Newsgroups: comp.robotics
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From: abrown@hpcvcec.cv.hp.com (Allen Brown)
Subject: Re: robot parts: info and question
Message-ID: <1993Feb2.170809.2097@hpcvca.cv.hp.com>
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Organization: Hewlett-Packard Company, Corvallis, Oregon USA
References: <C1sq2I.DLC@world.std.com>
Date: Tue, 2 Feb 1993 17:08:09 GMT
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: I recently got Tower Hobbies catalog where their 'standard' servos
: were on sale for $11.99.  I believe that regular price is ~$13.

They must have come way down.  I have bought them for $28 in the past.

:  Of course, you still need at least 12 of them for a six legged robot!

We are doing a beetle.  That means we use three degrees of freedom.
So we need a total of 18 motors.

: I believe Genghis used standard R/C Hobby servos, but I don't know how
: much he weighed.  

I have been able to find very little information on Genghis.  For
instance, how fast does he walk?  How much power does he consume?

: >    No feedback.  There is no way to to determine if the motor has
: >      stalled before reaching its target position.  This was what
: >      killed servos as far as I am concerned.
: True, and that also kills one of the main benefits of servos; that you
: don't need any feedback to move them to a specific place.

Using the approach I have outlined it looks like I will be able to
produce a three axis leg (including control) for less cost than three
of those servos.  And when you look closely you will notice that the
controller+motor really is a servo.  So what we have shown is that we
can design our own servo with better performance and at less cost.

: Of course, you could put a current sensor on the power supply to the
: servo;  When the motor stalls, the current draw will go way up.

And that drives the cost higher.  The servo already has this info.
Putting in an additional (redundant) sensor is not cost effective.

: Standard R/C (Radio Control) Hobby servos use 3 wires;  5V, GND, and Signal.

That's part of the problem.  5V is not a good choice for motors.

: No, the 68HC11 CPU has 8 A/D inputs on board.  It also has an RS-485
: serial network type connection which would be a good way to
: daisy-chain several boards.  Perhaps one Miniboard per 2 legs?  With
: one master controlling them all?  Also, the 68HC11 has 8 timing
: outputs which can be set-up to control the aforementioned R/C servos.

I need to take a closer look at this.  It might make a good central
CPU.  It is far to expensive to compete against the Pic in leg
control.  But there is only one central CPU.

: Another question;  Will you be using one of the PIC's for the 'master'
: of all the others?  Or will you be using a traditional CPU?
:	Greg A. Kulosa

Maybe.  I figure we have a lot of problems to solve before we need to
solve this one.  First major benchmark: move.  I have just bought a
house.  The robot will get reduced attention until I have settled in.
Second major benchmark: get one leg working.
--
  Allen C. Brown  abrown@cv.hp.com or hplabs!hpcvca!abrown or "Hey you!"
      Not representing my employer.
    Q: What is the opposite of Artificial Intelligence?
    A: Genuine Stupidity
