Abstract: Permanent magnet synchronous motors are increasingly widely used. This paper proposes a design scheme for a motor servo control system based on DSP and power modules; the hardware and software designs of the servo control system are completed. The system uses the TMS320LF2407A digital signal processor as the core unit to complete the hardware design of the motor servo control system. On this hardware platform, the software of the servo control system is written using C2000 series assembly language, thus completing the design of the control system.
Keywords: electric motor; servo system; DSP; power module
Abstract: At present, the permanent magnet synchronous machine is more and more widely used. A new motor servo-controller based on DSP and power module is designed in this paper. The system regarded the DSP TMS 320LF2407 as the core unit to complete the hardware design of the motor servo-controller, and then finish the software design using C2000 assembly language based on the hardware platform to implement the design of the controller.
Keywords: Motor; Servo-Control; DSP; Power Module
1 Introduction
With the development of science and technology, humanity has made tremendous progress in many fields such as microelectronics, computers, power electronics, and motor manufacturing, directly driving the rapid development and widespread application of servo control technology. Its applications are increasingly widespread in various CNC equipment, industrial robots, large-scale integrated circuit manufacturing, transportation, manned spacecraft, power tools, and household appliances. This paper proposes a design scheme for a motor servo control system based on DSP and a power module.
2. Overview of Motor Servo Control System
Functionally, the hardware of the control system consists of two main parts: the control board and the driver board. The control board is responsible for control calculations, variable acquisition, interface display, and data communication. Figure 1 shows the block diagram of the control system.
Figure 1. Hardware implementation block diagram of the servo control system
The main control unit on the control board is the TI TMS320LF2407A, a dedicated DSP for motor control. Its peripheral circuitry includes an ADC module for detecting three-phase AC current, a rotary transformer position detection module for detecting the motor's rotor position, a panel LCD control system, and a serial communication circuit. The driver board converts the weak current control signals output from the control board into strong current signals with a certain driving capability and outputs them to the motor. It mainly consists of a three-phase inverter bridge and its drive and protection current circuits. The use of an integrated intelligent power supply module (IPM) with integrated three-phase bridge arms and drive protection circuitry simplifies the design of the driver board's peripheral circuitry and ensures reliability.
3 Control System Hardware Design
The hardware of the control system mainly consists of two parts: the control board and the driver board. The control board is based on the TMS320LF2407A as its processing core. It comprises a minimum DSP system, a digital-to-analog converter (ADC), a rotary transformer position converter (RDC), an LCD control system, and an RS232 transceiver. The control board is the core of the entire control system.
3.1 DSP Minimum System
The DSP minimum system refers to the simplest hardware design that enables the DSP core to operate normally and allows for debugging of the DSP. It includes the DSP chip itself, power supply design, reset circuit design, JTAG interface design, and external RAM design for debugging. The DSP minimum system in this control system is designed according to the circuits recommended by TI, as shown in Figure 2.
Figure 2 DSP Minimum System Design
As shown in the figure, the power management chip selected is TI's TPS7333Q. It can not only provide +3.3V, 500mA power to the DSP and other low-voltage peripherals, but its pin 8 (/RS) can also output a low-pulse power-on reset signal with a width of 200ms to reset the DSP at the same time as power-on.
3.2 Rotor Position Detection Module Design
High-precision servo control systems for motors require high-precision rotor pole position detection signals to meet the high-precision control needs of the servo system . This necessitates the system being equipped with high-performance rotor pole position detection elements. Currently, commonly used detection elements include photoelectric encoders and rotary transformers. While photoelectric encoders offer high accuracy, their initial position is uncertain, and they are expensive, have lower reliability, and require sophisticated mechanical installation, thus limiting their application. Rotary transformers, on the other hand, have a fixed initial position, a robust and simple structure, low cost, and high detection accuracy. Furthermore, the signal transmitted between the transformer and the conversion chip is a low-frequency sinusoidal signal, making signal transmission less susceptible to noise and providing strong anti-interference capabilities. Therefore, they are increasingly used in high-performance AC servo systems.
The motor servo control system in this paper also uses a variable reluctance rotary transformer to detect the motor rotor position. To match the 12-pole permanent magnet synchronous motor, a 12-pole rotary transformer was also selected. The rotary transformer rotor is coaxial with the permanent magnet synchronous motor rotor and rotates with it, while the stator is pressed firmly against the motor housing and remains stationary. In this case, the rotor position of the rotary transformer reflects the rotor position of the motor in real time. Then, an Analog Devices AD2S83 rotary transformer signal conversion chip is used to convert the two sinusoidal analog signals output by the rotary transformer along with its excitation signal to obtain the actual rotor position values of both the rotary transformer and the motor. Furthermore, by collecting two rotor position values at specific intervals, the motor speed can be calculated.
3.3 Analog Input Acquisition, Forward Channel, and Digital-to-Analog Converter (ADC) Circuit Design
The motor servo control system requires real-time acquisition of the instantaneous values of the motor's three-phase current and DC-side voltage. Electrically, the instantaneous voltage and current values of the motor are high-voltage quantities and cannot be directly input to the control board; they must undergo strong-to-weak current isolation and corresponding conversion. In the servo control system design presented in this paper, multiple voltage and current Hall sensors are used to achieve strong-to-weak current separation; and the actual analog values are acquired by inputting them to a digital-to-analog converter (ADC) via an analog forward channel. Figure 3 shows the current Hall sensor circuit design.
Figure 3 Current Hall sensor circuit
In the diagram, pins 2 and 6 of the Hall sensor U17 are connected, and pins 3 and 5 are connected. The detection current flows in from pin 1 and out from pin 4. With this configuration, the maximum effective value of the primary-side detection current input is 8A, and the corresponding maximum effective value of the secondary-side conversion current output is 12mA. Pins 7 and 8 are the power supply pins of the Hall sensor, which have a separate power supply that is not grounded with the analog signal. A load resistor with a resistance of 68Ω is connected between pins 11 and 9. Its function is to balance the output and convert the output current signal into a voltage signal that is easy to detect. In addition, considering that the obtained voltage value is very low and the measurement error is large, a non-inverting amplifier composed of TL084 operational amplifiers is added to the Hall sensor.
3.4 Inverter Bridge with IPM and Isolation Design
For AC motor control systems, the inverter bridge is essential. Figure 4 shows the schematic diagram of a three-phase inverter bridge composed of six IGBTs. To simplify the design and increase the system's reliability, the inverter bridge in this servo control system uses a Mitsubishi Electric IPM, model PM50CSD060, with typical parameters: VCES=600V, IC=50A, Tdead=2.5μs.
Figure 4. Schematic diagram of a three-phase inverter bridge
The IPM contains the IGBTs and anti-parallel diodes required for a three-phase inverter, and the three phases are already connected. Simply connect the P and N leads to the two ends of the DC-side capacitor, and connect the U, V, and W terminals to the three-phase input terminals of the motor. For the PM50CSD060, in addition to six PWM signal inputs, there are four fault protection signal outputs: three for upper bridge arm faults and one for a total lower bridge arm fault.
The IPM driver requires four isolated power supplies. The three IGBTs on the upper bridge arm require three independent power supplies without a common ground, while the three IGBTs on the lower bridge arm are common-emitter and only need to share one power supply. Therefore, the isolated driver power supply for this system uses four 5V-15V DC-DC converters with a maximum output of 100mA. For the driver board design using the IPM, the design of the PWM drive section and the IGBT protection circuit is greatly simplified.
3.5 Other Design Elements
Because the control system needs to communicate with the host computer via an RS232 serial port and receive commands from the control system panel, a separate microcontroller W78E58P is used in the control system for ease of programming and debugging. This microcontroller enables LCD display, keyboard input, and coordinates communication between the DSP and the host computer. In terms of circuit connections, a double-pole double-throw relay is used between the DSP, W78E58P, and RS232 transceiver. Its open/closed state depends on whether an external serial communication line is connected to the control system. Specifically, when the control system is connected to the PC using an RS232 serial communication line, the relay connects the DSP and the RS232 transceiver, enabling communication between the control system and the PC, and sending commands to the control system via a dedicated host computer control program. When the communication line is disconnected from the control system, the relay disconnects the RS232 transceiver and connects the DSP and W78E58P, allowing a direct TTL level connection between them. In this case, the DSP main control system communicates with the W78E58P and receives commands via the panel keyboard.
Using DSP as the core of the control system and IPM to implement the inverter bridge main circuit design, a high-precision, high-performance, and reliable motor servo control system hardware design was achieved.
4. Control System Software Design
Leveraging the powerful computing capabilities and efficient instruction set of the TMS320LF2407A, the software for the control system can be easily designed. Functionally, the software of the motor servo control system mainly comprises four parts: a communication program with the host computer, an analog signal timing acquisition program, a core control algorithm program, and a PWM generation program. Its software structure diagram is shown in Figure 5. Considering the heavy computational load, and to achieve real-time performance and high efficiency in motor control, the entire software of the motor servo control system in this paper adopts the assembly language architecture based on the TMS320LF2407A.
Figure 5 Control System Software Block Diagram
The diagram above clearly illustrates the software architecture of the entire control system. First, the servo control system continuously scans for commands from the host PC or the front panel of the control system, and performs corresponding operations based on the command analysis. Commands are broadly divided into two categories: one is reading and writing motor parameters, i.e., writing specific motor parameters into the EEPROM for the control program to use during calculations, or reading motor parameters from the EEPROM and displaying them on the screen; the second category is motor control commands, including position control mode, speed control mode, torque control mode, and commands such as locking the motor, starting the motor, and stopping operation. When the control system receives a motor control command, it calculates the instantaneous three-phase voltage values of the motor and generates the actual three-phase voltage to control the motor by outputting PWM. Furthermore, all the current, voltage, and current motor speed values required for the calculations are obtained through a timed acquisition program.
The author's innovations are as follows: The hardware system of the servo control system was designed using the TMS320LF2407A digital signal processor and the intelligent power module (IPM), and the hardware debugging was completed; on this hardware platform, the software of the servo control system was written and debugged using the C2000 series assembly language, and the overall design of the control system was completed.
References:
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