Man plus machine
By Keith Byant
Having previously contrasted the strength of human decision-making against machines using sophisticated algorithms, I received some interesting feedback with regard to cooperation between man and machine and questions about the benefit of close loop systems. We are not talking cyborg here or even the gentlemen pictured right with the interesting headgear, but rather the situation where a machine gives feedback about the performance of itself or another machine within the system.
There is much talk of closed loop systems so let’s start with the definition of ‘closed loop’: “A complete path followed by a signal as it is fed back from the output of a circuit, device, or system to the input and then back to the output.” So no human interaction with the process, simply a signal traveling within a machine or from machine to machine and then returning to its origin.
You will see from this that what is referred to in our industry, as “closed loop feedback” is not actually fully closed. There are however areas where closed loop systems are effective but mainly within the same machine. Hybrid regulator technology is used to constantly monitor the air pressure within squeegee control systems on several advanced printers. Dependant on the position of the squeegee blade the pressure within the system will fluctuate, the system measures these fluctuations up to 500 times per second and controls the air pressure accordingly to maintain the selected print pressure. This provides dynamic print pressure correction that can compensate for deformed stencils, warped boards etc. This technology is real closed loop and replaces inferior spring based systems.
It could also be argued that the sophisticated camera systems within some printers are also closed loop as they register a fiducial position on the stencil and align every circuit board printed to it. However, feedback from a solder paste inspection system about the performance of the printer is simply feedback. It is of course very useful for the operator to know exactly how the printer, solder paste, stencil and his set up is effecting the results. But, I don’t think any printer manufacturer would be happy with an SPI system adjusting any settings on their machine based on what the algorithms of the SPI machine were reporting. This would be a real closed loop system but I don’t think anyone in the industry is ready to accept this level of automation. I may be wrong, and the way forward may include each machine within the line modifying settings of any or all of the others based on results calculated by its algorithms. For many of us this is either a nightmare scenario or our redundancy.
Those companies who promote closed loop feedback are taking us down a path where human intervention within the production process is almost zero. In my humble opinion this is not the solution. What is needed is the most accurate and complete feedback to the line operators or technicians giving them all the information required to make judgement calls on any machine adjustments and parameter modifications. Closed loop feedback works well within a machine but when it is reporting issues with other parts of the line human intervention may still be the best solution. There have been huge leaps forward in feedback reporting in the past few years and the increased level of automated inspection on the line allows almost real time improvement of process. But, the technician has to make a judgement call, as the feedback is not always complete or accurate because, as we all know one small change in the process can exhibit itself in different ways. So there is still a requirement for the skill of a person to interpret this feedback and make the correct adjustments or changes to improve the process yield and to make the correct call he needs access to complete an accurate process feedback.
So, man plus machine is the best process line solution however, closed loop feedback that improves the performance of an individual machine is also dramatically improving process repeatability. I use the word repeatability as most closed loop feedback is designed to ensure that a machine maintains its programmed settings. So its goal is stability rather than process improvement. Process improvement comes from judgement calls made by a skilled person who has all the required feedback data at his fingertips and I believe at the moment there are still one or two pieces missing from the puzzle. We have some intelligent feedback that is of real value, but due to gaps in the system we also have some feedback that is not definitive but simply points to possible process issues. It can report that something is not right but not that it is a failure. This is due to a lack of co ordinated or intelligent feedback within the process, it takes a measurement and says that it is not within spec, but not that the board will fail as the part or joint concerned is bad.
Again I finish with the same leap of faith as my previous article, the system says it is not within spec but not that it is Good or Bad, without full intelligent feedback there is still this area of uncertainty.