A servo amplifier is a controller used to control servo motors. Its function is similar to that of a frequency converter for ordinary AC motors. It is part of a servo system and is mainly used in high-precision positioning systems. Generally, it controls the servo motor through three methods: position, speed, and torque, achieving high-precision positioning of the transmission system. Currently, it is a high-end product in transmission technology.
The function of the servo amplifier is to compare, amplify, and process the input command signal (voltage) with the system feedback signal (voltage), and then output a control current proportional to the deviation voltage signal to the control coil of the servo valve torque motor to control the opening degree of the servo valve core and provide amplitude limiting protection.
Servo Amplifier Function and Principle
The function of a servo amplifier is to combine and amplify multiple input signals and feedback signals. Depending on the polarity of the combined signal, it outputs a corresponding signal to control the servo motor to rotate forward or backward. When the input signal and feedback signal are balanced, the servo motor stops rotating, and the output shaft of the actuator stabilizes at a certain position.
The servo amplifier group consists of a preamplifier, trigger, thyristor main circuit and power supply, as shown in the figure below.
To accommodate complex multi-parameter adjustments, the servo amplifier is configured with three input signal channels and one position feedback signal channel. Therefore, it can simultaneously accept three input signals and one position feedback signal. In simple single-parameter adjustment systems, only one input channel and one feedback channel are used.
In the servo amplifier, the preamplifier combines the three input signals and one feedback signal into a deviation signal, which is then amplified into a voltage signal U22-21 for output. This output voltage is simultaneously converted into trigger pulses by trigger 1 (or 2) to control the thyristor in the thyristor main circuit 1 (or 2) to conduct, thereby applying AC 220V power to the windings of the two-phase servo motor and driving the two-phase servo motor to rotate. When Δ1>0, U22-21>0, trigger 2 and main circuit 2 operate, and the two-phase servo motor rotates forward; when Δ1<0, trigger 1 and main circuit 1 operate, and the two-phase servo motor rotates in reverse. The circuit composition and parameters of the two sets of triggers and the two sets of thyristor main circuits are exactly the same. Therefore, when the input signal and the position feedback current If are balanced, the output U22-21 of the preamplifier is approximately 0, neither trigger outputs a trigger pulse, the thyristors in main circuits 1 and 2 are blocked, the power supply to the two-phase servo motor is disconnected, and the motor stops rotating.
Therefore, it can be seen that the servo amplifier is equivalent to a three-position contactless relay and has a large power amplification capability.
The servo amplifier consists of functional modules such as instruction and feedback comparison processing, zeroing circuit, current limiting circuit, preamplifier, and power amplifier. Its structural block diagram is shown in Figure 2.
The specific circuit schematic of the servo amplifier is shown in Figure 3.
The function of the preamplifier circuit is to compare and amplify the command and feedback input signals. K<sub>0</sub> and F<sub>bk</sub> are the input and feedback signals, respectively. The gain of the circuit is adjusted by potentiometer J<sub>b</sub> to meet the requirements of the power amplifier circuit, ensuring voltage matching between the preceding and following stages. The zero-adjustment circuit adjusts the circuit's reference voltage by superimposing an adjustable voltage onto the preamplifier circuit. Zero-bias compensation is performed by adjusting potentiometer J<sub>b</sub> to overcome the bias of the servo amplifier system.
The current-limiting circuit limits the maximum current flowing through the servo valve coil, preventing coil overload, protecting the servo valve, and limiting the maximum flow rate of the hydraulic system. This circuit consists of operational amplifiers U1D and U2A, diodes D1 and D2, and an adjustable voltage source. The amplitude of the input voltage to the power amplifier stage is adjusted via potentiometer R1 to limit the output current. The power amplifier circuit converts and amplifies low-power voltage signals into higher-power current signals to provide sufficient rated current for the servo valve to drive the load. It also requires good anti-interference capabilities and static and dynamic performance. This circuit utilizes the interconnected base and emitter of NPN and PNP transistors, with the signal input from the base and output from the emitter. The circuit can be viewed as a push-pull power amplifier circuit composed of two emitter followers, operating during the positive and negative half-cycles of the input signal. Additionally, a high-frequency dithering signal generated by an 8038 chip can be superimposed on the output current to improve the servo valve resolution and prevent valve core jamming due to Coulomb friction.
Servo amplifier parameter requirements
As an electronic device that drives electro-hydraulic servo valves, the servo amplifier has certain requirements regarding its parameters:
(1) The input voltage is within ±10V, which facilitates control by computer and programmable controller and other instruction elements;
(2) The output current is adjustable from ±10 to ±100n to adapt to various types of torque motor servo valves;
(3) The linearity error of the servo amplifier is less than 3% Fs;
(4) It has a feedback access terminal to form a closed-loop control system;
(5) To adapt to the high-frequency response characteristics of the servo system, the bandwidth of the servo amplifier is greater than 1200Hz;
(6) It has maximum output current limiting and output short circuit protection functions, which can limit the maximum flow of the servo valve and prevent the output line from short-circuiting and causing failure.
Servo amplifier output current calculation
The servo valve coil serves as the load for the servo amplifier, equivalent to an inductive impedance consisting of a 0.3H inductor and an 80Ω resistor. To ensure the output control current of the power stage is proportional to the input voltage signal, a resistor is used in series with the load coil. The voltage across this resistor is fed back to the inverting input of the amplifier, achieving closed-loop control and precisely adjusting the power stage output current. Because the power stage feedback voltage is generated by current, this is called current negative feedback. With the introduction of current negative feedback, within the rated load range, changes in load impedance have virtually no impact on the power stage output current, making the servo amplifier essentially a constant current source.
Based on the principles of virtual short and virtual open circuits of operational amplifiers, we can deduce that:
In the formula, K is the gain of the preamplifier circuit.
The output current of the servo amplifier is linear with the input voltage and is independent of the load, tending towards a constant current source.
