Tiny PLCs shine new light on automatic process control 01 November 2009
The power of today's smallest PLCs has been captured by a self-taught control engineer, using them to grow cell cultures in automated bioreactors.
Lee Pettitt of I4Innovation in Thetford says he has connected four Mitsubishi Electric Alpha mini controllers with a GT15 HMI (human machine interface) from the same company to monitor and control pumps, impellers, inlet and outlet valves and multiple sensors in his reactors.
The bioreactors have 32 inputs and 24 outputs, plus eight analogue outputs – all managed by the Alpha mini PLCs.
Pettitt says his system can store recipes for thousands of culture variants on the HMI, and use them for real-time process control and data logging, as well as mandatory digital signature management for the Food and Drug Administration's CFR21 part 11 electronic records standard.
In operation, he explains, his system feeds materials to the reactor under recipe control, via automatic inlet valves. Impellers, also under PLC control, keep the aqueous media circulating, while the system continuously optimises temperature, pressure, dissolved oxygen and pH, under feedback loop control.
It also manages a pressure swing absorption system to separate and compress nitrogen and/or oxygen for bleeding into the reactors, at rates to promote culture growth.
"Most other bioreactors use bottled gases," comments Pettitt, "but this leads to an extra cost, plus a considerable bottle management effort."
Pettitt says he now rarely needs more complex PLCs. "When used to their full capacity they are very powerful. There is also clarity in the way they are programmed and I have converted several older control systems from larger PLCs to Alphas – usually improving performance," he says.
And he adds that today's HMIs are also very versatile. "I work hard to develop user-friendly graphics, trying to see the machine operation and process sequence from the operators' point of view, rather than the system developers' position."
Pettitt says he is now assessing the latest generation Mitsubishi HMIs, which have massive memory and higher resolution screens. "We are going to do a video version of the user manual and embed it into the GT16," he explains.
"This will allow bioreactor operators complete autonomy so, in theory, anybody will be able to plug them in and teach themselves everything they need to know."
Mitsubishi Electric Europe BV
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