Design of robot multi-axis motion controller based on DSP and voltage feedback
introduction
For robot control technology, real-time and stability are the focus of research. At this stage, the main method of robot control is to plan the gait offline, and to compensate for the robot's motion on the host computer in real time. This processing method puts forward a high calculation speed and processing capacity of the processor. Requirements. Most traditional robot controllers use 80C196 series single-chip microcomputers as processors. When a 12MHz crystal oscillator is used, the state cycle is 167ns and the machine cycle is 1ms, which cannot meet the needs of robot control.
The processing speed of DSP chip can reach several nanoseconds or even higher, which is very suitable for robot control. Therefore, in this paper, DSP is used to replace the original single chip microcomputer, and at the same time, with the help of the underlying voltage feedback technology, a distributed robot multi-axis motion controller is designed.
Control system structure and function
The structure of the robot control system is a typical "PC + motion controller" mode, in which the main control computer requires small size and fast operation speed, and usually uses an embedded industrial control computer. Mainly responsible for the online motion planning, action-level motion control, voice interactive control, visual guidance control and human-computer interaction of the entire system. The bottom-level control unit takes the controller as the core and uses voltage feedback to control each motion axis system. The specific structure is shown in Figure 1. The controller and main control computer are connected via CAN bus. This communication method reduces the complexity of connection, improves the speed of communication, and enhances the stability of the system. In addition, since only two wires are used for communication, the structure is also very flexible.
The controller is the core of the entire control system. By receiving the control command of the main control computer, it controls the axis system of each joint, and at the same time feeds back the underlying information to the main control computer to realize large loop feedback, so that the main computer can coordinate planning and unify management. The control performance of the controller is directly related to the movement ability of the robot. This article specifically designed a multi-axis motion controller based on DSP and voltage feedback.
Controller design
The various movements of the robot are completed by each joint axis system, each axis system has one degree of freedom, and can complete the rotation task in a certain direction, and all axis systems can complete relatively complex motions by coordinating work at the same time. All the shaft systems studied in this paper are driven and controlled by PWM pulse signals.
The structure of this multi-axis motion controller is shown in Figure 2. The entire controller takes DSP as the core and is divided into four parts: the DSP main processor module mainly completes information processing and various control functions; the A / D conversion module provides the position information of the motion axis system for the main processor; the peripheral circuit completes the voltage Monitoring, parameter storage, circuit decoding, photoelectric isolation and other functions; the communication module is responsible for information exchange with the main control computer.
DSP main processor module
The DSP main processor is the core of the entire controller and is responsible for the processing of various information. At the same time, it communicates with the main control computer. Its calculation speed and information processing capability directly affect the performance of the controller. This article selects TI's TMS320LF2407A chip, which integrates real-time processing capabilities and controller peripherals in one, and is very suitable for industrial control.
In the controller designed in this paper, the working voltage of TMS320LF2407A is 3.3V, the system clock is 40MHz, the data bus is connected to the A / D conversion module, and the A / D conversion result is received; the address bus and the control bus and the decoding part in the peripheral circuit Connected to complete various decoding and control functions; PWM output port provides control signals for the motion axis system; the communication interface is connected to the communication module and is responsible for interacting with the main control computer; the interrupt interface is connected to the decoding circuit and accepts A / D conversion The external interrupt triggered by the end signal; the reset interface is connected to an external watchdog circuit, and the main processor is reset when the external voltage exceeds the specified range; the serial interface is connected to an external memory, and the parameters required for the control algorithm are read from the external memory.
A / D conversion module
The external voltage sensor converts the position information of the moving shaft system into a voltage signal, and the A / D conversion module performs A / D conversion on this voltage signal to provide a digital signal that can be recognized by the main processor. The A / D conversion circuit designed in this paper is shown in Figure 3. The ADG508A is a multi-selector with 8 selections, selects the input multi-channel voltage, and selects a voltage to send to the converter. AD622 is a voltage amplifier and has a filtering function. It can amplify the input voltage of -5V ~ + 5V to -12V ~ + 12V to improve the accuracy of A / D conversion. AD976 is a 16-bit A / D converter that converts the amplified voltage into a + 5V digital signal. 74LVTH245 is an 8-bit level converter, which converts + 5V digital signals into + 3.3V digital signals that the main processor can accept and sends them to TMS320LF2407A. When using it, two parallel blocks are required to form a 16-bit level converter.
Peripheral circuit module
The peripheral circuit module is mainly responsible for functions such as voltage monitoring, parameter storage, circuit decoding, and photoelectric isolation of the controller. Among them, the task of voltage monitoring is completed by an external watchdog circuit. This article uses the DS1834A chip. This chip can monitor + 5V and + 3.3V voltage on the circuit board at the same time, the voltage safety range can be adjusted, and it also has a manual reset function. If the voltage exceeds the safe range, a low-level reset signal is generated at the corresponding pin, and the reset pin does not return to the high level until 350 ms after the voltage returns to normal. If manual reset, two reset pins generate reset signal at the same time.
The parameters required for the DSP main processor to execute the control algorithm are stored in the external memory X25650. The DSP is connected to the X25650 through a serial peripheral interface (SPI port) module to read the required parameter data from it.
The circuit decoding function is implemented by a CPLD. The main functions include providing a port selection signal for ADG508A, a conversion start signal for AD976, an output enable signal for 74LVTH245, receiving an AD976 conversion end signal, and providing an A / D conversion end interrupt signal for DSP , Receive the reset signal of DS1834A and provide reset pulse signal for DSP and so on.
The optoelectronic isolation device uses high-speed optocoupler 6N137, which mainly provides isolation between the controller and the external environment, including the isolation of PWM pulse control signals and the isolation of CAN bus communication.
Communication module
The communication module mainly communicates with the host computer, receives commands from the host computer and provides necessary data to the host computer. Because the TMS320LF2407A has a CAN bus communication module inside, it only needs an external CAN transceiver to communicate with the outside world. The transceiver uses the SN65HVD230 chip, which is compatible with the traditional CAN transceiver PCA82C250 and supports 3.3V voltage, which can be directly connected to the TMS320 LF2407A. In order to improve the communication quality, an optocoupler should be used for isolation.
Control flow and analysis
This controller can provide control functions for 8-way shafting at the same time. Its working process is shown in Figure 4. The functions enclosed by the dotted lines in the figure are completed by the controller.
The specific working process is: the system starts to run and completes the initialization work; the voltage sensor converts the position information of the execution shaft system into a voltage signal; the DSP main processor sends a selection signal to the ADG508A, and the gated voltage signal is amplified by the amplifier AD622 and sent to the AD976 A / D conversion. Which voltage is selected is controlled by the address line of the main processor after decoding the ADG508A port selection bit, which is usually an eight-way alternate selection. The conversion start of AD976 is also controlled by the main processor. When the conversion ends, an end signal is sent out. After decoding, it is provided to the main processor to generate an external interrupt. The main processor reads the conversion by starting the level conversion circuit in the interrupt subroutine. The data. The highest conversion rate of AD976 is 100KSPS. At this rate, 8 channels are converted at the same time. The conversion rate of each channel is 12.5KSPS, which can fully meet the needs of control.
Conclusion
The controller designed in this paper adopts the way of first selecting the route, then amplifying and finally converting to provide the required position information for the DSP main processor. The whole controller has a simple and flexible structure, stable and reliable operation, and is very suitable for robot control. This method has been proved to be effective by practice. This controller can control up to 8 axis systems at the same time, and can be selected according to the actual situation. The single channel conversion rate is up to 100KSPS. At the same time, this design can also be easily transplanted to other similar control mechanisms to control various PWM pulse driven shafting, it is a multi-functional universal controller.
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