Hardware Introduction…

Presently the stabilizers for airborne reconnaissance system which foreign companies invented are usually two or four axes gimbal mechanism. Figure 3.2 shows three present airborne stabilizer products, which (a) and (b) are for EO sensor application, and (c) is for radar application. The stabilizer should stabilize the sensor or antenna LOS regardless of aircraft motion while a map is being made.

Likewise we design an antenna servomechanism. It is the most important elements of the antenna assembly and composed of three parts. The gimbal mechanism supports the antenna. The motor and associated drive provide the driving force. The gyroscope senses the antenna pointing angles and provides an input for feedback control. The stabilization loop ensures proper feedback control while antenna is pointing a designated direction.

3.2.1 D.C. Servomotor

The motor we chose in the system as actuator is SmartMotor 2315D.

Traditionally, an entire servomotor system includes: encoder, motion controller, driver, servomotor, and cables etc. The SmartMotor integrates these components in a single module, and it becomes an integrated servo system. It includes a very high quality, high performance brush-less D.C. servomotor which has a rotor with extremely powerful rare earth magnets and a stator (the outside, stationary part) that is a densely wound multi-slotted electro-magnet. There are seven I/O channels available on a SmartMotor 2315D. They can be assigned as either inputs, outputs, or 10 bit analog inputs individually. Besides, there is also an encoder which resolution is 2000 counts/revolution coupled to the motor shafts to provide feedback signals for the position feedback loops. The servomotor

works with a D.C. power supplied 24 volts and its communication are under the RS232 interface.

The major advantage for our selection is system volume reduction. With all components integrating in a single servomotor module, we can efficiently reduce system volume. The SmartMotor is shown in Figure 3.3.

3.2.2 Gyroscope

The gyroscope measures the changing attitudes of aircraft which are absolute roll, pitch, and yaw angles. The gyroscope we chose is MicroStrain 3DM-G. It is a self-contained sensor system that measures the three degrees of its orientation in space with respect to Earth. When we say Earth, we are referring to the coordinate system established by the cardinal axes of our planet Earth itself. We define a coordinate system that is “fixed” to the Earth with the Z-axis pointing down through the center of the Earth, the X-axis pointing North and the Y-axis pointing East. By ‘fixed’ we mean that this coordinate system is stationary and provides us with a reference to measure against. The Earth’s Coordinate System is shown in Figure 3.4.

Likewise we define a local coordinate system that is fixed to the gyroscope.

The MicroStrain 3DM-G is shown in Figure 3.5. The faceplate is imprinted with the gyroscope’s coordinate system for reference during use. The measurements output by the gyroscope give the orientation of the gyroscope’s local coordinate system with respect to the Earth’s coordinate system. If we orient the gyroscope such that its Z-axis is pointing down through the center of the Earth, its X-axis is pointing north and its Y-axis is pointing east, we have aligned the gyroscope with Earth’s coordinate system. At this orientation the gyroscope will be outputting zero pitch, zero roll and zero yaw angles. If we turn it from there, we

will start getting non-zero pitch, roll and yaw angles.

The 3DM-G incorporates:

‹ accelerometer sensor to measure Earth’s gravity

‹ magnetometer sensors to measure magnetic fields

‹ rate gyroscope sensors to measure the rate of rotation about their sensitive axis

‹ a temperature sensor

‹ signal conditioning amplifiers to condition the raw output of the sensors

‹ a signal multiplexer to route the sensors’ signals to the A/D converter

‹ a 12-bit A/D converter that converts the conditioned output of the sensors into the digital domain

‹ a microprocessor that carries out the processing algorithm

‹ non-volatile EEPROM to store calibration, filter and other parameters

‹ a data communications port

3.2.3 Two-axis Gimbal Mechanism

Referring to most stabilizer design, the antenna is fitted with a roll-stabilizing system followed by the servo-loop in azimuth carrying an elevation servomechanism. We use a two-axis gimbal mechanism equipped two D.C. servomotors to stabilize antenna. The gimbal in Figure 3.6 and Firgure3.7 are the prototype of antenna gimbal mechanism designed by Yong-Chieng Tong in our laboratory. It is designed for the purpose of high efficiency and least power loss in a small workspace. That is just suitable for airborne side–looking radar.

We define the gimbal rotation angles as follows:

‹ Azimuth angle (AZ): The angle between the projection of the antenna centerline onto the x-y plane of the S frame, often termed the azimuth component of the look angle.

‹ Elevation angle (EL): The angle between the antenna centerline and its projection onto the x-z plane of the S frame, often termed the elevation component of the look angle.

The azimuth range is 15 degrees and the elevation range is 40 degrees for our gimbal. The gimbal support assembly has been designed to specifically match the UAV structure, but that can be easily modified to fit other aircraft mounting configurations. It can be looked as an actively inertial stabilized platform to stabilize antenna’s inertial position. Its SolidWork sketch and actual picture are shown in Figure 3.8 and Figure 3.9.

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