Machine vision systems can only realise their full potential if they are seamlessly integrated into the overall system concerned. Depending on the application, the inspection unit needs to be connected not only to the camera but also to a higher-level control system, and possibly to trigger sensors. Because the modern panel-PC used by an image processor provides only one free PCI slot to cover all process interfaces, the hunt was on for a practical solution.
Picking racing rims accurately from the conveyor belt
To take a concrete example, a manufacturer of racing rims needs to pick up rims from the conveyor-belt during the production process, in order to feed them correctly into the next machine in the process. The rims have previously been placed on the belt by a pneumatically operated system that does not allow exact positioning. The image processor now has the task of determining the undefined position of the rims, to enable the gripper system to grip them accurately. As the project manager explains: "With such extremely light and finely tooled aluminium racing rims, a so-called 'gripper crash' can very easily result in a deformation, making the component unusable. At the high speeds reached in motor races, even the smallest imbalance can have serious consequences. These rims have to be selected out and written off by the manufacturer as loss items."
The vision processing software runs on a modern panel PC with flat panel monitor and touchscreen display, which is small enough to be fixed in a more or less free floating manner in the most suitable position. Of course, this compact size results in restricted internal dimensions of the computer system. There are just two free PCI slots available for the whole image processing system; one of these is taken by the frame grabber board, which leaves a single PCI slot for any other interfaces.
As the PLC system in this application has no fieldbus connection, it can only communicate with the image processing system via parallel digital 24 V inputs and outputs. The computer also needs a number of analog connections. To transmit the orientation of the rim, a 0 to 10 V analog signal is passed to the gripper system. All in all, the determination of the position of the rims is dependent on both digital and analog ports, which must be implemented on a single PCI board.
Rugged, isolated industrial interface board
The solution to the problem came from the manufacturer of measuring equipment and PC boards, Addi-Data, which had also developed industrial-strength interface hardware for other projects. The APCI-3122, provides four analog outputs with a voltage range of -10 to +10 V and 12-bit resolution, plus 10 digital 24 V inputs and 12 digital 24 V outputs. The digital and analog sections are optically isolated, allowing high loads in parallel with analog output. The opto-isolated digital outputs deliver up to 500 mA per channel, up to a maximum total current of 3 A.
The PCI board also offers all the performance features and meets all the safety criteria required by the most demanding industrial applications. These include protection against overheating and voltage surges, protection against high-frequency interference, and special output capacitors to protect against electromagnetic discharges or the accidental disconnection of the PC power supply or the external 24 V supply. There is also shutdown logic that kicks in when the supply voltage drops below 5 V, and a watchdog to reset the outputs.
Addi-Data is represented in South Africa by Westplex, which also markets Matrox Image processing software and frame-grabbers, Basler digital cameras for machine vision and Phlox back-lights. Westplex is also a system integrator of computer-based measurement and machine vision systems.
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