Synchronize sensors and cameras

Trigger circuits provide measurements and images at the right time.

By Shih-Jie Chou, Rui-Cian Weng, and Tai-Shan Liao, National Applied Research Laboratories, Instrument Technology Research Center, HsinChu, Taiwan -- Test & Measurement World, 9/1/2010 12:00:00 AM

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Measurement systems often use cameras and other sensors that must be synchronized. We developed an aerial-photography system that uses a camera built from CCD image sensors, an IMU (inertial measurement unit), and a GPS unit. We then built circuits to provide trigger signals to synchronize the measurements at the optimal rate. The GPS provides information on spatial location, while the IMU provides information on spatial azimuth; the IMU combines a gyroscope, a magnetometer, and an accelerometer to produce angular and acceleration measurements of a three-axis vector.

Figure 1 shows the system we developed for making aerial photographs. It consists of four Atmel area-scan CCD image modules, one linear image-sensor module, two Dalsa PCI frame-grabber cards, the IMU, a clock-adjustment circuit, and a microcontroller. We used a Tektronix digital oscilloscope to view the trigger signals during development.

Figure 1. The clock-adjustment circuit generates a series of pulses that trigger the frame grabbers and the IMU.

The trigger signals that synchronize the sensors are the key 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 consisting of the 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.

Figure 2. This aerial photograph was taken from a height of 7000 ft during a flight over Mailiao Township, Yunlin Country, Taiwan.

The external trigger pulses also control the sensors, GPS, and IMU. Figure 2 shows a photo taken at 7000 ft in Mailiao Township, Yunlin County, Taiwan, using the external trigger circuit to drive and combine with the linear sensor and the IMU.

We needed to design a circuit to 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, where each pixel is 5 μm x 5 μm in size. That produces images of about 500 lines per frame. The CCD image sensors have a maximum output rate of 320 Mpixels/s. 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.

Figure 3. Microcontroller software adjusts the trigger pulse frequency that drives the measurements.

The clock-adjustment circuit generates the external trigger clock pulses. We implemented it on an Altera CPLD using Altera's development software to simulate the trigger signals and design the circuit. The clock-adjustment circuit provides up to 15 trigger-signal frequencies to the system.

The system's Atmel microcontroller contains 256 bytes of RAM plus 8 kbyte of programmable flash memory for program storage. The microcontroller communicates to a PC over RS-232 so 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 that change the pulse frequency of the external trigger.

We optimized the frame rate of the CCD image module using the 15 trigger frequencies. The external trigger signal also triggers the IMU 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.

The IMU is a key sensor in the system, and there must be a direct correlation between the IMU and the frame grabbers. If the external trigger frequency is 1 kHz, then each of the two frame grabbers will capture 1000 frames/s and the IMU will sample at 1 ksamples/s.

Through experimental results using aerial photography, we found that the system successfully synchronized all of the sensors.