Working principle of servo motor
The diagram below shows a servo motor control circuit using an LM675 power operational amplifier. The motor is a DC servo motor. As shown, the LM675 is powered by 15V. This 15V voltage is applied to the non-inverting input of the LM675 via RP1. The output voltage of the LM675 is applied to the input of the servo motor. A speed signal generator is installed on the motor to detect its speed in real time. This speed signal generator is essentially a generator; its output voltage is proportional to the speed. The voltage output from the speed signal generator G is divided by a voltage divider circuit and fed back to the inverting input of the operational amplifier as a speed error signal. The voltage value set by the speed command potentiometer RP1 is divided by resistors R1 and R2 and applied to the non-inverting input of the operational amplifier, serving as a reference voltage.
Servo motor control principle diagram
The servo motor, represented by the letter M, is the power source of the drive system. The operational amplifier, indicated by its circuit name (LM675), is an amplifying device in the servo control circuit that provides drive current to the servo motor.
Speed command potentiometer RP1: Sets the reference voltage for the operational amplifier in the circuit, i.e., speed setting. Amplifier gain adjustment potentiometer RP2: Used in the circuit to fine-tune the amplifier gain and the magnitude of the speed feedback signal, respectively. When the motor load changes, the voltage fed back to the inverting input of the operational amplifier also changes, i.e., the speed...
When the motor load increases, the speed decreases, and the output voltage of the speed sensor generator also decreases. This causes the voltage at the inverting input of the operational amplifier to decrease, increasing the difference between this voltage and the reference voltage, and thus increasing the output voltage of the operational amplifier. Conversely, when the load decreases and the motor speed increases, the output voltage of the speed sensor generator rises, increasing the feedback voltage applied to the inverting input of the operational amplifier. This reduces the difference between this feedback voltage and the reference voltage, causing the output voltage of the operational amplifier to drop, which in turn causes the motor speed to decrease, thus automatically stabilizing the rotational speed at the set value.
Advantages of servo motors
1. Precision: It achieves closed-loop control of position, speed, and torque; overcoming the problem of stepper motor step loss;
2. Speed: Good high-speed performance, generally rated speed can reach 2000-3000 rpm;
3. Adaptability: It has strong overload resistance and can withstand loads three times the rated torque, making it particularly suitable for occasions with instantaneous load fluctuations and requirements for rapid start-up.
4. Stability: Smooth operation at low speeds, without exhibiting the stepping motion characteristic of a stepper motor. Suitable for applications requiring high-speed response;
5. Timeliness: The dynamic response time of motor acceleration and deceleration is short, generally within tens of milliseconds;
6. Comfort: Heat generation and noise are significantly reduced.
Servo motor selection steps
Each servo motor model's specifications include parameters such as rated torque, maximum torque, and servo motor inertia. These parameters are necessarily related to the load torque and load inertia. The output torque of the selected servo motor should meet the motion requirements of the load mechanism, such as the rate of acceleration, the weight of the mechanism, and the motion mode of the mechanism (horizontal, vertical rotation, etc.). Motion conditions are not directly related to the output power of the servo motor, but generally, the higher the output power of the servo motor, the higher the relative output torque will be.
Therefore, not only does the weight of the mechanism affect the selection of the servo motor , but the motion conditions also change the choice of servo motor. A larger moment of inertia requires a larger acceleration and deceleration torque, and a shorter acceleration and deceleration time also requires a larger servo motor output torque. The following steps should be followed when selecting servo motor specifications.
(1) Clarify the motion requirements of the load mechanism, namely, the speed of acceleration/deceleration, motion speed, weight of the mechanism, and motion mode of the mechanism.
(2) Calculate the load inertia of the mechanism by selecting an appropriate load inertia calculation formula according to the operating conditions.
(3) Select the appropriate hypothetical servo motor specification based on the load inertia and the servo motor inertia.
(4) Calculate the acceleration torque and deceleration torque by combining the initial selected servo motor inertia and load inertia.
(5) Calculate the load torque based on the load weight, configuration, friction coefficient and operating efficiency.
(6) The maximum output torque of the initially selected servo motor must be greater than the acceleration torque plus the load torque; if it does not meet the requirements, other models must be selected for calculation and verification until they meet the requirements.
(7) Calculate the continuous instantaneous torque based on the load torque, acceleration torque, deceleration torque and holding torque.
(8) The rated torque of the initial servo motor must be greater than the continuous instantaneous torque. If it does not meet the requirements, other models must be selected for calculation and verification until they meet the requirements.
(9) Complete the selection.
Simplest servo motor selection calculation method
When selecting a servo motor, the first thing to consider is its power. Generally, the following two points should be noted:
1. If the motor power is selected too small, a "small horse pulling a big cart" phenomenon will occur, causing the motor to be overloaded for a long time, which will damage the insulation due to heat, or even burn out the motor.
2. If the motor power is selected too large, a "large horse pulling a small cart" phenomenon will occur, where its output mechanical power cannot be fully utilized, resulting in low power factor and efficiency. This is not only detrimental to users and the power grid, but also leads to energy waste.
In other words, the motor power cannot be too high or too low. To correctly select the motor power, the following calculations or comparisons must be performed:
P = F * V / 100
(Where P is the calculated power in kW, F is the required pulling force in N, and V is the linear velocity of the working machine in m/s)
In addition, the most common method for selecting motor power is by analogy. The analogy method involves comparing the motor's power with that of motors used in similar production machinery.
The specific procedure is as follows: find out the power rating of the motors used in similar production machinery in your own unit or other nearby units, and then select a motor with a similar power rating for trial operation. The purpose of the trial operation is to verify whether the selected motor is compatible with the production machinery.
The verification method is as follows: drive the production machinery with the motor, measure the operating current of the motor with a clamp meter, and compare the measured current with the rated current marked on the motor nameplate.
If the actual operating current of the motor is not significantly different from the rated current marked on the nameplate, it indicates that the selected motor has appropriate power.
If the actual operating current of the motor is about 70% lower than the rated current indicated on the nameplate, it means that the motor power has been selected too high, and a motor with a lower power should be selected instead.
If the measured motor operating current is more than 40% higher than the rated current indicated on the nameplate, it indicates that the motor power has been selected too low, and a motor with higher power should be selected instead.
In reality, it should take into account torque, motor power, and torque calculation formulas.
That is, T=9550P/n
In the formula:
P – Power, kW; n – Rated speed of the motor, r/min; T – Torque, Nm.
The output torque of the motor must be greater than the torque required by the working machinery, and a safety factor is generally required.
Mechanical power formula: P=T*N/97500
P: Power (W); T: Torque (g/cm); N: Rotational speed (r/min).
Considerations for servo motor selection
1. Some systems, such as conveyor systems and lifting devices, require servo motors to stop as quickly as possible. However, in the event of a malfunction, emergency stop, or power failure, the servo motor lacks regenerative braking and cannot decelerate the motor. At the same time, the system's mechanical inertia is relatively large. In such cases, the selection of dynamic brakes must be based on factors such as the load and the motor's operating speed.
2. Some systems require a large output torque from the motor to maintain the stationary position of mechanical devices, and the stopping time is relatively long. If the self-locking function of the servo is used, it will often cause the motor to overheat or the amplifier to overload. In this case, a motor with electromagnetic braking should be selected.
3. Some servo drives have a built-in regenerative braking unit, but when regenerative braking is frequent, it may cause the DC bus voltage to be too high. In this case, a regenerative braking resistor needs to be added separately. Whether a regenerative braking resistor is needed and what size it should be can be determined by referring to the user manual in the corresponding sample.
4. If a servo motor with an electromagnetic brake is selected, the motor's moment of inertia will increase, which must be taken into account when calculating the torque.
