K008 streak camera. Evolution of features. Part II
The original ICC camera K008 has the following drawbacks, which required priority elimination:
- The inability to remotely control the camera due to the manual control with mechanical switches;
- The need to monitor matching of hardware and program settings;
- Single-window software with limited image scaling capabilities;
- Large amount of hardware-dependent data stored in the PC, and, as a result, a unnecessary binding of the camera to a specific PC;
- Impossibility of work with several cameras on a single PC;
- Lack of automatic protection of the ICT cathode from excess radiation, in case of emergency situations;
- Lack of metadata describing shooting conditions in image files;
In order to eliminate these shortcomings and expand functionality, a significant architectural change was made to the camera - the executive and control units were separated from each other. Control elements with communication through unified digital interfaces were introduced in executive units, such as high voltage sources, pulse generators, bias voltage regulators, etc. The control part of the camera is implemented using a modern digital element base - microcontrollers and FPGAs. Hardware settings are stored in microcontrollers memory and hardware-dependent data used in image processing can now be stored in the built-in FLASH drive.
Thanks to the separation of the executive and control parts of the equipment, and also unification of commands for managing nodes, it became possible to improve the interchangeability of units and maintainability of the camera. In addition, with this approach, it is much more convenient to adapt the camera to new modes of operation, and this allows more flexibility when meeting the requirements of different consumers and taking into account the specifics of their shooting conditions.
When upgrading the camera's execution units, special attention was paid to solving the following two most important issues: increasing temperature and long-term stability of the analog parts parameters and significantly increasing maximum triggering frequency of the camera.
First issue is related to minimizing the drift of the image on the surface of the screen of the ICT so that the image processing algorithms that depend on the location of the image on the screen work correctly. To ensure that, it is required to provide a temperature drift of the image that does not exceed one pixel of the CCD camera in the entire range of ambient temperature requirements (20 ± 15 ° C). And this requires temperature stability of voltages applied to the tube to be not worse than ± 30 ppm/oС. Solution of this problem is complicated by the fact that constant voltages applied to ICT are sufficiently high and can reach several kilovolts. High stability requirements are also applied to amplitude and shape of the pulses used for exposure and sweeping of the image.
Hardware of the traditional camera K008 was designed to achieve the limiting time parameters with single sweep at a low frequency - no more than 10 Hz. This approach was to some extent justified, due to the limited bandwidth of the USB 2.0 channel, used to transmit images to the computer. With USB 3.0 transfer speed can be increased greatly, but camera parameters will still be the same. It might be much more efficient to use the approach in which a series of camera starts is performed with a high frequency with a single image reading from the screen of the ICT at the end of the series. In this approach, the camera can work in single-frame or in the streak-mode. Due to the high frequency of starts (dozens of kHz) a certain resulting image is generated on the screen of the ICT, which, depending on the way the camera is triggered, can be interpreted in various ways. For example, when working in the frame mode, it becomes possible to implement stroboscopic survey that captures the various phases of the movement of an object in one frame. If the fast-flowing process is of a recurring or periodic nature, it is possible to perform recording in different phases of their flow by synchronizing the camera's start in combination with a controlled delay. To implement such modes, and also for the purpose of accurate temporal adjustment of the high-speed modules of the camera, it has built-in controllable digital delays with an adjustment step of 10 ps. This helps reducing the set of external equipment needed to implement the special synchronization described above, and simplify the shooting process.
High frequency of camera startup can be useful also when registering low-luminous periodic processes in the mode of image accumulation on the screen of the image converter.
Modules forming the pulse signals have been modified to substantially reduce the relaxation time in order to achieve high frequency of the camera triggering. This required the use of more powerful power supplies and voltage regulators with active feedback elements.
A lot of attention was paid to redesigning of the camera trig-in unit. In its previous version there was only adjustment of the trigger level threshold. In the new version, input trigger pulse polarity, trigger front (rising / falling) and AC/DC settings were added. The trig-in unit is digitally controlled, implemented using a high-speed comparator with a 1 ns response speed, and provides galvanic isolation from the trigger source.
The optical input is equipped with a FC connector and can be used to trigger the camera through an optical fiber cable coming from either an external trigger device or directly from the process under investigation if it provides visible or near infrared light.
Here is the list of 8 major improvements:
- Control of all camera operating modes is performed by commands from the PC.
- Temperature and temporal stability increased.
- Trigger frequency increased from 10 Hz to 1kHz (with image accumulation on the CCD).
- Adjustable internal delay was added, with ability to trigger external devices with additional delay.
- Optical trigger source added.
- Remote control of the camera, including local network.
- Multiple cameras can be connected to one PC.
- Significantly improved software. In particular, an API (Application Program Interface) has been added to integrate the camera software with consumer software and hardware complex.