Hardware Triggering and Strobe Synchronization

A camera running in free-run mode captures images at a fixed interval defined by its frame rate. For many applications, that is sufficient. For inspection on a moving conveyor, position-dependent measurements, or applications where images must be captured at a specific moment, free-run acquisition may not provide the required timing accuracy. Hardware triggering coordinates the camera's exposure with an external signal (typically from a photoelectric sensor, an encoder, or a PLC) so that the image is acquired when the part is in the correct position, rather than at the next scheduled frame.

Strobe synchronization extends this coordination to the illumination system. A high-intensity light pulse, synchronized with the camera exposure, can effectively freeze motion while minimizing motion blur. Because the illumination is active only for a short duration, LED strobes can often be driven at a higher peak current than would be permissible during continuous operation, producing significantly higher light output.

How Hardware Triggering Works

In a hardware-triggered system, an external device detects a physical event and sends a digital signal to the camera. The camera responds to the configured trigger conditions by initiating an exposure. Industrial cameras provide one or more digital input lines on their I/O connector for receiving trigger signals. When the configured signal edge (rising or falling) is detected, the camera initiates an exposure cycle. The key parameters to configure are:

Trigger source: The input line connected to the external signal. Many industrial cameras provide multiple I/O lines; the GenICam SFNC parameter "TriggerSource" is typically set to the appropriate line designator, such as "Line0" or "Line1".

Edge polarity: Whether the camera responds to the rising or falling edge of the incoming signal. This needs to match the output signal of the trigger device.

Trigger delay: A configurable offset between the trigger event and the start of exposure. Trigger delay is used to compensate for mechanical latency, such as the time between a sensor detecting a part and the part reaching the camera's field of view.

Debounce time: This setting filters short-duration signals to prevent electrical noise or contact bounce from generating unintended trigger events.

Trigger Sources in Industrial Systems

The trigger signal can originate from several types of devices, depending on the application's timing and synchronization requirements.

Software triggering, in which the host PC sends a command to initiate image acquisition, is available on most industrial cameras but introduces variable latency from the operating system and network stack. For applications requiring deterministic timing relative to a physical event, hardware triggering via a dedicated I/O line is generally the preferred approach.

Strobe Synchronization

Continuous LED illumination is suitable for many static or slow-moving inspection tasks. For faster moving objects, or in applications or where short exposure times are required to minimize motion blur, pulsed strobe illumination synchronized with the exposure window provides two practical benefits.

First, a brief, high-intensity flash from a strobe-capable LED controller can deliver more light during a short exposure than continuous illumination at the LED's rated current. Second, LED drivers can operate at significantly higher instantaneous current during short pulses than in continuous mode, often several times the rated continuous current, provided the duty cycle remains low enough to avoid thermal overload. A common guideline is to keep the LED on-time below 10% of the total cycle period.

The camera's I/O connector typically provides an output signal, often labeled "strobe output" or "exposure active," that indicates when the sensor is in its exposure interval. Connecting this output to the input of a strobe controller provides a hardware-level synchronization path between the camera's exposure timing and the light pulse, without relying on software timing.

The behavior differs between shutter architectures:

For rolling shutter sensors, the exposure timing and strobe pulse must be configured so that the light pulse occurs during the period when all sensor rows are simultaneously exposing.

Practical Timing Considerations

Achieving accurate trigger and strobe timing requires accounting for the latency at each stage of the signal chain.

The total delay between a physical trigger event and image acquisition includes: sensor detection and output latency, signal transmission, camera input processing, configured trigger delay, and exposure time. These delays can typically be characterized during system commissioning and compensated using the camera's trigger delay parameter.

For applications involving high or variable conveyor speeds, encoder-based triggering is generally more robust than using a fixed photoelectric sensor because it maintains image acquisition at a consistent physical position along the conveyor. Rather than triggering after a fixed time delay, the camera acquires an image after a defined number of encoder pulses, corresponding to a fixed physical distance regardless of conveyor speed.