In modern measurement systems, cameras and other sensors often need to be synchronized for accurate data collection. This example describes an aerial photography system that includes a camera equipped with a CCD (charge-coupled device) image sensor, an inertial measurement unit (IMU), and a GPS (Global Positioning System) module. The system uses a synthesized circuit to generate a trigger signal that ensures synchronization at the optimal frame rate. GPS provides geographic location data, while the IMU offers spatial orientation information through a combination of a gyroscope, magnetometer, and accelerometer, which measure angles and acceleration along three axes.
Figure 1 illustrates the overall system setup for aerial imaging. It consists of four Atmel area scanning CCD modules, a linear image sensor module, two Dalsa PCI frame capture cards, a measurement unit, a clock conditioning circuit, and a microcontroller. During development, the trigger signal was monitored using a Tektronix digital oscilloscope.
The trigger signal is essential for coordinating all components in this system. The clock conditioning circuit sends an external trigger pulse to the frame capture card, generating a synchronized signal for the entire system. A video module containing the image sensor receives this trigger and captures images accordingly. Each frame capture card stores the captured image in onboard memory before acquiring the next frame.
External trigger pulses also control the operation of the sensor, GPS, and IMU. Figure 2 shows an aerial photograph taken from 7,000 feet above Maizi Township in Yunlin County, Taiwan, using the external trigger circuit to synchronize the linear sensor and measurement unit.
The circuit must adjust the frequency of the external trigger clock to achieve the best possible frame rate. The CCD sensor in the linear image sensor module has 12,288 pixels, each measuring 5 mm x 5 mm, and produces approximately 500 lines per frame. Its maximum output rate is 320 Mpixels/second. Image data is transmitted to the frame capture card via a Camera Link interface and then sent to a PC through the PCI bus.
The clock conditioning circuit generates an externally triggered clock pulse. It uses an Altera CPLD (Complex Programmable Logic Device), which is programmed using Altera's development software to simulate trigger signals and design the circuit. This circuit can provide up to 15 different trigger frequencies for the system.
The Atmel microcontroller in the system has 256 bytes of RAM and 8 KB of programmable flash memory for storing programs. It communicates with the PC via an RS-232 interface, allowing it to receive commands and report status updates. This communication process involves generating decoding and encoding parameters for the trigger signal. The microcontroller also sends instructions to the digital timing adjustment circuitry, enabling changes to the external trigger pulse frequency.
With 15 available trigger frequencies, the system can fine-tune the frame rate of the CCD image module. The external trigger signal also initiates the measurement unit to record and store spatial parameters. Figure 3 demonstrates the algorithm used to determine the optimal trigger frequency, where frame rate is directly proportional to the trigger frequency.
The inertial measurement unit plays a crucial role in the system and must be closely aligned with the frame capture card. For example, if the external trigger frequency is set to 1 kHz, each frame capture card can capture 1,000 frames per second, and the IMU can collect 1,000 samples per second. Experimental results from aerial photography confirm that the system successfully synchronizes all sensors, ensuring accurate and reliable data acquisition.
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