and fast response, the servo motor should have a small moment of inertia and a large locked-rotor moment, and have as small a time constant and starting voltage as possible, and should also have a longer overload capacity. To meet the requirements of low speed and high torque, can withstand frequent starting, braking and positive, reverse, if blindly choose large specifications of the motor, not only increase the cost, but also make the design equipment volume increase, the structure is not compact, so the selection of the motor should fully consider all aspects of the requirements, in order to give full play to the performance of the servo motor; The following describes the selection of servo motor precautions.

1. Several braking methods commonly used by servo motors
We are easy to confuse the role of electromagnetic braking, regenerative braking, dynamic braking, and choose the wrong accessories. The following concepts are clarified.
The dynamic brake is composed of dynamic brake resistance, which can shorten the mechanical feed distance of servo motor by energy braking in case of failure, emergency stop and power failure.
Regenerative braking refers to when the servo motor decelerates or stops, the energy generated by braking is fed back to the DC bus through the inverter circuit and absorbed by the resistance-capacitance circuit.
Electromagnetic braking is a mechanism that locks the shaft of the motor.

The difference between the three:
Regenerative braking must be effective when the server is working normally, and the motor cannot be stopped in the case of failure, emergency stop, power failure, etc. Dynamic brakes and electromagnetic brakes do not require power when operating.
The work of regenerative braking is carried out automatically by the system, while the work of dynamic braking and electromagnetic braking requires external relay control.

Electromagnetic braking is generally activated after SV OFF. Otherwise, the amplifier may be overloaded, and the dynamic brake generally starts after the SV OFF or main circuit is powered off, otherwise it may cause the dynamic brake resistance to overheat.

Dynamic braking and regenerative braking are accomplished by the internal excitation of the servo motor, which is to increase the current in the direction opposite to the direction of rotation.

Electromagnetic braking, also known as the holding brake, is controlled by the peripheral DC power supply and is normally closed. After the power is generated, the brake is opened, and the power is lost, which is pure mechanical friction braking.

Points to note in choosing accessories:
(1) Some systems such as transmission devices, lifting devices, etc., require the servo motor to stop as soon as possible, and in the case of failure, emergency stop, and power failure, the server has no regenerative braking and cannot slow down the motor. At the same time the mechanical inertia of the system is large. At this time, the dynamic brake should be selected according to the weight of the load and the working speed of the motor.

(2) Some systems to maintain the static position of the mechanical device, the motor needs to provide a larger output torque, and the stop time is longer. If the self-locking function of the servo is used, it often causes the motor to overheat or the amplifier to overload, in which case the motor with electromagnetic braking should be selected.

(3) Some servo drivers have built-in regenerative braking units, but when regenerative braking is more frequent, it may cause the DC bus voltage to be too high, then it is necessary to match another regenerative braking resistance. Whether the regenerative brake resistance needs to be configured separately and how large it should be configured can be configured according to the usage instructions of the corresponding sample.

(4) If the servo motor with electromagnetic brake is selected, the moment of inertia of the motor will increase, and the torque should be considered when calculating.

2. Speed/position detector
The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. It is used to measure the working speed or rotation position of the motor.

The commonly used rotary encoder is incremental, the encoder disk is composed of A lot of grating engraving lines, there are two (or four) optical eyes to read A, B signals, the density of the engraving line determines the resolution of the incremental encoder, that is, the minimum change of Angle value can be distinguished. The parameter that represents the resolution of the incremental encoder is PPR, which is the number of pulses per revolution. Some incremental encoders, the original engraving line is 2048 lines (2 to the 11th power, 11 bits), through 16 times (4 bits) subdivision, to get 15 bits PPR, and again 4 times the frequency (2 bits), to get 17 bits (Bjt) resolution, generally using "Bit, bit" to express the resolution. This encoder at a faster speed, the internal use of unsubdivided low signal to process the output, otherwise the response can not keep up, so do not
Be confused by its "17 bit", pay attention to the design and selection of servo motors.

3. Regenerative braking frequency
The regenerative braking frequency represents the allowable frequency of the motor from rated speed to deceleration stop when the element is loaded.
It should be noted that the number of braking times on the general sample list is the data of the motor when it is unloaded. In the actual selection, the inertia ratio should be calculated first according to the load inertia of the system and the motor inertia on the sample. Then divide the number of braking times on the sample list by (inertia ratio +1), so that the data is the number of braking allowed.

4. the radial and axial load on the servo motor shaft
Ensure that the radial and axial loads applied to the shaft of the servo motor during installation and operation are controlled within the specified values of each model, otherwise it will accelerate the wear of the servo motor, reduce the life of the motor, and even affect the required accuracy.

5. Tolerance of output shaft
Pay attention to the relationship between motor shaft extension and other parts.

Source: Operations Control CMCIA