manual control of a victor?

[Jeff Pahl]

Originally Posted by Joe Johnson

Would it be unacceptable to use a 555 in a billion dollar satellite project? Yes, of course.

However, I have used them to test billion dollar satellite projects...

In particular, I needed a PWM stream with two specific pulse widths, and used two 555's as one shots, fired by logic that picked which one based on if I needed a "1" or a "0". The logic was driven by clocked logic that followed all the "golden rules", but the 555 was far and away the easiest way to get the two pulse widths. Took 10 minutes to put it all together, which was far less time than I spent listening to various design engineers tell me I needed a PIC, or a FPGA, or ..... (all of which, in addition to having the added time of writing the software, in the spaceflight test world have the added burden of having to place all of the code used into configuration management, so the <$1 PIC now costs considerably more to use once in a test fixture).

There is almost always more than one solution to any engineering problem. One part of elegant design is picking the one that makes the most sense for a particular design problem, even if that means "breaking" some normally established "rules".

[KenWittlief]

your examples of being able to put together a one shot circuit quickly and easily assumes you already have experience with 555 timers, or similar devices, and you dont have to consult a data sheet to look up the equations, figure out whether to use ceramic or electolytic or tantilum caps, variable resistors, carbon, wire, precision resisitors, to get the performace you need under all conditions

I could equally argue that with my 10 years of experience using PIC chips I can pull a PWM or timer subroutine out of my existing libraries, compile the SW in a minute or two, and all I need is power and gnd to the 8 pin PIC chip, and it will be producing a very accurate, very precise waveform in a matter of minutes

and with the in-circuit programablilty of PIC chips, I can change any of the parameters faster than you can unsolder and solder caps and resistors.

I dont know where those 1 billion one shot chips are going each year. I can honestly say that in my 20 years of electrical and computer design experience, on both military and commercial products, I have never used one, and I have never seen any product at any of the companies Ive worked for use one either.

[billbo911]

Originally Posted by KenWittlief

I can honestly say that in my 20 years of electrical and computer design experience, on both military and commercial products, I have never used one, and I have never seen any product at any of the companies Ive worked for use one either.

And there in lies the rub!


I can say in my 20+ years working for one major medical imaging company, among the dozens of other thing GE is into, that I have seen multiple systems and circuits, from various vendors and corporations, that do use 555's.

I guess my point is: Until you have seen everything, every situation, every design, every possibility, making blanket statements and setting hard and fast rules, with no room to bend, is just not a good idea.

BTW, I did a quick Google search and found a great little 555 circuit that will suffice perfectly to generate a 1 to 2 ms pulse at a period of 17ms. One 555, one cap, one pot and a resistor. It even had charts on the site for the mathematically impaired to help decide what component values to use.

Don't get me wrong, I understand the Prof.'s reasoning for doing things his way. He has every right and privilege as a Prof. to do so in his class room. But unless he meets the "Until you have seen everything, every situation, every design, every possibility...." rule, his ways outside the class room aren't always the best ways.

[KenWittlief]

taking a step back from this discussion, Dr Schmidt taught the senior level Digital Design class

the focus of my career has been digital design and programming

and the 555 timer and other one-shot chips are analog devices

so this is really a discussion of the merits of digital systems vs analog systems - and the merits of hybrid digital/analog systems

so from Dr Schmidts persepective, he was correct. Analog timers have no place in a digital design class, and they have no place in a digital system (ie, you would never plop down a 555 timer to generate the CAS and RAS signals to an SDRAM bank on a pentium motherboard, you would use digital counters and state machines

I concede that if you are designing an analog system, then things like one shot timers are acceptable

and Im certain that analog designers have their own set of golden rules

but back to the original question of this thread, I think the debate that has been raging here is really a question of which approach is better, a digital or an analog solution?

BTW, part of the problem is that the PWM control of the victors is a carry over of the PWM of radio control model servos. Originally all RC systems were analog, but now here we are with these digital FIRST controllers interfaced to Victor speed controllers, with a legacy-analog PWM signal format. :^)

[Al Skierkiewicz]

Ken and Joe,
Let us not forget that no matter what the individual benefits or pitfalls, I can go down to the local Radio Shack and pick up a 555 usually with a schematic on the back and have something useful, albeit inaccurate, within an hour or so. I would love to use a PIC, but don't have any at a local store or my junkbox or even at work. And the fishing timer I built for my wife's aunt, works great and no one cares if it times out at 15 minutes or even at sixteen minutes. As long as it times out and reminds us to move on to a new spot, everyone is happy.
BTW, I used a dual 555 to make a really annoying alarm output for my Heathkit digital clock. It has lot's of distortion, dual frequencies and darn near knocks me out of bed at 4:44 AM each morning. (for an audio guy, the distortion does the trick) My wife likes that better than having 220 volts hooked up to the bedsprings.

[KenWittlief]

15 minute fishing timer? thats an interesting idea. How does it let the fish know its time to move to a new spot? :^)

[Al Skierkiewicz]

I don't know but it works really well. I haven't caught a fish on their lake yet. As a matter of fact I was presented with a plaque two years ago with the words "Fish laugh at the mention of your name".

[billbo911]

Originally Posted by Al Skierkiewicz

I haven't caught a fish on their lake yet. As a matter of fact I was presented with a plaque two years ago with the words "Fish laugh at the mention of your name".

You may want to change your fishing tactics. May I suggest Dynamite!
Worst case, the fish won't laugh any longer. They'll be deaf and never hear your name mentioned again.

[Adam Y.]

the focus of my career has been digital design and programming

and the 555 timer and other one-shot chips are analog devices

Does it affect anything that the motor is an analog device and needs an analog signal (PWM)??

BTW, part of the problem is that the PWM control of the victors is a carry over of the PWM of radio control model servos. Originally all RC systems were analog, but now here we are with these digital FIRST controllers interfaced to Victor speed controllers, with a legacy-analog PWM signal format. :^)

It's pulse position modulation for the incoming signal not pulse width modulation. Pulse width modulation is the signal that controsl the motor.

[Al Skierkiewicz]

Originally Posted by Adam Y.

Does it affect anything that the motor is an analog device and needs an analog signal (PWM)?? It's pulse position modulation for the incoming signal not pulse width modulation. Pulse width modulation is the signal that controsl the motor.

Adam,
That's kind of the beautiful things about this control. It is a digital signal, running from +12 to ground, that is converted to an average mechanical power with no real active circuitry. The motor does all the conversion with a little help from physics (conservation of momentum) and a little from electrical (inductance) etc.
The control signal and the output of the controller are both PWM. Pulse position signal uses the same size pulse but varies it's position in relation to a reference. In PWM the pulse width varies over the time of a single cycle but the cycle length doesn't vary. That makes it sort of self clocking in that for the most part there is a start and end to the pulse cycle. (about 2 kHz) The output PWM is not an amplified version of the input. The input pulse width varies over 255 steps where the value (width) of 127 represents no motor current at all. A "0" will make the output turn on to full 12 volts (no PWM) and a 255 will do the same but reverse the leads (+12 becomes common and common becomes +12 to the motor) which reverses the direction of the motor.