Design and implementation of digital dynamic target controller

A target is a device used to detect photoelectric tracking and measuring equipment indoors, and is divided into dynamic targets and static targets. In general, static targets are used to detect the measurement accuracy of equipment, and dynamic targets are used to detect the tracking performance of equipment. Combined with the requirements of range tests and operator training tasks, the self-developed digital dynamic target can not only be used for calibration and detection of photoelectric tracking and measuring equipment, but also for physical training of operators. The
digital dynamic target consists of a target frame and a target controller. The target frame consists of a target bracket, a target (collimator, light source, star point plate, reflector) and an actuator (DC motor, speed measuring motor); the target controller is used to control the target to move according to a pre-set working mode. The target light source passes through the star point plate and forms parallel light after passing through the collimator, and then is reflected to the theodolite lens by a plane mirror, generating an infinite light spot that can be imaged by the photoelectric tracking and measuring equipment, that is, simulating a spatial moving target for tracking and performance testing of photoelectric tracking and measuring equipment such as photoelectric theodolites, infrared tracking and measuring systems, and television tracking and measuring systems.
1 Hardware design of the system
The digital dynamic target controller is based on a single-chip microcomputer, and its hardware principle is shown in Figure 1. The system is mainly composed of a drive circuit, a signal conditioning circuit, a light source brightness control circuit, an alarm circuit, a display circuit, a clock circuit, and various interface circuits.

In addition to calculating the control quantity and controlling the target to rotate according to the set mode, the single-chip microcomputer also needs to scan the keyboard, display the system status and communicate with the time system terminal or computer. In order to meet the design requirements, this system selected the 16-bit single-chip microcomputer 80C196KB. Compared with the 8-bit single-chip microcomputer, the 80C196 KB has higher computing performance and richer software and hardware resources, such as A/D converter, pulse width modulator PWM and high-speed output HSO. The driving device is a high-power transistor PWM power amplifier, the actuator motor is a DC torque motor J130LYX02B, and the speed detection element uses a DC tachometer generator 130CYDN02C. The display screen uses a wide-temperature graphic LCD display module MGLS24064-21C, with a large display window and more display content. In order to realize the custom working mode, a clock circuit (using the chip DS12C887) is also designed in the system, which can pre-set the training time of different modes, so as to simulate point targets with complex motion characteristics. The system also has a self-test function, and the alarm circuit will give an alarm in time when a fault is found. In order to further improve the function of the dynamic target, the system also designs RS-232 and RS-485 serial interfaces for communication with computers or B-code terminal devices.
1.1 Driving circuit
In order to reduce peripheral circuits and make full use of microcontroller resources, the pulse width modulator inside the microcontroller is used to directly output the driving signal (PWM), and the target rotation direction signal (DIR) is output by the I/O pin of the microcontroller. In order to drive the power level, the TTL level PWM signal and DIR signal should be preprocessed first, and the circuit is shown in Figure 2.

The photocouplers U1 and U2 are used to eliminate the common impedance coupling interference caused by the ground loop, realize the isolation of different voltage signals, and suppress the interference transmission. After the photoelectric coupling isolation, the DIR signal first passes through the D flip-flop U3 to form two control signals of the analog switch U4. After the photoelectric coupling isolation, the PWM signal directly enters the data input terminal of U4. After the buffering, the output signal of U4 forms the control signals HL, LR, HR and LL of the H-bridge drive circuit (as shown in Figure 3). When the direction is set to "forward", the HL and LR signals are valid, so that the tubes Q1, Q5 and Q4, Q8 are turned on, and the current flows from the S2 end of the motor to the S1 end, and the motor rotates forward; when the direction is set to "reverse", the HR and LL signals are valid, so that the tubes Q3, Q6 and Q2, Q7 are turned on, and the current flows from the S1 end of the motor to the S2 end, and the motor reverses.

1.2 Signal conditioning circuit
The DC tachometer generator is an analog speed measuring device that can convert the shaft speed signal into a DC voltage output. From the electromagnetic theory, the relationship between the induced electromotive force E and the speed n of the DC tachometer generator can be deduced as follows:

Where: C is a constant related to the generator structure; Φ is the magnetic flux.
When the tachometer generator is working, it must be connected to a load resistor. The terminal voltage U of the load resistor R is the output voltage obtained. This terminal voltage is equal to the induced electromotive force minus the voltage drop on its internal resistance r (generator winding loop resistance), that is:

a voltage divider is formed, R5 and C9 form a filtering link, U11 and U12 are in-phase and inverting amplifiers respectively, and D1, D2, D3 and D4 play a protective role. Adjust the position of W3 so that ACH0 or ACH1 (corresponding to forward and reverse rotation, respectively) of the tachometer generator is +5 V at the maximum speed, and adjust the amplifier parameters at the same time to ensure the linearity of the signal.

1.3 Protection circuit
In order to prevent the motor from being burned by excessive current, a protection circuit as shown in Figure 5 is designed, in which R22, R23 and R24 divide the voltage to form a comparison level, U6 is a comparator, U3 is a D flip-flop with clear and set terminals, point A and point C are connected to point A and point C in Figure 2 respectively, and point B is connected to point B in Figure 3. In Figure 3, R20 is a sampling resistor. The greater the current flowing through the motor winding, the greater the voltage at point B (VB). When VB is less than the comparison level, U6 outputs a high level, then U5 outputs a low level, and pin 12 of U3 remains high, so the protection circuit has no effect on the drive circuit; when VB is greater than the comparison level, U6 outputs a low level, and after being inverted by U5, U3 is set to 1, and pin 12 of U3 becomes a low level, pulling point A in Figure 2 to a low level, then LL and LR become high levels at the same time, that is, Q2, Q7 and Q4, Q8 are turned off at the same time in Figure 3, the current flowing through the motor winding becomes zero, the motor stops rotating, and the motor is protected.

1.4 Light source brightness control circuit
In order to simulate the dynamic changes in the brightness of the target, a light source brightness control circuit is designed, as shown in Figure 6. The high-speed output device HSO of the single-chip microcomputer is used to generate the pulse width modulation output PWML, which controls the conduction time of the field effect tube U8 after the photoelectric coupler U7, thereby realizing the control of the light source brightness. The change in the light source brightness is manifested as the change in the brightness of the target when the theodolite is imaging.

2 System software design
In addition to completing the initialization of the hardware, the system software also needs to complete the real-time control of the hardware circuit, perform input and output operations on data, and analyze and process numerical values. The software is written in MCS96 assembly language and adopts a modular structure design. Each functional submodule is independent, easy to debug, and easy to expand as needed. The software has functions such as timing, keyboard scanning processing, display, speed sampling, and alarm. Figure 7 is a schematic diagram of the system main program.

2.1 PID controller
PID controller has a simple and fixed form, and can maintain good robustness in a wide range of operating conditions; at the same time, because the PID controller allows engineers to adjust the system in a simple and direct way, PID control has become the most widely used form in industrial process control. The discrete PID expression is as follows:

It can be seen from the above formula that if the single-chip control system adopts a constant sampling period T, once KP, I, and D are determined, the control quantity can be recursively calculated by formula (6) by using the deviation of the three measured values ​​before and after.
In this system, in order to eliminate the adverse effects of integral saturation, the limit weakening integral method is adopted. The specific process is: before calculating uk, first determine whether the previous control quantity uk-1 exceeds the limit range. If it exceeds, it means that it has entered the saturation zone. At this time, according to the positive or negative deviation, it is determined whether the control quantity makes the system increase or reduce the overshoot. If it is to reduce the overshoot, the integral term is retained; otherwise, the integral term is cancelled. The program flowchart is shown in Figure 8.

2.2 Generation of PWM waveform
The output of the PID controller is converted into a PWM control voltage with a certain duty cycle to control the rotation of the motor. In the 8096 system, two methods can be used to provide analog output: one is to provide it through HSO. The other is to provide it through the internal pulse width modulator. The PWM control voltage is generated by the second method. The program is as follows:
LDB IOC1,#25H; //Select P2.5 pin as PWM signal output;
LDB IOC2,#00H; //Set the PWM repetition period to 256 state cycles (42.75 μs for 12MHz crystal oscillator);
LDB PWM_CONTROL,Uk; //Write the control quantity Uk into the PWM register to output the PWM control voltage.
From the above analysis, it can be seen that by setting the rotation speed and acceleration of the target, targets with different motion modes can be generated, including uniform motion, uniform acceleration motion, uniform deceleration motion and custom modes, which are used to train the operator's target capture ability. By adjusting the brightness of the target light source, the dynamic light and dark changes of the target can be simulated. On the basis of increasing the difficulty of training, the parameter setting ability of the equipment operator can also be trained. At the same time, by setting the training parameters in combination with specific test conditions, the working conditions of the theodolites at different sites can be simulated, providing a basis for the site selection of the theodolite before the test.
When the digital dynamic target is used as a high-precision measurement target, it is necessary to accurately determine the spatial angle of the target at any time, that is, the rotation accuracy of the target must be guaranteed, and the influence of the axis runout of the rotating target must be eliminated. Therefore, it is necessary to add a position feedback link (such as a high-precision encoder). At the same time, the axis system and mechanical properties of the target must be further improved and perfected. Only then can the digital dynamic target be used for the precision measurement of optoelectronic tracking and measurement equipment, thereby improving the calibration and detection capabilities of our optical measurement equipment, so that our department can take an important step in the repair, maintenance and detection of equipment based on the application of equipment.
References
[1] Xu Aiqing. Intel 16-bit single-chip microcomputer [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2002.
[2] Wang Xiaoming. Single-chip microcomputer control of electric motors [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2007.
[3] Wang Furui. Design of single-chip microcomputer measurement and control system [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2000.