Re: Risetime calculator (in tekwiki)


Hi Tim,
It is an ANALOG computer. These were very commonly used for many things but that was probably before you were born.
I got my first real job in 1967 working in a huge lab at Allied Aerospace that designed airplane autopilots and flight control systems. Until a year later these were always analog computers. To design the airplane's analog flight control computer to have the right characteristics a team had to measure the airplane's response to various (controlled) inputs such as how it responded when commanded to make a sharp turn.

The job of the airplane's analog computer was to take that raw response which was converted into an equation and create another equation that had the response the customer (an airline) wanted. Presumably that would be to turn the sharp bumpy turn into something gentle and smooth. This was all done on a precision analog computer that had a plug board that was at least 6ft by 10ft long. Various modules (OpAmps, inductances, capacitances, resistances, etc.) could be easily connected together and modified until the engineers were satisfied with the results.

The final configuration was converted back into an equation and then a different group of engineers took those equations and using OpAmp modules, from companies like Philbrick Research, put it all together in a small box that ultimately became the autopilot for that plane. By the time I arrived Allied Aerospace was using newly developed OpAmp ICs such as the uA709. I remember when I was only on the job for a few weeks and I blew one of those uA709s out. I was really scared that I would be chewed out. Those things cost $50 each because they were so new. The OpAmp ICs revolutionized everything we did. Suddenly the autopilot could be smaller and do more things.

2 years later digital ICs were beginning to be reliable enough that they could be used in parts of the autopilot. I worked on the Concorde Autopilot that was done with this new family of DIGITAL logic called DTL. It had a lot of details you had to be careful about like needing pull up resistors in certain cases. Those caused trouble for a while. The problems were solved eventually.
For a few years after that the autopilots were a mixture of analog and digital ICs. Most of the autopilot was analog, and parts like VOTER circuits, which decided which of the three autopilot results (everything is triple redundant in an autopilot) is correct, were digital. This was done with analog comparators and digital logic. If something should go bad with one of the three autopilots the voter circuit disconnects it and relies on the other two autopilot results.

Dennis Tillman W7PF

-----Original Message-----
From: [] On Behalf Of Tim Phillips
Sent: Thursday, January 16, 2020 11:32 PM
Subject: [TekScopes] Risetime calculator (in tekwiki)

from Tim P (UK)
What is this, please? It seems to be a simple analog computer, maybe with Log pots.
In case the link doesn't work, the device is in tekwiki Other Instruments.

Appears to give f = sqrt(a^2 + b^2 + c^2)

Tim P

Dennis Tillman W7PF
TekScopes Moderator

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