Circuit Synchronizes Sensors and Cameras

Measurement systems often use cameras and other sensors that require synchronization. This Design Idea describes an aerial-photography system that uses a camera comprising CCD (charge-coupled-device) image sensors, an inertial-measurement unit, and a GPS (global-positioning-system) unit. The resulting circuits provide trigger signals to synchronize the measurements at the optimal rate. The GPS provides information on spatial location, and the inertial-measurement unit provides information on spatial azimuth. The unit combines a gyroscope, a magnetometer, and an accelerometer to produce angular and acceleration measurements of a three-axis vector.

Figure 1 shows a system for taking aerial photographs. It comprises four Atmel area-scan CCD image modules, one linear image-sensor module, two Dalsa PCI (Peripheral Component Interconnect) frame-grabber cards, the measurement unit, a clock-adjustment circuit, and a microcontroller. A Tektronix digital oscilloscope views the trigger signals during development.


Figure 1

The trigger signals that synchronize the sensors are the keys to this measurement system. The clock-adjustment circuit sends an external trigger pulse to the frame-grabber cards, which generate trigger signals for the system. Video modules comprising image sensors receive trigger signals from the frame grabbers. Each frame grabber captures an image and stores it in onboard memory before capturing the next image.

The external trigger pulses also control the sensors, GPS, and inertial-measurement unit. Figure 2 shows a photo taken at 7,000ft in Mailiao Township, Yunlin County, Taiwan, using the external trigger circuit to drive and combine with the linear sensor and the measurement unit.


Figure 2

The circuit must change the external trigger clock’s frequency to obtain the best frame rate. The CCD sensors that go into the linear-image-sensor module have 12,288 pixels, and each pixel measures 5×5 microns, producing images of approximately 500 lines/frame. The CCD image sensors have a maximum output rate of 320M pixels/sec. They use a Camera Link interface to send image data to the frame grabbers, which transfer the images to a PC over the PCI bus.

The clock-adjustment circuit generates the external trigger clock pulses. The circuit employs on an Altera CPLD (complex programmable-logic device) using Altera’s development software to simulate the trigger signals and design the circuit. The clock-adjustment circuit provides as many as 15 trigger-signal frequencies to the system.

The system’s Atmel microcontroller contains 256B of RAM and 8kB of programmable flash memory for program storage. The microcontroller communicates to a PC over an RS-232 interface so that it can also receive commands and report its current state. This handshake process includes the decoding and encoding parameters for generating the trigger signal. The microcontroller also sends commands to the digital-timing-adjustment circuit; these commands change the pulse frequency of the external trigger.

You can adjust the frame rate of the CCD image module using 15 trigger frequencies. The external trigger signal also triggers the measurement unit to record and store spatial parameters. Figure 3 shows the algorithm for finding the optimal trigger frequency. The frame rate and the trigger frequency are linearly proportional.


Figure 3

The inertial-measurement unit is a key sensor in the system, and there must be a direct correlation between it and the frame grabbers. If the external trigger frequency is 1kHz, then each of the two frame grabbers captures 1,000fps and the unit samples at 1k samples/sec. Through experimental results using aerial photography, the system successfully synchronizes all of the sensors.