High reliability analysis and design of intelligent controller based on DSP

　　High reliability design of hardware system

　　1 Hardware Circuit Design

　　The hardware circuit principle is shown in Figure 1. In the specific design, each part should consider the anti-interference problem to minimize the impact of interference on the performance of the entire system and ensure that the system has sufficiently high reliability.

　　Figure 1 Schematic diagram of the hardware circuit of the intelligent controller

　　①DSP part

　　This controller is based on TI's TMS320F2812 (hereinafter referred to as F2812), which is a high-performance, multi-functional, cost-effective 32-bit fixed-point DSP chip dedicated to control. The circuit design of F2812 focuses on the following issues:

　　● Power supply power-on sequence. F2812 is a low-voltage, multi-power DSP. It must meet the order that the I/O power supply is powered on before the CPU core power supply, and the power-on time difference between the two cannot be too long (generally no more than 1s), otherwise it will affect the service life of the device or even damage the device. This article uses TPS75733KTT and TPS76801Q power chips to design a power module, which meets the special requirements of the above power-on sequence.

　　● System clock. F2812 requires the input clock signal level to be 1.9V (at this time, the main frequency can reach up to 150MHz) or 1.8V (at this time, the maximum main frequency is 135MHz), while the output level of ordinary crystal oscillators is 5V or 3.3V, so the crystal oscillator cannot be used directly to design the system clock. In order to improve the stability and reliability of the overall system operation, this design uses a crystal and two capacitors to form an oscillation circuit with the F2812 on-chip clock module to meet the clock requirements.

　　● Handling of unused input/output pins. Unused input pins cannot be left floating. Key control input pins (such as Ready and Hold) should be fixed to high or low levels. Non-key input pins should be pulled up or down to a fixed level. Unused output pins can be left floating.

　　②Power supply

　　This design takes the following measures for the DC side:

　　● The power supply is divided into two categories: internal and external, and technical measures such as isolation, filtering and grounding are adopted. The internal power supply is responsible for the power supply of the F2812 core system, and a voltage monitor is set up for power supply abnormality protection; while the external power supply is only connected to the external interface.

　　● The analog power supply and digital power supply are separated and powered by independent power supplies respectively.

　　● A two-stage voltage stabilization method is adopted for the rectified DC voltage to ensure that the previous stage voltage stabilizer can still output the specified voltage after being affected.

　　③Input and output channel part

　　The input and output channels are connected to the process, which is the main channel for process interference to enter the DSP system and is also one of the important contents of the anti-interference design of the DSP system. The anti-interference design of the input and output channels mainly adopts isolation measures, which can greatly improve the signal-to-noise ratio on the process channel.

　　④Communication part

　　The F2812 chip has two serial communication interfaces, which can be freely configured as standard serial ports RS-232 or RS-485 according to specific needs. This design uses RS-232, and in order to improve the anti-interference ability of the entire system, a high-anti-interference driver chip MAX3160 is selected, and a high-speed optical coupler is used for isolation.

　　2 PCB circuit board design and production

　　At present, electronic equipment is generally assembled using PCB circuit boards. With the rapid development of integrated circuits and related technologies, the density of components on PCBs is getting higher and higher, and the quality of PCB design and production has an increasingly greater impact on the reliability of DSP systems. Therefore, when designing and producing PCBs, not only the layout of components and circuits should be considered, but also the relevant anti-interference design rules should be met.

　　①PCB layout

　　PCB layout is very important. It not only determines the visual effect of the circuit board and the routing rate of automatic routing, but more importantly, it affects the overall performance of the instrument. Therefore, the layout must be considered comprehensively and follow certain rules, including:

　　● The geometric dimensions of the PCB board should be appropriate. If the size is too large, the line impedance will increase and the anti-noise ability will be reduced. If the size is too small, the heat dissipation will be affected and adjacent lines will be easily interfered.

　　● Components and signals should be partitioned reasonably, strong and weak signals should be separated, digital and analog signals should be separated, and interference sources should be separated from sensitive components;

　　● Arrange the positions of each functional module according to the signal flow as much as possible to make the signal direction consistent;

　　● Component layout should be centered around the core components of each functional module, and the arrangement and welding of components should be considered, and they should not be too dense;

　　②PCB wiring

　　In the PCB design process, wiring is a very important step that requires great skill. The following rules should be followed when wiring:

　　● The wiring directions of two adjacent layers should be as vertical as possible, and ground wire isolation can be added when necessary;

　　● The ground wire and power wire should be as thick as possible to reduce voltage drop and coupling noise;

　　● Digital circuits have high frequencies and analog circuits have high sensitivity. When wiring, analog devices should be kept as far away from digital signal lines as possible, and ground wires should be used to isolate the digital area from the analog area.

　　● The entire PCB board has only one ground node to the outside, while inside the PCB board, the digital ground and analog ground are separated. Usually, the digital ground and analog ground can be connected together at the analog ground pin of the D/A converter;

　　③Power cord design

　　The solution to the interference problem is to place the power supply components separately together, and then completely isolate them from other parts with two thicker ground wires, and then place bypass capacitors and decoupling capacitors near the power supply components to minimize the interference on the output power line. In addition, the power line should be widened as much as possible according to the current size, and the direction of the power line and ground line should be consistent with the data transmission direction as much as possible to improve the system's anti-noise ability.

　　④Ground wire design

　　The noise and interference of electronic systems are closely related to their grounding methods. Good grounding can often solve most interference problems.

　　For low-frequency circuits, the inductance between wiring and components has a relatively small effect, while the loop current formed by the grounding circuit will have a greater impact on interference. At this time, a single-point grounding method should be used to minimize the potential difference on the ground wire; for high-frequency circuits, the ground wire impedance will become very large. At this time, shortening the ground wire length to reduce the ground wire impedance becomes a key issue, so a multi-point grounding method should be used nearby. In addition, the ground wire should be as thick as possible to reduce the ground wire resistance, otherwise, the signal level will be unstable due to changes in the ground potential, thereby reducing the anti-noise ability.

　　⑤Filter capacitor design

　　Select a capacitor of 1uF to 10uF to bridge the power line and ground line at the entrance of the circuit board, which can effectively eliminate low-frequency interference. For high-frequency interference signals, 0.01μF and 0.1μF capacitors can be placed next to the power and ground pins, especially 0.1μF high-frequency capacitors should be directly connected between the power line and ground line of each integrated circuit chip. In addition, ferrite beads can also be used for high-frequency filtering, which is equivalent to a series connection of a resistor and an inductor. Its AC impedance at high frequency is very large, while its DC impedance is very small (close to 0Ω). In this way, high-frequency interference signals are absorbed and consumed in the form of heat.

　　3. Spatial anti-interference problem

　　The main measure to resist spatial interference is shielding. This design adopts the commonly used shielding method, that is, using low-resistance materials to make a shielding cover to surround the interference source or the part susceptible to interference. In this way, it not only prevents the interference source from exerting interference to the outside, but also avoids the part susceptible to interference from receiving external interference.

　　High reliability design of software system

　　(1) Anti-interference design of software

　　In addition to the above-mentioned hardware anti-interference measures, anti-interference design should also be done in software.

　　①Application of watchdog interrupt

　　When designing a program, a watchdog counter reset instruction is inserted every other program segment. In this way, if the program enters a dead loop or illegal code area during execution, the counter cannot be cleared. When the counter overflows, the system will be reset and run again. At this time, if the interference or fault has been eliminated, the system will return to normal from the fault state.

　　② False interrupt processing

　　When designing a program, a program should be written for each interrupt, and the interrupt flag should be cleared in the interrupt service program and the program should return normally, which ensures the stable operation of the program.

　　③Instruction redundancy technology

　　For important instructions such as turning interrupts on and off, interrupt initialization, system register initialization, and timer timing value setting, instruction redundancy technology is adopted, that is, repeated write operations are performed one more time to ensure the correct execution of these important instructions.

　　(2) Reliability issues of control algorithms

　　The control algorithm design of this controller mainly considers the following two aspects:

　　The first is the accuracy of the control rules. We repeatedly examine and analyze the surgical experience data obtained from cataract surgery experts and combine them with relevant experimental conditions to extract representative data as the basis for writing control rules.

　　The second is the completeness of the controller, that is, for any input, the controller can give a suitable control output, which mainly depends on the database and the rule base. In terms of the database, for any input, a fuzzy set can always be found so that the membership value of the input to the fuzzy set is not less than a positive number ε (usually ε can be taken as 0.5); in terms of the rule base, the phenomenon of system out of control due to the inference engine failing to search for a suitable control result should be avoided, that is, for any input, it should be ensured that there is at least one applicable rule, and its applicability should be greater than a positive number (taken as 0.5). At the same time, the number of control rules should not be too few, and of course not too many. In practice, the number of control rules should be minimized while meeting the completeness requirements of the controller.