Alt-BEAM Archive

Message #09940



To: beam@sgiblab.sgi.com
From: "Ben Hitchcock" beh01@uow.edu.au
Date: Wed, 09 Feb 2000 23:44:35 +1100
Subject: [alt-beam] Re: The Problems In My Head?


I haven't built a power smart head yet, but I think I understand what the
circuit is doing...

From the graphs that you have given the list, I would suspect the motor
drivers. Notice how the output, although noisy, still is centered around
the right place? ie. +5 volts when grounded, and 0 volts when the input is
tied high? It seems to me that the chip is doing its best, but something is
making the power rails go screwy when you turn the motor on.

Have you tried putting a capacitor (0.1 uF or so) ACROSS the power rails for
the chip?

Here's what I'd do:

Take the 0.2 uF capacitor and motor out of the circuit, and try again. If
the noise goes away, then you will need a better motor driver, or some of
the inverters are connected backwards. If the noise stays, then read on...

Remove the link between the 10 Hz oscillator stage and the input to the
motor driver stage. See what the oscillator does now. If it still bursts
into static when the input is put high or grounded, then you have a problem
with the oscillator. I doubt that this will happen though, your oscillator
looks to be working fine.

If the output is stable, then the problem is with the motor driver.

Check the connections to the motor drivers that you have in parallel. I
would suspect that you mistakenly put an input where an output should be,
and an output where an input should be. This could result in high frequency
oscillation, exactly as you describe.

I would rub out the circuit diagram on your whiteboard, grab yourself a copy
of the 74x240 pin out (Yes yes I know you know it off by heart, but just do
it anyway) and REDRAW the whole circuit out, component by component.

See if the two schematics match.

And try not to take short cuts when drawing out the schematic!

Here's the pinout, as described by http://www.blee.net/chipdir/chipdir.htm.

+----------+
/1OE |1 +--+ 20| VCC
1A1 |2 19| /2OE
/2Y4 |3 18| /1Y1
1A2 |4 17| 2A4
/2Y3 |5 74 16| /1Y2
1A3 |6 240 15| 2A3
/2Y2 |7 14| /1Y3
1A4 |8 13| 2A2
/2Y1 |9 12| /1Y4
GND |10 11| 2A1
+----------+

Note half the inverters go left to right, while the other half go right to
left.

Good luck!

Ben
----------
>From: Elmo
>To: Beam Mailing List
>Subject: The Problems In My Head?
>Date: Wed, 9 Feb 2000 14:37
>

> Greetings and salivations everyone.
>
> I have returned to my PSH project (Power Smart Head for those who have
> just tuned in), after much cursing and swearing at it last year.
>
> I am looking for some advice on whats going wrong. I have attached
> several images of my oscilloscopes output and the circuit i am using.
>
>
>
> The Problem In Brief:
> When the oscillator stage reaches a point at which it is DC enough to
> run the motor all i get is noise in the oscillator and no motor
> movement.
>
> The Details:
> Without any optical front end attached (which basically is pulling the
> input to the oscillator stage toward either Vcc or Ground), i can get a
> nice 10Hz symetrical square wave. If i then try to connect the input to
> either Vcc or ground to simulate a full rail swing in the front end, my
> oscillator stage goes into a noisy state and i get no motor movement
> becuase of it. I have a 10Hz oscillation stage with a 5Hz power smart
> filter stage behind that, plus i have used all the left over inverters
> in my 74HC240 chip in parellel as an extra current kick for the motor,
> which is a small pager motor at present. I got a 0.22uf Cap across the
> motor to get rid of extra noise and i have done Wilfs trick with the
> filter stage of putting the resistor between the two caps in the circuit
> to break any HF (high frequency) noise paths. I have tried a 74HCT240
> and a 74ACT240 as well but i get the same results.
>
> Any ideas or suggestions in the BEAM collective?
>
> Elmo
>



9941 Wed, 9 Feb 2000 11:25:03 -0500 [alt-beam] Great gearmotors for sale "Dennison Bertram"
I hate to bring this up again, as I am selling something, but I know several
people had contacted me about where they could get good gearmotors for
walkers. So for those people, I'm still selling two never used Mac Eject
Motors on ebay. At
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=255893547 So far no
one's bid and there is only three day's left. Right now the price is $20 for
two, which is cheaper than servos. So, if anyone needs some gearmotors for a
two motor walker, here's a good deal.


dennison

Home