Abstract: Servo motors are increasingly widely used as actuators for precise positioning control. Servo amplifiers, as the drive and control components of servo motors, form a complete servo system. Servo motors offer advantages such as high starting torque, high control precision, fast dynamic response, low inertia, low noise, high efficiency, and easy maintenance, making them increasingly important in industrial automation and showing a strong trend of replacing AC motors. This article uses Mitsubishi's servo system as an example to explain the application methods of servo motors in industry.
Keywords: servo motor; Mitsubishi servo system; servo parameter setting
Research of using Mitsubishi servo system
ZHAO Pei-qing
(SHANDONG University, Jinan 250061, China)
Abstract: Servo motors are generally used as precise positioning actuator, and which are used more and more widely. Servo amplifiers are some sections which can drive servo motors and can form a complete servo system with servo motors. Servo motors have big starting torque, high control accuracy, fast dynamic response, small inertia, low noise, high efficiency, easy maintenance and so on, so that servo motors play an increasingly important role in the production of industrial automation, which will replace AC motor. In this essay, the author will use Mitsubishi servo system as an example to explain the method of servo motor applications in industrial production.
Key words: servo motor; Mitsubishi servo system; servo parameter setting.
Introduction: Servo motors are increasingly widely used in industrial production, making the study of their usage methods and techniques imperative. This article first introduces the overview of servo motors, and then focuses on the usage experience and precautions of a specific Mitsubishi servo motor and servo amplifier model. Since Mitsubishi's servo systems are structurally similar, it is feasible to extend this experience to the entire Mitsubishi servo system.
1. Overview of Servo Motors
1.1 Definition and Classification of Servo Motors
The word "servo" originates from the Greek word for "slave." People envisioned servo mechanisms as docile, obedient tools that act according to control signals. Before a signal arrives, the rotor remains stationary; after the signal arrives, the rotor immediately begins to rotate; and when the signal disappears, the rotor stops immediately. It is named for its "servo" performance. A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value). The main task of a servo is to amplify, transform, and regulate power according to control commands, making the torque, speed, and position control of the drive device highly flexible and convenient.
The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, it serves as an actuator, possessing characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. It can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are divided into two main categories: DC servo motors and AC servo motors. Their main characteristic is that they do not rotate when the input signal voltage is zero, and their speed decreases uniformly as the torque increases.
The advantages of DC servo motors are: precise speed control, strong torque-speed characteristics, simple principle, ease of use, and low price. The disadvantages are: a series of problems caused by brush commutation, speed limitations, additional resistance, and the generation of wear particles (especially in cleanrooms).
The advantages of AC servo motors are:
1. It has excellent speed control characteristics, achieving smooth control with virtually no oscillations across the entire speed range;
2. It has high efficiency, reaching over 90%, and does not generate heat;
3. Enables high-speed control;
4. Enables high-precision position control;
5. Capable of achieving constant torque within the rated operating range;
6. Low noise;
7. No brush wear, maintenance-free;
8. It does not produce abrasive particles or sparks, making it suitable for cleanrooms and explosive environments;
9. Low inertia.
1.2 Development History and Current Status of Servo Motors
Since the Indramat division of Rexroth GmbH of MANNESMANN in Germany was established at the 1978 Hanover Trade Fair...
Figure 1 Servo Motor
The official launch of the MAC permanent magnet AC servo motor and its drive system marked the entry of the new generation of AC servo technology into the practical application stage. By the mid-to-late 1980s, various companies had complete product series. The entire servo device market shifted to AC servo systems. Early analog control systems had shortcomings in areas such as zero drift, anti-interference, reliability, accuracy, and flexibility, and could not fully meet the requirements of motion control. In recent years, with the application of microprocessors and new digital signal processors (DSPs), digital control systems have emerged, and the control part can be entirely controlled by software.
To date, most high-performance servo systems employ permanent magnet synchronous AC servo motors, and their control drives utilize fully digital position servo systems for fast and accurate positioning. Typical manufacturers include Siemens (Germany), Kollmorgen (USA), and Panasonic and Yaskawa (Japan).
2. Brief Introduction to the Working Principle of Servo Motors
Since AC servo motors are the most widely used, the following section mainly introduces their structure and working principle. AC servo motors are typically single-phase asynchronous motors, with two structural forms: squirrel-cage rotor and cup rotor. Like ordinary motors, AC servo motors also consist of a stator and a rotor. The stator has two windings: an excitation winding and a control winding, which are spatially separated by 90° electrical degrees.
The working principle of an AC servo motor is not fundamentally different from that of a single-phase induction motor. However, an AC servo motor must possess one crucial characteristic: it must overcome the so-called "self-rotation" phenomenon. This means it should not rotate without a control signal, and especially if it is already rotating, it should stop immediately if the control signal disappears. In contrast, a regular induction motor, once started, often continues to rotate even after the control signal is lost.
When the motor is initially stationary, if no control voltage is applied to the control winding, only the excitation winding is energized, generating a pulsating magnetic field. This pulsating magnetic field can be considered as two circular rotating magnetic fields. These two circular rotating magnetic fields rotate in opposite directions with the same magnitude and speed. The resulting forward and reverse rotating magnetic fields cut the cage winding (or cup-shaped wall) and induce electromotive forces and currents (or eddy currents) of the same magnitude but opposite phases. The torques generated by these currents interacting with their respective magnetic fields are also equal in magnitude and opposite in direction, resulting in a net torque of zero, preventing the servo motor rotor from rotating. Once the control system receives a deviation signal, the control winding must receive a corresponding control voltage. Under normal circumstances, the magnetic field generated inside the motor is an elliptical rotating magnetic field. An elliptical rotating magnetic field can be considered as the resultant of two circular rotating magnetic fields. These two circular rotating magnetic fields have unequal amplitudes (the forward rotating magnetic field, rotating in the same direction as the original elliptical rotating magnetic field, is larger, while the reverse rotating magnetic field, rotating in the opposite direction, is smaller), but they rotate in opposite directions at the same speed. The electromotive force and current induced by the cutting of the rotor windings, as well as the resulting electromagnetic torque, are in opposite directions and of unequal magnitude (larger for forward rotation, smaller for reverse rotation). The resultant torque is not zero, so the servo motor rotates in the direction of the forward magnetic field. As the signal strengthens, the magnetic field approaches a circle. At this point, the forward magnetic field and its torque increase, while the reverse magnetic field and its torque decrease, resulting in a larger resultant torque. If the load torque remains constant, the rotor speed increases. If the phase of the control voltage is changed, i.e., shifted by 180°, the direction of the rotating magnetic field reverses, and therefore the direction of the resulting torque also reverses, causing the servo motor to reverse. If the control signal disappears, and only current flows through the excitation winding, the magnetic field generated by the servo motor will be a pulsating magnetic field, and the rotor will quickly stop.
To enable the AC servo motor to stop rotating immediately upon the disappearance of the control signal, its rotor resistance is made exceptionally high, resulting in a critical slip greater than 1. During motor operation, if the control signal drops to "zero," the excitation current still exists, generating a pulsating magnetic field in the air gap. This pulsating magnetic field can be considered as a combination of a forward-rotating magnetic field and a reverse-rotating magnetic field. Once the control signal disappears, the motor generates a braking torque opposite to the rotor's original direction of rotation. Under the combined action of the load torque and the braking torque, the rotor stops rapidly. It must be noted that ordinary two-phase and three-phase asynchronous motors normally operate in a symmetrical state; asymmetrical operation is a fault condition. However, AC servo motors can achieve control through varying degrees of asymmetrical operation. This is the fundamental difference between AC servo motors and ordinary asynchronous motors in operation.
A servo motor is a typical closed-loop feedback system. The rotor inside the servo motor is a permanent magnet. The three-phase electricity (U/V/W) controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of a servo motor depends on the accuracy (line count) of the encoder.
3. Mitsubishi Servo System Hardware Introduction
Servo motors are increasingly widely used as actuators for precise positioning control. Servo amplifiers, as drive and control components of servo motors, form a complete servo system with servo motors. Servo amplifiers can receive pulse drive signals from PLCs, amplify the signals, and then output the processed signals to servo motors.
Figure 2 Connection diagram of Mitsubishi PLC and Mitsubishi servo system
Figure 2 shows a wiring example for a PLC to send pulses to a servo amplifier, which then processes the signal and outputs it to a servo motor. As shown in the figure, the PLC model is FX3U, the servo motor model is HF-KN23J-S100, and the servo amplifier model is MR-E20A-KH003. This servo amplifier must be used with the aforementioned servo motor. For specific usage instructions, please refer to the Mitsubishi servo amplifier user manual. The CNP1 terminal block of the servo amplifier should be connected to the power input; the CNP2 terminal block is connected to the servo motor via a power cable to drive the servo motor; the CN2 terminal block is connected to the servo motor via an encoder cable, and the encoder of the servo motor feeds back the signal to the servo amplifier via the encoder cable to achieve closed-loop control of the servo motor; the CN1 terminal block is the control signal terminal block. The above-mentioned servo amplifier models have three control modes for the servo motor: position control mode, speed control mode, and position/speed control mode. The actual functions of each control signal terminal of the servo amplifier vary slightly depending on the control mode. Please refer to the Mitsubishi servo amplifier user manual for details (some Mitsubishi servo amplifiers have six control modes for the servo motor, such as the MR-J2S-A series). In Figure 1, in position control mode, terminals 1 and 2 of CN1 are VIN (power input for digital I/F) and OPC (open collector power input), respectively. These two terminals need to be connected to a +24V power supply. Terminal 3 is RES (reset). Terminals 8 and 13 are EMG (external emergency stop) and SG (common terminal for digital I/F), respectively. To ensure that the servo motor can work normally, the EMG terminal must be connected to 0V, and SG, as the common terminal for digital I/F, must also be connected to 0V. Terminals 23 and 25 are PP (forward pulse train) and NP (reverse pulse train), respectively.
In order for the servo system shown in Figure 2 to work properly, five points need to be explained.
1. The COM terminal of the PLC's output Y0 must be connected to the common COM terminal of the entire system; otherwise, the pulse train sent by the PLC cannot be recognized by the servo amplifier.
2. Terminal 4 of CN1 in the servo amplifier is SON (servo ON). If terminals 4 (SON) and 13 (SG) are short-circuited, the main circuit is powered on and enters the runnable state (servo ON state). If terminals 4 (SON) and 13 (SG) are disconnected, the main circuit is open-circuited, and the servo motor idles (servo OFF state). To short-circuit terminals 4 and 13, the common method is not to connect them together externally, but to change the P41 parameter value of the servo to "□□□1", which will automatically make the servo always ON internally.
3. The servo amplifier has three selectable pulse train input formats and can select positive or negative logic. The pulse train input format can be selected through parameter P21. The specific meaning of parameter P21 is shown in Figure 3.
Figure 3. Specific meaning of parameter P21
In the example above, the pulse train sent by the PLC to the servo amplifier is a signed pulse train with negative logic. Therefore, terminals 23 and 25 of CN1 are the pulse input terminal and pulse direction terminal, respectively. Thus, the P21 parameter value should be set to "0011", as shown in Figure 4. In actual servo motor debugging, it is common to encounter situations where the servo motor's rotation direction is exactly opposite to the desired direction. If the servo motor is already installed and the wiring is complete, using a signed pulse train format only requires changing the pulse positive/negative logic selection in the P21 parameter to change the servo motor's rotation direction, without needing to modify the already installed mechanical and electrical components.
Figure 4. Pulse train input format and P21 parameter diagram
4. Terminals 6 and 7 of CN1 are LSP (end of forward stroke) and LSN (end of reverse stroke), respectively. These two terminals need to be used in conjunction with limit switches to fulfill the limit requirements. During operation, LSP and SG, and LSN and SG should be short-circuited. If they are disconnected, an emergency stop will be initiated and the servo motor will be locked, as shown in Figure 5. If parameter P41 is set according to Figure 6, it will internally be changed to automatic ON.
5. Terminal 9 of CN1 is ALM (fault). When no alarm occurs, ALM and SG are connected within 1 second of power-on. When an alarm occurs, ALM and SG are disconnected.
4. Introduction to Mitsubishi Servo Parameter Settings
4.1 Mitsubishi Servo Parameter Settings
In the servo amplifier described in this article, the basic parameters (P0-P19) differ from extended parameters 1 (P20-P49) and extended parameters 2 (P50-P84) in usage, based on parameter safety and operating frequency. The basic parameters can be set and changed by the user at the factory default state, while the extended parameters cannot be set or changed. For more advanced operations, the extended parameters can be modified by changing the value of parameter P19. It is important to note that after setting the P19 parameters, the power must be disconnected and then turned on again for the settings to take effect.
As mentioned above, although servo amplifiers have many parameters, only a few are frequently used. Below are some of the most commonly used parameters.
1. Parameter P0. The function of parameter P0 is to select motor power, select optional regenerative components, select motor series, and select control mode.
2. Parameter P1. The function of parameter P1 is to select the input filter. If the external input signal oscillates due to noise or other reasons, the input filter is used to suppress it.
3. Parameter P2. Parameter P2 is used to select the response speed during auto-tuning and to select the gain adjustment mode. Regarding the response speed during auto-tuning, choose a smaller value when mechanical vibration or loud gear noise occurs, and choose a larger value when it is necessary to shorten the stopping and settling time to improve performance.
4. Parameter P3. Parameter P3 is the numerator value of the electronic gear ratio.
5. Parameter P4. Parameter P4 is the denominator of the electronic gear ratio. The electronic gear ratio is P3/P4, and it should satisfy: 1/50.
6. Parameter P5. Parameter P5 sets the range of the output positioning completion signal. It is set in units of the command pulses before the electronic gear calculation. The initial default value is 100 PULSE.
7. Parameter P6. Parameter P6 sets the gain of position loop 1. A larger gain results in better tracking of position commands. During automatic adjustment, this parameter will be automatically set to the result of the automatic adjustment.
8. Parameter P19. The function of parameter P19 is to select the readable and writable range of the extended parameter.
9. Parameter P21. The function of parameter P21 has been explained previously; it selects the waveform of the pulse train input signal.
10. Parameter P41. The function of parameter P41 has been introduced earlier; it sets SON, LSP, and LSN to be automatically ON.
4.2 Electronic Gear
As shown in Figure 7, assuming the electronic gear ratio of the servo amplifier is set to N, then if the PLC (or host computer) sends one pulse to the servo amplifier, the servo amplifier will send N pulses to the servo motor. If the electronic gear ratio is set to 30, and the PLC sends 100 pulses, the actual number of pulses sent to the servo motor after passing through the servo amplifier should be 100 * 30 = 3000.
Figure 7 Definition of electronic gear ratio
The specific function of an electronic gear ratio is to set the workpiece movement amount, equivalent to one pulse output from a command controller, to any value. In practical applications, it connects different mechanical structures, such as ball screws and worm gears. Due to differences in parameters such as pitch and number of teeth, the amount of motor rotation required for the minimum unit movement varies.
The following simple example illustrates the application of electronic gear ratios. Assume a servo motor drives a roller, with the roller rotating 1 revolution for every 5 revolutions of the servo motor. The roller's diameter is 120mm, and the encoder resolution of the servo amplifier is 131072p/r. The requirement is that each pulse sent by the PLC should rotate the roller by 0.01mm. In this case, the electronic gear ratio should be set as shown in the following formula:
5. Servo Motor Selection Calculation
1. Confirmation of rotational speed and encoder resolution.
2. Calculation of load torque on motor shaft and calculation of acceleration and deceleration torque.
3. Calculate the load inertia. The smaller the inertia, the better, as this improves accuracy and response speed.
4. Calculation and selection of regenerative resistors. Generally, for servo motors with a power of 2kW or higher, a regenerative resistor should be installed externally.
5. Cable Selection. The encoder cable uses a twisted-pair shielded cable. For Japanese products such as Mitsubishi servo encoders, the absolute encoder has 6 cores, while the incremental encoder has 4 cores.
6. Conclusion
This article introduces the usage methods and experience of a certain model of servo motor and servo amplifier from Mitsubishi Corporation. Since Mitsubishi's servo systems are structurally similar, these methods and experiences can be applied to other models of Mitsubishi's servo systems.
