A servo system was chosen primarily for precise positioning control. The servo driver operates in position control mode, and the control block diagram is shown in Figure 1.
Figure 1 Position Closed-Loop Control
I. Currently, the most commonly used servo motor is the three-phase AC permanent magnet synchronous servo motor. This type of motor features good dynamic characteristics and high positioning accuracy. It is called a "servo" motor because an encoder is installed at the end of the motor shaft, and its feedback signal can form a closed-loop control with the servo driver. Commonly used encoders are photoelectric incremental encoders, which can be divided into 6-channel standard encoders, 3-channel low-line encoders, and serial communication bus encoders based on the different characteristics of their output signals.
Standard encoder output signals are: A, B, Z, U, V, W. A and B are orthogonal coded pulses with a 50% duty cycle and a 90-degree electrical phase difference. Based on the lead and lag relationship, the forward and reverse rotation of the motor can be distinguished. Extracting both the rising and falling edges of the A and B signals yields a 4x frequency pulse signal. For example, the commonly used 2500C/T encoder in China, after 4x frequency multiplication, generates 10,000 feedback pulses for one revolution of the motor.
Z is the zero-position pulse (also called the index pulse), which outputs one pulse per revolution as a positioning signal;
U, V, and W are signals used to detect the rotor's magnetic pole position, providing the driver with the servo motor's rotor position information to determine the initial phase of the injected stator winding current, ensuring the servo motor has sufficient starting torque. After normal startup, the servo driver can use the Z pulse to correct the rotor position. The precise rotor rotation position is obtained from the A and B signals. At this point, the servo driver no longer needs the U, V, and W signals, which is the reason for the development of line-saving encoders.
Line-saving encoder: A, B, Z signals and U, V, W signals are output in a time-division manner on the same output line. Although the lead wires are only marked with A, B, Z signals, U, V, W signals are still output on the signal lines marked with A, B, Z first during startup, and then A, B, Z signals are output. This reduces the number of signal output lines of the photoelectric encoder by half, greatly reduces the workload of soldering the plugs, and greatly reduces the probability of errors and failures.
Bus-type incremental encoders require only one pair of differential signal lines, namely PS+ and PS-, which are serial communication signals. They output information such as position, speed, and direction according to proprietary protocols. The wiring is particularly simple, which is a symbol of technological progress. Yaskawa servo systems have been using this technology for a long time, but its application in China is relatively late.
The output signal of a photoelectric encoder is generally transmitted as a differential signal via a line driver chip, such as MC3487 or AM26LS31. Differential driving is a good method for signal transmission because it is not sensitive to noise, can eliminate or reduce common-mode interference, and can also suppress its own electromagnetic interference, thus improving the reliability of long-distance transmission. At the servo driver end, a line receiver is used to receive the differential output of the encoder. The signal transmission diagram is shown in Figure 2.
Figure 2. Schematic diagram of encoder signal transmission
2. Referring to Figure 1, in addition to receiving encoder feedback signals, the servo drive must also receive command pulses from the upper-level controller in position control mode. The upper-level digital controller generally provides pulse and direction signals. Each pulse corresponds to one step of motor feed, and the direction signal controls the forward and reverse rotation of the motor through high and low levels. Correspondingly, the servo drive's position command pulse input method should be set to: pulse + sign.
The position command output by the controller is generally a differential pair signal processed by the wire driver, and the driver generally uses a high-speed optocoupler to receive the position command pulse, as shown in Figure 3.
Figure 3 Position command pulse transmission
The position commands output by the controller (such as CNC systems, PLCs, positioning modules, etc.) are executed by the servo drive unit to drive the servo motor. The number of pulses determines the angle of rotation of the servo motor (or the distance the worktable moves), the pulse frequency determines the motor speed, and within the rated load capacity, the motor output torque is determined by the load; the greater the load, the greater the motor torque. In various specialized and general-purpose CNC machine tools commonly used in industry, the servo system operates in position control mode.
III. The commonly used digital (switch) output signals for servo drives include the following:
SRDY: Servo ready signal. The output is ON if the main power supply is normal and the driver has no alarms; otherwise, the output is OFF.
COIN: Positioning complete signal. ON if the position deviation is less than the parameter setting value, OFF otherwise.
ALM: Alarm signal. When the servo driver detects an abnormality, it outputs ON; otherwise, it outputs OFF (there may be an option where it outputs ON when there is no alarm and OFF when there is a fault).
Digital output ports, commonly opto-isolated outputs, completely isolate the output stage from the internal circuitry of the servo unit. Servo output ports include both fixed-function output ports and programmable-function output ports, which vary between manufacturers.
The output circuit structure is also slightly different. It can output + and - signal terminals independently from the collector and emitter of the optocoupler (as shown in the circuit on the left side of Figure 4), or it can connect the emitter of the output optocoupler internally to the driver, leading out a common terminal for use by all digital output ports (as shown in the circuit on the right side of Figure 4). When the optocoupler output stage is on, there is approximately a 1V voltage drop between the collector and emitter, which does not meet the TTL low-level standard and cannot be directly connected to a TTL circuit.
Figure 4 Digital Output Interface
IV. For digital input interfaces, an independent external switching power supply is often used to power the signal circuit. The controller uses normally open contacts of relays or open-collector transistors and optocouplers for control, as shown in Figure 5.
Figure 5 Digital Input Interface
Miniature relays with low contact resistance should be selected. Two normally open contacts can be connected in parallel to reduce contact resistance and increase reliability.
The commonly used signal for digital input ports is the enable signal SON. Without the enable signal, the servo motor shaft is not locked and can rotate freely; with the enable signal, the motor shaft is locked, and sending a position pulse will cause the motor to move the worktable; sending a pulse without the enable signal will not cause the motor to rotate. The SON signal can be used to control the rotation and stop of the motor, but pulse control is generally used to control the motor's start and stop. Continuous pulses will cause the servo motor to rotate continuously; no pulses will stop the motor. If the CNC system detects an alarm signal from the servo driver or an alarm from other system functional circuits, it will stop sending pulses and disconnect the enable signal.
V. Precautions for using a servo system:
1. The U, V, and W power lines of the servo motor must be connected one-to-one with the U, V, and W terminals of the driver. It is forbidden to change the direction of the motor by swapping the terminals, otherwise the motor may run away or not turn at all;
2. The main power supply for commonly used servo drives is three-phase AC220V. It must be stepped down by a three-phase transformer to convert the three-phase AC380V to three-phase AC220V. Do not connect it directly to a three-phase AC380V power supply.
3. The servo drive plays a crucial role, receiving input signals from the servo motor encoder and simultaneously dividing the received A and B signals and outputting them along with the Z signal to the upper-level controller for position control loop configuration (Note: only some devices require this output signal). One is the input, and the other is the output; don't confuse them.
4. If the main circuit power supply is connected and disconnected using an AC contactor, the contactor should be a model with a resistor-capacitor absorber to absorb the surge voltage generated by the coil.
