Image acquisition and trigger

The acquisition and trigger features are interrelated and are therefore be considered together below. The term Acquisition basically describes a state of cameras in which images can be taken, see ImageAcquisition. Whether and when images are actually taken and sent is determined by the configuration of the Trigger parameter.

The various configuration options and relationships between acquisition and trigger settings are described in more detail in the following chapters, which are arranged according to the AcquisitionMode.

By far the most commonly used image capture options are:

• Continuous Image Acquisition - the camera streams, it captures images at regular intervals without any other triggers.
• Single image trigger with continuous image acquisition - the camera takes exactly one image after a trigger signal (hardware or software). The camera remains in the state ACQUIRING, therefore it is not necessary to write the AcquisitionStart command repeatedly before each recording.

Checking the features of the camera Please refer to the manual for the camera to see whether the camera supports corresponding features. The same applies to the feature names that are actually used. The procedure using the feature names according to the GenICam Standard Features Naming Convention (Version 2.4) is described below.

Acquisition Mode

The AcquisitionMode feature mainly specifies how many images are taken during an acquisition and refers to the TCVN_CS_ACQUIRING state of the camera state machine. To end a recording, the state must be exited with a AcquisitionStop command before the next recording can begin again with a AcquisitionStart command. Therefore, the Continuous mode is the most frequently selected setting, since you can then remain permanently in the acquiring state.

• Continuous
• Image acquisition begins with the AcquisitionStart command and is only terminated by the AcquisitionStop command. In between, images are continuously captured depending on the trigger settings.
• Continuous image acquisition
• MultiFrame
• Images are captured until the number of images specified in AcquisitionFrameCount is reached. A AcquisitionStart command is required for each image acquisition sequence.
• Multi-image acquisition
• SingleFrame
• A single image is captured. A AcquisitionStart command is required for each image acquisition.
• Single image acquisition

Trigger Source

The TriggerSource feature specifies which internal or physical signal is to be interpreted as a trigger. The list of possible sources is long and varies depending on the camera. Some samples are provided below:

• Software
  A software command sets the trigger. The PLC function blocks FB_VN_GevCameraControl and FB_VN_SimpleCameraControl provide the method TriggerImage for sending the command.
• Line0, Line1, ...
  A physical camera input is the trigger source. Pay attention to the necessary edge steepness with the signal source connected to it. This may vary depending on the camera.
• Encoder0, Encoder1, ...
  Line scan cameras are often connected to an encoder to trigger image acquisition. If no physical encoder is available, but the corresponding axis values are available in TwinCAT, the encoder signal can be output via an EL2521 or EL2522.

Trigger Activation

• RisingEdge
  The trigger is activated with a rising edge of the TriggerSource.
  

• FallingEdge
  The trigger is activated with a falling edge of the TriggerSource.
  

• AnyEdge
  The trigger is activated with a rising or falling edge of the TriggerSource.
  

• LevelHigh
  The trigger is activated as long as a signal is present at the TriggerSource.
  

• LevelLow
  The trigger is activated as long as no signal is present at the TriggerSource.