Early in my career I decided to channel my inner entrepreneur into an electronic design consultancy. One of my first projects was to design a control for a coffee packaging machine. It was in the late 70s and microprocessors were on everybody's lips, but few actually knew anything about them. I had set myself up as an expert, on the basis that the one-eyed man was king in the land of the blind.
In principle the pneumatically powered packaging machine was going to have a two-stage fill. The empty can was first going to be tared on a load cell, and then the coffee powder was going to be bulk filled at a high rate till the can was about 80% full by weight. In the second stage, the partially filled can was then going to be shifted off the first load cell onto a vibrating table to settle the contents. The third step was to move the partially filled can onto a second load cell and then trickle filled at a low rate to 100% of its target weight. Of course with automation, both the bulk and trickle feeds happened simultaneously (on different cans of course) and in fact there were two stages of vibration between the bulk and trickle. There were also four lanes in parallel. The trick was to fine tune the bulk and trickle so that both took about the same time and on each channel.
The feeds consisted of vibrating channels and the accuracy of the fill was determined by the amount of coffee “in flight” when the vibrator was turned off. It was a function of the density of the coffee powder and its self-adherence and would obviously vary from batch to batch.
The microcomputer had to provide eight digital outputs for the eight feed vibrators and a single output to index the machine from one step to the next. It also had a 12 bit A/D with an eight-channel input multiplexer for each of the eight load cells. It also controlled 4 x 4-digit seven-segment displays and several thumbwheel switches for target weight, and percentage bulk fill. For the computer we opted for an Intel Multibus based chassis using an 8085-based computer board, but for some reason added a wire wrap board for the expansion which we were probably intending to convert to a customised PCB. However we only made 4 machines and the intention evaporated.
The expansion board included DG202 analog switches for the multiplexers and an Intersil ICL7109 as the A/D. Also included were some 2716 EPROMs, display drivers and other sundry drivers. (Remember the LED display drivers, SN75491 and SN75492? The former was made obsolete right about then if I remember correctly, and I had to scramble to discover the ULN2003A to do the same job.)
The load cells were of the LVDT type (linear variable differential transformer) and I decided that I was a digital engineer and wanted nothing to do with the signal conditioning needed, so the machine designers contracted someone with more courage than I did. He knocked together an amplifier using a 741, each housed in a plastic module with an octal relay base and then disavowed all responsibility.
The project was developed in assembler with a minimal development aid- an SDK85 development board. Given that there was a lot of buffering of data with the pipeline of coffee cans certainly arrays would have been great to use, but this was all coded the hard way. Debugging was with a logic analyzer and any changes were coded on the fly using “jumps” to free memory locations and the new code inserted there.
Initially I had some trouble with the input multiplexers blowing up — they were relatively expensive and I had to find a solution in a hurry. Fortunately some series resistors and diodes solved the problem, but that was only the first in a long series of problems. As you can imagine the 741 op-amps would drift, along with the other circuitry. I evolved a method of calibration as detailed in my blog, Calibration, Part 1, on my own. This was all new stuff, remember. An article appeared in EDN from National Semiconductor using the same approach after I had come up with my ideas and I felt quite pleased with myself for being capable of original thought. In the end we prepared 32 cans of known weight that we would shift through the machine as part of the calibration cycle that was done every four hours or so.
With this we managed to get the machine mostly operational. At one stage though, the machine started working backwards. I remember talking on the phone with the mechanical designer who categorically stated (and I am paraphrasing to significantly less profane language) that there was absolutely no way that this could happen. I took some delight in informing him that I was looking at evidence to the contrary.
With all that resolved there was still a problem that really was a show stopper. At random points during a fill, the micro would go “nuts” and one or more of the feed channels would deposit a mountain of coffee over everything. The only thing that would cure this was a reset to the micro. No one had ever heard of a watchdog timer back then and I really doubted my abilities and my future. I suddenly had an inspired thought- I connected the chassis earth to the negative voltage of the internal power supply and suddenly everything was going right.
If I was a coffee drinker before this experience, I am pretty certain that I wouldn't have been after. Instant coffee is only a very small part real coffee (they wouldn't tell me how much). The rest is bulked up with maltose, dextrose and sucrose and probably several other “oses” as well. It is extremely hygroscopic and if there was any powder left standing, there would be this really sticky, gritty residue. At that time NASA was having trouble with the adhesive for the tiles on the space shuttle (it was not operational yet). I was sure that instant coffee would have done the trick. The only way to clean it was to wash the machine down with soapy hot water. When we started the installation, the air filtration and conditioning system had not been completed, so there was plenty of coffee dust in the air. It would get in your eyes, up your nose, down your ears and into your socks. Taking a shower after work was like standing in a percolator. If you had placed a cup in the shower drain you would have got a strong cup of (instant) coffee!
With time I moved into some analog design and although I still fell out of my depth, I do know now a bit about instrumentation amplifiers and other factors like drift, long signal wires, surge suppression, ground loops, and noise.