[ODE] Re: Torque settings for simulated servo

[Garvin Haslett]

I'm not sure if this is correct, after all g is acceleration due to gravity 
and all bodies fall with the same acceleration in a vacuum.  Furthermore 
the SI units of acceleration are m/s^2.  It seems then that altering the 
constant for g should only affect units for distance in my simulation.

As to why I do not use SI units in my simulation, if you check the ODE 
documentation it recommends that units should be chosen that they lie in 
the range 0.1 to 10 to ensure the engine's stability.

That said thanks for the suggestion and does anyone else have a suggestion 
as to what is going wrong here?

Garvin.

Damián Lezama wrote:
>
> Hello,
>
>     I think your mistake is with the constant "g". Supose you use grams
> for mass, is "g" still 98,1 ? I think not. My guess is that with your
> units g=981, so when you multiply by 1 unit of mass you get 981
> dm.hg/s^2  (dm=decimeter, hg=hectogram). I hope that helps. Why don't
> you use a standar system ? I use SI.
>
> Regards,
>     Damián Lezama
>     Proyecto de Grado - Construcción de Robots Bípedos
>     http://www.fing.edu.uy/~pgrobip
>
> Garvin Haslett wrote:
>
>> I am using ODE to simulate a robot that I am building.  To model the
>> servo motors I am using hinges but I seem to be making an error in
>> setting the servo torque.  I have set gravity to -98.1, thus implying
>> that one unit of distance is 10cm.  I have chosen my unit of mass to
>> be 0.1 kg.
>>
>> The data sheet for my servo lists the torque as being 4.1 kg cm or
>> 0.041 kg m.  I convert this to Nm by multiplying by 9.81 giving 0.402
>> Nm.
>>
>> I seem to be getting lost in choosing the correct unit of torque in
>> simulation.  I've taken the view that a real Newton is the kgm/s^2 so
>> therefore one of my simulated "Newton" shoud be 0.01 N (0.1m x
>> 0.1kg).  Continuing this logic my unit for torque should be 0.001Nm so
>> my the value I set the Max Torque for my servo should be 402.
>>
>> However once I do this I find my simulated servo lifting weights the
>> real one cannot.
>>
>> Is there any obvious flaw in my reasoning here?
>>
>> Thanks in anticipation of any help,
>>
>> Garvin.
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>>
>>
>
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> End of ODE Digest, Vol 12, Issue 2
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