While many machine vision applications use fixed distances and lighting conditions, there is also a trend towards GigE Vision and USB Vision cameras in outdoor applications such as traffic monitoring, security or sports with different object distances. It must be possible to adjust the focus here. The advantage of these solutions is that they only require a programming interface from the camera to the host PC and therefore no additional USB port, which is then free for other purposes.

Lenses with fixed focal lengths normally have two settings. One sets the focus, the other the aperture.

It is very common to change the focus using motors. This makes it possible to control the focus remotely, whether with switching commands or by programming. The disadvantage is that these solutions are slow and bulky and have a limited service life. Liquid lenses with electrowetting are now being successfully produced that can change their curvature based on the voltage applied. Varioptic, a division of Corning, is a manufacturer of these lenses. The image on the left (both images are from Varioptic) shows the functional principle, the one on the right shows an example in the form of a C-mount lens with a liquid lens element. These liquid lenses have been tested with over 100 million cycles without any deterioration. They have also been found to be very robust against vibrations and are suitable for applications that require a quick change of focus.

A liquid lens requires an RS232 port in the camera and a DC voltage of 5 to 24 volts. With firmware version 2.24.975.0, this is supported by the BVS CA-GX0 (incl. POE), BVS CA-GX2 and BVS CA-SF camera families. The pin assignment of the Hirose connector of the cameras is shown in the following table on the left. How the lens must be connected is shown on the right.

The lens uses the camera's power supply. This means that the POE models of the BVS CA-GX0 as well as the USB 3.0 camera must be supplied with power via the Hirose connector. Suitable lenses can be found in the accessories section.

• SelectInveniosLiquidLens as "mv Serial Interface Mode".

• With "mv Liquid Lens Power Mode" mvLiquidLensActive, the "mv Liquid Lens Set Focus Value" can be entered directly, which adjusts the focus immediately.

To save power, StandBy can be selected as "mv Liquid Lens Power Mode". The focus can be set to any value, whereby the change is carried out within milliseconds and is therefore faster than the typical frame rates of a camera. With "mv Liquid Lens Save Focus Value", the focus that is to be set after the camera is started can be specified. This value is saved in the EEPROM of the lens itself.

Figure 1: ImpactControlCenter - mv Serial Interface Mode

The properties of the "mv Serial Interface Control" for RS232 is Balluff specific but appears in the XML file of the camera according to GigE Vision Standard and SFNC (Standard Feature Naming Convention). This allows this feature to be used in third-party software and applications, as shown in the following example of HALCON image acquisition.

Figure 2: Lens control via third-party libraries and APIs

The following list contains the lenses that can be used. There are two different lenses that allow writing via the C-mount interface. Option R2 must be used. This includes the RS232 interface with 12 V operation.

All BVS CA-SF cameras and all BVS CA-GX0 / BVS CA-GX2 cameras with sensor diagonals up to 2/3".