Alt-BEAM Archive

Message #02694



To: beam beam@corp.sgi.com
From: Richard Piotter richfile@rconnect.com
Date: Tue, 27 Apr 1999 16:41:13 -0500
Subject: [alt-beam] Re: Bicore head targeting?


PERRRRRRRRRFECT!!!

This is EXACTLY what a switching power supply does. It creates a pulse
that can have a variable duty cycle. A filter (caps, inductors) filter
it into DC.

You can cut the negative portion of the output with a diode, since there
is never an inactive state. Filter the output using a filter that you
might use for an average Switching powersupply. You should have a
varying voltage level.

Duty Cycle controlled variable voltage output (how a switching supply works)

50% duty cycle
+ | ________ ________
| | | | | |
0 |-|--------|--------|--------|--------|-
| | |________| |________|
- |

Volts out (after filter and diode using 5 volt scale for example)
5 |
4 |
3 |_______________________________________
2 |
1 |
0 '---------------------------------------


30% duty cycle
+ | ____ ____
| | | | | |
0 |-|----|------------|----|------------|-
| | |____________| |____________|
- |

Volts out (after filter and diode using 5 volt scale for example)
5 |
4 |
3 |
2 |_______________________________________
1 |
0 '---------------------------------------


70% duty cycle
+ | ____________ ____________
| | | | | |
0 |-|------------|----|------------|----|-
| | |____| |____|
- |

Volts out (after filter and diode using 5 volt scale for example)
5 |
4 |---------------------------------------
3 |
2 |
1 |
0 '---------------------------------------


If desired, attach that filtered output to a comparitor. Have the
variable resistor set up to allow you to adjust the "center point" (find
the area where you get a 50% duty cycle). You'll get a high or a low
output depending on whether the head is facing left or right (or
up/down) of the center point, where the Bicore is at 50% duty cycle.

If you want to take it further, you can use two comparitors, on it, one
tuned just below 50% duty cycle, and the other tuned just above. When
set up properly, the one set below 50% duty cycle (say 45% for an
example) will output high when the value is lower, and low when higher.
The other one is set just above 50% duty cycle (say 55% for example). It
will output low when the value is lower, and high when higher. The
result is as follows...

(note, I havn't actualy determined the polarity. I'm going by memory,
but you get the idea, I hope. Play around with it!)

The output from the + comparitor represents <55%, and the - comparitor
represents >45% (percents would be adjustable, just using the example
numbers). Say that a low duty cycle is left, and a high duty cycle is
right. then the + comparitor will output a low when the head is either
in the center "boundary" or lower duty cycle. In other words, 55% or
less. The - comparitor outputs a low when it's in the center threshold
or has a greater duty cycle. The result is as follows:

+ | -
---|---
0 | 1 - configured comparitor with a value below threshold
| + configured comparitor with a value below threshold
|
0 | 0 - configured comparitor with a value above threshold
| + configured comparitor with a value below threshold
|
1 | 0 - configured comparitor with a value above threshold
| + configured comparitor with a value above threshold


0% 45% 55% 100%
|_____________|___|_____________|
| | | |
| Left |Cen| Right |
|_____________|___|_____________|
|
Center threshold is adjustable

The function of all this is simple. A diode cuts the negative value out
of the output of the bicore. the pulse is put into a filter (like used
in a switching power supply). What you've essentialy created, is a
bicore driven switching power supply. The voltage varies higher or lower
depending on the duty cycle, which is determined by the diference of the
2 inputs. The 1 comparitor version just tells you whether the head is
above or below an adjustable threshold. The 2 comparitor verison creates
an adjustable boundary between left and right. There is a point where
the circuit registers the head as being BOTH left and right. When both
signals go low, the head has a fairly equal duty cycle.

Anyone want to try???


> Dennison wrote:
>
> I found out from dave that the controll system for his and Mark T's
> Heads are not Bicores. They use some other "poppernets" or something
> like that to control things. So that brings me back to bicores, how
> does one tell if the bicore has 'targeted' something? You should be
> able to compare pulse durrations, when the pulse durrations are equal
> in length you know that the head has 'locked on' and is no longer in
> motion. But how to compare these pulse's is the question. My first
> Idea was to connect both outputs in some fashion to the +v or gnd side
> of a sizable cap. The idea goes that, while one output charges the
> cap, the other output discarges. When the pulse durations are equal,
> then in theory the average net change in voltage across the cap should
> be zero. That would mean that at any other point, when the avereage
> net change was a positive value, the head was still in motion. I know
> it's confusing, but think about what would happen if the cap was being
> charged and discharged.
>
> Dennison

--


Richard Piotter
richfile@rconnect.com

The Richfiles Robotics & TI web page:
http://richfiles.calc.org

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