Design of gyro-stabilized platform for airborne pod based on digital servo controller

In the civilian market, most of the existing airborne pod gyro stabilization systems in China use analog servo controllers. Servo controllers have many defects, such as large size, heavy weight, easy drift, difficult to adjust, poor servo control effect, and inability to achieve digital communication. Therefore, it is impossible to use chips such as FPGA to process motion signals, which has become a bottleneck for performance improvement and cannot meet market demand well.

Elmo's Whistle series digital servo controllers are small in size, light in weight, and provide digital input and output interfaces. They offer two communication methods: RS232 and CAN bus, and are programmable. By using the Elmo Whistle digital servo controller, you only need to design a relatively simple part of the peripheral circuit through programming to achieve very complex functions, which reflects great superiority. However, there are relatively few manufacturers in China that use this controller to realize airborne pod gyro-stabilized platforms, so the experience that can be learned is very limited. This article designs an airborne gyro-stabilized platform that meets the performance indicators by carefully reading the relevant documents of Elmo.

1 Pod and gyro-stabilized platform

A pod is a payload container device suspended outside a moving carrier (such as an aircraft or ship). Its main function is to isolate the influence of the carrier's attitude changes and mechanical vibrations on the pointing direction of the photoelectric sensor. The pod system consists of seven parts: a gyro-stabilized platform servo platform, a TV tracking system, a cockpit display and control system, an infrared measurement system, a laser rangefinder and a GPS positioning and ranging data link system, a data acquisition and recording system, and a pod environmental control system.

The gyro-stabilized platform system is mainly used to stabilize the visual axis of the TV and infrared cameras on the airborne pod and eliminate the interference torque caused by the swaying during the flight of the helicopter. The platform designed here belongs to a two-axis four-frame system, which is divided into azimuth axis and pitch axis. A single-degree-of-freedom fiber optic gyroscope is installed on each axis to sense the interference torque. After amplification and filtering, the gyro output signal is sent to the Elmo servo controller, which intelligently processes the gyro signal. After the signal processing is completed, the output part of the controller drives the DC servo motor to achieve the stability of the entire system.

2 Stable platform design

2.1 System overall design block diagram

The gyro-stabilized platform designed in this article is mainly to ensure the stability of the line of sight of each optical sensor. Combined with the entire pod system, it mainly realizes the following 6 functions:

1) Stable control of the pod; 2) Realize the motion control of the pod; 3) Limit signal input; 4) Error indication circuit; 5) LOCK circuit; 6) Serial communication.

Figure 1 is the overall design block diagram of the system.

2.2 Hardware Circuit Design

2.2.1 Gyro signal processing circuit

Since the output signal of the Russian Fizoptika VG94l-3AS fiber optic gyroscope is very weak, the output proportional factor is only 3.3 mV/deg/s. For such a weak signal, it must first be processed by the small signal amplification circuit before it can be transmitted to the Elmo servo controller for the next step of processing. One point to note here is that the gyro output signal is not filtered, because the Elmo servo control already has a digital filtering circuit inside, which can be set during debugging to achieve the filtering purpose.

The Elmo digital servo controller originally has two analog input ports, which can directly connect the output signal of the fiber optic gyroscope to the digital servo controller. However, due to the large random drift of the gyroscope, there is basically no regularity to follow. Every time the gyroscope is turned on and powered on, the random output of the gyroscope is different. Therefore, a gyroscope signal processing circuit should be designed. On the one hand, the output signal of the gyroscope can be amplified by a certain proportion and then input into the Elmo digital servo control, reducing the proportional factor in the program, thereby reducing the impact of the noise inside the Elmo digital servo controller on the entire system. On the other hand, by connecting an external adjustable resistor, the drift compensation for each startup can be achieved, so that the pod can remain in a stable state.

The output voltage of the Fizoptika VG941-3A fiber optic gyroscope is 2.5 V when the carrier is stationary. Therefore, to ensure that the voltage input to the analog input port of the Elmo digital servo control is 0 V when the carrier is stationary, a precise voltage chip must be used to generate a voltage of 2.5 V, and a subtraction circuit must be implemented through the amplifier 4558. In this subtraction circuit, REF02CZ is used to generate a voltage of 5 V, and then a 2.5 V reference voltage is obtained through a resistor divider. Figure 2 is the schematic diagram of the gyroscope signal processing circuit (subtraction circuit).

2.2.2 Power supply circuit design

The main purpose of the power supply circuit design here is to provide reference voltage for each chip. TSMl212D is used to generate ±12 V reference power supply to provide reference voltage for amplifier 4558 and REF02CZ, while REF02CZ is used to generate +5 V reference voltage to provide reference voltage for the amplifier. Figure 3 is the schematic diagram of the power supply circuit.

2.2.3 Pod motion signal processing circuit

In addition to the basic gyro stabilization function, the pod system must also have functions such as cruising and tracking. Therefore, the entire pod system also has a processing circuit for rotation signals and drift signals. These two signals are controlled by the corresponding switch buttons on the control panel (HCU). Figure 4 is a schematic diagram of the pod motion signal processing circuit.

2.3 System software design

The software structure of the Elmo Whistle digital smart drive can be generally divided into two parts: 1) The drive's own program, which includes the boot program, firmware and personalized settings. These programs can be downloaded from the official website and then burned according to the specific drive model; 2) The user's own program to realize the functions designed by the user.

In the software design of this system, the main function is to complete the gyro stabilization function. By collecting the voltage signal AN[1] fed back by the fiber optic gyro at the analog input port of the Elmo Whistle controller, the corresponding follow-up ratio AG[2] is set in the program to achieve the corresponding gyro stabilization function. The key here is the determination of parameter AG[2]. This parameter first has an estimation process. After the estimation is completed, it can be fine-tuned in the later debugging stage to finally achieve accurate gyro stabilization function. Parameter AG[2] can be estimated as follows:

1) In the Smart Terminal interface, set the input AN[1] to 1 V, measure the speed of the pod at this time, set it to N, and check the motor speed in the Smart Terminal interface to see that it is S1 in count/s;

2) The maximum inductive output voltage of the fiber optic gyroscope is 2.5 V. At this time, the corresponding pod speed should be M. The value of M has been set when the pod is designed, which is 60 (°)/s. At this time, the motor speed is S2, and the value of S2 is: S2=(60/N)xS1;

3) Scale factor AG[2]=S2/2.5;

Elmo Whistle has callable functions inside. Through the corresponding setting statements, the controller can judge the status of the six digital input ports and execute the corresponding internal functions. In this system, it is reflected in the LOCK signal function, limit signal function and indication output, etc. Figure 5 is a partial software flow chart of the gyro stabilization system.

In order to truly realize the digitization of the airborne pod, in addition to realizing the above functions, this system also makes corresponding attempts to control the movement of the pod by command. In the original software module, a serial communication module is added by judging the status of input port 3. If the controller digital input port 3 is detected to be low level, the serial communication module subroutine is triggered to send the control status word to the controller to realize the command control pod function. Of course, this function can also be realized by sending corresponding instructions to the controller by the PC.

3 Conclusion

With Elmo's Studio interface and Recorder software, it is possible to analyze whether the onboard pod gyro-stabilized platform meets the technical requirements, and to modify the system hardware circuit design and program parameters when necessary to achieve the desired goals.

The airborne pod gyro-stabilized platform finally designed by this system is applied to the current pod system. The stability performance of the pod reaches 50μrad, the pitch rotation angle is -120°～+15°, the azimuth rotation angle is 360° continuous, the maximum rotation speed is 60(°)／s, the maximum rotation acceleration is 200(°)／S2, and the power consumption is less than 240 W.