Alt-BEAM Archive
Message #09946
To: beam@sgiblab.sgi.com
From: Ben Hitchcock beh01@uow.edu.au
Date: Thu, 10 Feb 2000 07:39:19 +1100 (EST)
Subject: [alt-beam] Re: The Problems In My Head?
One more thing I thought of last night...
Remove the 0.2 uF cap altogether, and try the whole circuit with just that
component removed. It could be that this cap is causing the motor drivers
to change state, by going through the schmitt-trigger feedback portion of
the inverters. I don't know how the 240 is constructed internally, so I
could be talking through my hat here, but it's a possibility.
I would put that 0.2 uF cap across the power rails for the chip, rather
than across the motor.
Just a thought,
Ben
> 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
> >
>
--
Nothing is foolproof to a sufficiently talented fool.
9947 Wed, 9 Feb 2000 16:04:43 EST [alt-beam] RoachBot Photovore beam@sgiblab.sgi.com BUDSCOTT@aol.com
content-transfer-encoding: 7bit
Heres the file i tried to send, its a jpeg of my RoachBot photovore, could be
helpful if you want to see how you can attach on tacticle sensors. Sorry for
the inconvinience.
-Spencer
Attachment: Roachbotsml1.JPG
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