Development of control system for automatic stripping device for high voltage live working

In order to improve the automation level and safety of live-line working, reduce the labor intensity of operators and the personal threat of strong electromagnetic fields to operators, many countries have carried out research on live-line working robots since the 1980s. In 2002, my country also developed a product prototype of a high-voltage live-line working robot. The stripper used in live-line working is a special working tool for high-voltage live-line working robots. Its main function is to strip the insulation of 10 kV distribution lines to prepare for other work tasks [1]. Most of the strippers currently used are manually controlled. In order to meet the requirements of high-voltage live-line working robots, the design of the PWM power drive device studied in this paper adopts a highly integrated module and is controlled by an ATMEGA128 single-chip microcomputer. It has overcurrent, overload and overvoltage protection functions. This stripper remote control system is remotely controlled by a remote controller. It is safer and more reliable than a manual stripper and is more convenient to operate, meeting the requirements of high-voltage live-line robot work tasks.

1 Overall design of control system

As shown in Figure 1, the hardware design of the remote control peeler is divided into 5 layers, and the functions of each layer are as follows: Layer 1 is the execution structure, which is the basis of robot movement; Layer 2 is the drive system, including the motor driver, system power supply and current feedback circuit. Since the overload current of the remote control peeler motor can reach 10 A, SA60 with H bridge is selected. SA60 is a PWM power output chip. The power supply voltage provided by the circuit to the motor can reach up to 80 V, and it can continuously provide 10 A current to the load. The maximum analog input voltage is 8 V, the PWM carrier frequency can reach 250 kHz, and the efficiency can be as high as 97%. The direction and speed of the motor are controlled by the 12 V serial DA conversion chip, and the DISABLE pin controls the power on and off of the motor. Layer 3 is the system interface part, including the DA conversion interface. The output voltage is 4 V~8 V. Layer 4 is the ATMEGA128 core board part, including JTAG, reset circuit and crystal oscillator; Layer 5 is the system function expansion board to realize communication with the remote control. Since the super-regenerative receiving module has the advantages of simple circuit, low cost, strong temperature adaptability, higher receiving sensitivity, stable and reliable operation, and strong anti-interference ability, the wireless receiving module uses a superheterodyne receiving module with strong anti-interference ability [2].

2 Power supply circuit design

Electromagnetic interference must include three elements, namely, the source of electromagnetic interference, the transmission path of electromagnetic interference, and the responder that receives electromagnetic interference. These three elements are quite complex and have different manifestations in different occasions. As shown in Figure 2, in order to prevent the motor drive circuit and the control circuit from interfering with each other, the inductor L2 is used to achieve non-common ground. In the circuit layout, due to the impedance of the ground wire, a certain potential difference will be generated. Due to the existence of the potential difference, it will inevitably affect the operation of the circuit. Therefore, in the circuit board layout, one point of grounding is required. In order to prevent the control part and the motor drive part circuit from short circuiting, a self-resetting fuse is added to the pre-order circuit. The self-resetting fuse is a device made of a new type of polymer material. When the current is lower than the rated value, its current resistance is only a few tenths of an ohm. When the current reaches a certain level, its resistance value rises rapidly, causing heat, and the hotter it is, the greater the resistance value, thereby blocking the power supply current. During the operation of the peeler, due to uneven force, the voltage will be pulled down, so a 12 V boost chip MAX734 is added to the power supply part. When the battery is fully charged, the voltage can reach 14.4 V, which exceeds the supply voltage of the MAX734. Therefore, a 3.3 V Zener diode is reversely connected at the front end of the MAX734.

3. Design of driving circuit

The PWM power drive uses the switching characteristics of high-power tubes to modulate the DC power supply so that it turns on and off at a fixed frequency. Changing the length of the on and off time within a cycle, that is, changing the "duty cycle" of the output voltage, thereby changing the average voltage and controlling the output power. Its structure can be divided into two parts: the circuit that transfers energy from the main power supply to the load is called the power conversion circuit, and the rest is the control circuit. Changing the pulse duty cycle can achieve the regulation of the motor speed, but first it is necessary to convert the speed control command signal into a pulse signal with a corresponding duty cycle. The basic method of generating a PWM signal is to compare the control command signal with a fixed-frequency triangular wave or sawtooth wave signal to generate a pulse signal with a duty cycle proportional to the control command voltage. As shown in Figure 3, the SA60 has a built-in 555 timer with an external 270 pF capacitor to generate a 45 kHz triangular wave. When the control command signal voltage is greater than or equal to the triangular wave voltage, the output signal is the comparator power supply positive voltage VCC; when the control command signal is less than the triangular wave voltage, the output signal is the power supply ground signal 0 V. When the motor is in a regenerative braking state due to deceleration or other reasons, the mechanical energy stored in the transmission system will be converted into electrical energy by the motor and fed back to the DC bus side through the power device. This energy is generally stored in the energy storage element of the power main circuit. If there is no energy release circuit, the voltage on the DC bus side will increase. This increased voltage is called pump-up voltage. In this case, if no protective measures are taken, the power device or energy storage element may be damaged. An RC resistor-capacitor network is added to the circuit to suppress the generation of instantaneous pump-up voltage. The resistor is 100 Ω 2 W and the capacitor is 1 000 pF 100 V [3].

Set up an overcurrent protection circuit. The overcurrent protection circuit consists of a current detection link, an operational amplifier, a comparator, etc. The current of the ISENSE A and ISENSE B pins of the SA60 is sampled through a 0.01 Ω precision resistor. The sampled voltage is added and amplified 10 times by the single-power operational amplifier LM324, and then input together with a 2.5 V reference value into the voltage comparator LM393 to compare whether the output current exceeds 25 A.

The following is the formula for calculating the gain of the voltage amplifier:

4. Single chip microcomputer control circuit design

4.1 Wireless receiving circuit

Super regenerative receivers are widely used due to their simple circuits and low cost. Although superheterodyne receivers are more expensive, they have strong temperature adaptability, higher receiving sensitivity, stable and reliable operation, and strong anti-interference ability. Therefore, the wireless receiving module uses a superheterodyne receiving module with strong anti-interference ability. The remote controller uses a long-distance remote controller that matches the receiving module. The battery uses A23 batteries, and it uses a white high-quality plastic shell, silicone buttons, and a pull-rod antenna. There is a movable battery compartment cover on the back of the remote controller, which can easily replace the battery. The transmission distance is 300 m~500 m.

4.2 Analog Output Circuit

As shown in Figure 4, LTC1257 is a single-power supply, 12-bit output DA conversion chip. The LT1021 reference chip is used to provide an 8 V reference voltage. Since VCC is 2.7 V greater than VREF, LTC1257 can normally output 0 V~8 V. The interface with the controller uses the SPI bus to communicate, and the interfaces are CLK, Din, and Dout, which is convenient for control.

.3 ATMEGA128 control unit

The ATmega128 control unit is responsible for controlling the output of analog signals, the collection of overcurrent information and wireless communication. ATmega128 is an 8-bit low-power CMOS microprocessor based on the AVR RISC structure. It has many advantages such as fast, flexible, high integration, strong encryption and easy implementation. ATmega128 has 128 KB of in-system programmable Flash, 4 KB of E2PROM, 4 KB of SRAM, 53 general-purpose I/O lines, 32 general-purpose working registers, real-time clock RTC, 4 flexible timers/counters (T/C) with comparison mode and PWM function, 2 USARTs, byte-oriented two-wire interface TWI, 8-channel 10-bit ADC, programmable watchdog timer with on-chip oscillator, and SPI serial port. Due to its advanced instruction set and single-cycle instruction execution time, the data throughput of ATmega128 is as high as 1 MIPS/MHz, which is 10 times higher than that of ordinary complex instruction set microprocessors, thereby alleviating the contradiction between power consumption and processing speed in the system [4].

5. Software Design of Bolt Cutter System

The software design mainly includes: decoding program of the received control command code, DA conversion program of motor speed regulation and overload protection program.

5.1 Decoding Procedure

The address code, data code and synchronization code of the coded signal sent by the encoding chip PT2262 form a complete code word. After the decoding chip PT2272 receives the signal, its address code is compared and checked twice, and the VT pin outputs a high level. At the same time, the corresponding data pin also outputs a high level. As shown in Figure 4, the interrupt method is used to receive the transmission code and decode it to obtain the control command.

5.2 DA conversion procedure

As shown in Figure 5, since the starting and stopping current of the reduction motor used is very large, acceleration and deceleration can be achieved by changing the voltage at both ends of the loading motor. Based on simplicity and practicality, the DA analog speed control method is adopted. The software method is achieved by setting the register inside the DA conversion chip, and the software adjustment amount index is higher, and the adjustment is more reliable, convenient and accurate. In order to realize the protection of the power module, when the overcurrent signal is collected, the DA output method is used to control the input voltage of the motor to adjust the output current of the power module SA60.

The PWM power drive device controlled by the ATMEGA128 microcontroller introduced in this article adopts an integrated chip design, so the reliability and integration of the entire system are greatly improved. The output of the PWM power drive device meets the electrical characteristics requirements of the TEC module. In addition, the system is controlled by the ATMEGA128 microcontroller, which has good programmability and is easy to operate, and improves resolution and accuracy. The real-time operation information of the system can be fed back to the ATMEGA128 control system to monitor the system and handle faults. In practical applications, the automatic peeling device has a good peeling effect and has great practical value.