Cameras and synchronization 150

In this section it is assumed that the lens aspects of the cameras (aperture, focal length, etc.) are known or can be found in other sources. The streaming system, which collects data from the cameras, is the most complex portion of the PIV system at UWO. This system was not purpose built for PIV and thus offers many settings for the wide

possibility of applications for such a system. The successful use of this system for PIV measurements rely on certain settings. Ideally these settings do not get changed or reset; however, it is inevitable that this will not be the case. First, we start by opening the Streams software and the FASTCAM settings box as well as understanding how settings are applied.

• To begin, open Streams and select the previously used *.vl library

• Open the FASTCAM settings box by:

o Right click one of the cameras on the left side of the screen in Streams o Configure device → Camera settings

• The upper right of the FASTCAM settings shows which camera is being manipulated.

• If the status bar on the lower left displays “Unknown” (or a variant), it means that the applied settings will not be registered with the camera and indicates that one of the frame grabber boards is not connected properly. There is no feedback to Streams from the Photron cameras so it does not guarantee that these settings are applied.

Data can be acquired with one camera or both cameras depending on the desired results. When operating with both cameras it is imperative to ensure that they are synchronized with each other or the clock drift of each camera will ruin the sensitive timing required for PIV. It is assumed that the reader can infer how to use the cameras in single camera mode from the following synchronized mode.

• Settings common to the two cameras in the FASTCAM settings box

o Gain File: PixelGainData_Default_1024*.gdf (camera specific) o Calibrate shading: This should be done with an 18% Gray card

• Settings for Camera 18 (or Master camera)

o Mode box (upper left corner of FASTCAM settings box)

ƒ Frame rate: 1000

ƒ Sync Mode: Free Run

ƒ Output: 2x8x10 bit

o Triggering box (lower right corner of FASTCAM settings box)

ƒ Sync In: Disable

ƒ Sync Out: Cam VSync Pos

ƒ General Out: Exposure Pos

ƒ Trig Out: t_ttl_in

• Settings for Camera 19 (or Slave camera)

o Mode box

ƒ Frame rate: 1000

ƒ Sync Mode: External Sync Mode

ƒ Output: 2x8x10 bit

o Triggering box

ƒ Sync In: Hirose Sync_In Pos

ƒ Sync Out: Cam VSync Pos

ƒ General Out: Exposure Pos

ƒ Trig Out: t_ttl_in

• Ensure that the hirose connectors (the ones with BNC connectors to the frame grabbers) are connected to the OUT for Camera 18 and the IN for Camera 19.

• Connect SYNC_OUT from Camera 18 to TRIG_IN on Camera 19

• It is often worth the time to ensure that the cameras are synchronized using an oscilloscope and the TRIG OUT serial connection on the back of the PC (this should have two BNC cables attached to pins 6 and 7).

The whole PIV system is synchronized to the timing of the cameras. However, the settings in Streams that allow for this synchronization seem to be reset each time the software is started. To set the correct values:

• Right click camera → Configure device → Click the Miscellaneous tab → Signal Manager button

o Ensure that the “Secondary FVAL” is enabled (checkbox in lower left

corner)

o Set the FVAL Skip value to 1 (this performs the signal masking)

The synchronization between the cameras and the laser is accomplished through the use of TSI’s Insight software and their synchronizer. There are 6 BNC ports on the back of the synchronizer labelled A through F. A and C are used for flashlamp lasers which are irrelevant for this PIV system. Ports E and F are ordinarily for cameras, but for this system are unnecessary. The following needs to be connected on the synchronizer:

• Port B and D are connected to the two BNC connections on the front of the laser power supply (does not matter which)

• RS-232 (Serial port) connection needs to be connected to the controlling PC

• External trigger is connected to the TRIG OUT serial connection on the back of the PC. This serial connection has two BNC cables connected to it. Depending on which camera is being used will depend on which cable to connect to. Pin 6 corresponds to Camera “18” and Pin 7 corresponds to Camera “19”. The camera naming is based on the last two digits of their serial numbers.

The synchronization is performed using TSI’s Insight 3G. The following needs to be performed to ensure that the system is synchronized:

• Open the “Timing Setup” box on the Capture tab

• The frequency should be set to somewhere between 500 – 1000 Hz. This value can be set by Tools → Component Setup → Laser Setup

• The camera trigger is high when on so it is a rising edge trigger; however, the synchronizer, by default, is triggered by a falling trigger. To change this

behaviour go to Tools → Component Setup → Synchronizer setup and ensure that the toggle is set to “External Trigger Edge Rising” in the lower right

• Once again open the “Timing Setup” box on the Capture tab

• The mode should be on single

• The delta T should be set using the following,

ion magnificat U nt displaceme pixel t × = ∆ ∞

displacements for interrogation windows of 32 x 32 pixels² are ideally 8;

however, sometimes this is not possible. One should strive to get as close to 8 as possible to reduce the effect of peak locking errors.

• The external trigger box should be clicked

• The laser pulse delay should be in the range of 920-980 µs. This is the parameter which controls when the first laser pulse of a PIV cycle is fired. It is set to ensure that the first pulse appears in the first image of an image pair.