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From: sharkey@robots.ox.ac.uk (Paul Sharkey)
Subject: Re: Distance to Mars
Message-ID: <1993Jun11.155202.8666@duncan.robots.ox.ac.uk>
Originator: sharkey@duncan.robots
Keywords: virtual reality, mars rover, speed of light
Organization: Robotics Research Group, Engineering Science Dept, Oxford, UK.
References: <1v5t56INNlkm@flop.ENGR.ORST.EDU> <1993Jun10.145007.12268@mksol.dseg.ti.com>
Date: Fri, 11 Jun 1993 15:52:02 GMT
Lines: 61

<1993Jun10.145007.12268@mksol.dseg.ti.com> writes

>> In article <1v5t56INNlkm@flop.ENGR.ORST.EDU> brittar@research.CS.ORST.EDU (Robert Wade Brittain) writes:
>> >
>> >Hmmm, just one question:  doesn't speed-of-light restrict communication
>> >speed?  You've probably got delays of a few minutes from here to Mars;
>> >control like that either implies a really slow robot, or that you can
>> >anticipate moves so far ahead...
>> >
>> >Uh, yes, isn't Mars approximately 26 light-minutes away?  That would translate
>> >to a 52 minute Minimum Sense-to-Action time.  I've noticed that on several
>> >occasions while riding my bicycle that turning my head to look at distractive
>> >scenery for only a few seconds has a tendency to put me in near- catastophic
>> >predictaments.  I can't imagine going anywhere with a 52-minute reaction time.
>> >(except very very slowly!!)  Can anyone enlighten me ?

>> At closest approach to Earth, Mars is roughly 30 million miles away, which
>> works out to about 160 light-seconds.  At farthest, Mars is some 200 million
>> miles away, or about 1100 light-seconds, or about 20 light-minutes away.

>> Obviously, you are not going to do real-time closed-loop manual control,
>> relying the operator to close the feedback loop with hand/eye coordination.

>> The EASY answer is to close the loop from Mars orbit, but that requires us
>> to do something very politically incorrect in certain circles: manned 
>> spaceflight.


Cycling you are using your eyes as part of a closed loop feedback
system for collision avoidance (and to a large extent balancing
from the hoorizon). Thus you need constant update from your eyes.

On a remote manipulator or rover, the loop can closed *locally* (ie on
Mars) using machine vision or, more commonly, Polaroid/laser sensors
for the menial tasks of collision avoidance/getting to a goal/et
cetera.  Now the *remote* instruction set reduces to commands like "get
to that rock 200m away and let me know when you get there".

>> However, there is currently a fair amount of amateur interest in

 and a lot of professional interest in 

>> teleoperation in the presence of time lags, and a fair amount of work
>> being done.  The work I have been following has been more interested
>> in 2-second time lags.

As mentioned everything is not that simple and there are considerable
problems using master-slave systems --- even over short distances.

P.


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Dept Eng Sci, Univ of Oxford        Email:   sharkey@uk.ac.ox.robots
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