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Posted by pogo on August 19, 2008, 1:01 am
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I read an article today in Wired magazine about a plan to get an electric car /
battery infrastructure going in Israel as a beta
test. Somewhere in the article it mentioned something like "don't need gears
because to go faster you just apply more electricity"
... I've seen this same statement mentioned in other articles about electric
vehicles.
But is this true? Would gearing up / gearing down help to preserve the battery's
power once the vehicle gets rolling - just as
gearing up/down does for internal combustion engines ? These statements seem a
bit ignorant ...
I figure this topic applies just as much to robotics, hence my posting it here.
JCD
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Posted by Curt Welch on August 19, 2008, 2:16 am
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> I read an article today in Wired magazine about a plan to get an electric
> car / battery infrastructure going in Israel as a beta test. Somewhere in
> the article it mentioned something like "don't need gears because to go
> faster you just apply more electricity" ... I've seen this same statement
> mentioned in other articles about electric vehicles.
>
> But is this true?
Yes.
> Would gearing up / gearing down help to preserve the
> battery's power once the vehicle gets rolling - just as gearing up/down
> does for internal combustion engines ? These statements seem a bit
> ignorant ...
>
> I figure this topic applies just as much to robotics, hence my posting it
> here. JCD
DC motors draw the power needed/used. If you drop the voltage (take you
foot off the "gas") to the level required to maintain your speed, the DC
motor will be drawing exactly what it needs to overcome air resistance and
all the friction effects and nothing more. No energy is wasted as long as
your voltage regulator is not wasting energy (which they all do to some
extent). Gears are mostly needed in cars because internal combustion
engines have very low torque and power and low speeds. Gears are added to
adjust for this fact. DC motors on the other hand have max torque at the
low speed so they have just what is needed. Their max torque is the stall
torque (the torque provided when the motor is not turning at all). Gears
are typically added to DC motors when the max RPM of the motor is higher
than the application needs - so you add gears to increase the torque and
reduce the max RPM delivered (you shift the power curve down to a lower RPM
point). But seldom is there a need to have more than one gear ratio (aka
multiple gears like a car). The only examples I can think of where DC
motors have multiple gears are where the gears are used as a cheap speed
control device. This is, instead of an expensive power control circuit,
they just used gears with no power control other than an on/off switch.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
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Posted by cadcoke4 on August 19, 2008, 7:34 pm
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On Aug 19, 1:16 am, c...@kcwc.com (Curt Welch) wrote:
> DC motors on the other hand have max torque at the
> low speed so they have just what is needed.
> Curt Welch
So, why do all those robotics stores sell gear motors?
Joe Dunfee
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Posted by Gordon McComb on August 19, 2008, 8:15 pm
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cadcoke4 wrote:
> So, why do all those robotics stores sell gear motors?
Couple quick reasons: The typical small hobby DC motor turns in excess
of 9000 RPM; most folks aren't as interested in robots going at freeway
speeds. Something a little slower is usually called for.
From a price standpoing, a set gear reduction is cheaper than complex
PWM driver circuitry as you'd find in an electric car.
-- Gordon
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Posted by John Nagle on August 19, 2008, 10:37 pm
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Gordon McComb wrote:
> cadcoke4 wrote:
>> So, why do all those robotics stores sell gear motors?
>
> Couple quick reasons: The typical small hobby DC motor turns in excess
> of 9000 RPM; most folks aren't as interested in robots going at freeway
> speeds. Something a little slower is usually called for.
>
> From a price standpoing, a set gear reduction is cheaper than complex
> PWM driver circuitry as you'd find in an electric car.
Motors capable of both high speed and controlled slow speed are
quite feasible. The classic example is elevator control. Gearless
traction elevators classically have a direct drive DC motor connected to the
traction sheave, and are able to level the car within 1/8 inch or so.
That's a nice piece of control, especially since it's been done with
relays, resistors, and motor-generator sets.
Adept's SCARA robots are gearless direct drive, with "pancake" brushless
servomotors directly driving the axes. Positioning to 0.001 inch is possible.
Motors capable of fine positioning like that usually have a large
number of poles, and are designed to avoid "cogging" problems.
While the control electronics becomes more complex, gearless
servomotors have some substantial advantages. For one thing, you
can't strip the (nonexistent) gears. At the large end of the scale,
they're preferred for shock loads like rolling mills for just that reason.
At the small end, there's no backlash, which is useful when
doing precise positioning.
John Nagle
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> car / battery infrastructure going in Israel as a beta test. Somewhere in
> the article it mentioned something like "don't need gears because to go
> faster you just apply more electricity" ... I've seen this same statement
> mentioned in other articles about electric vehicles.
>
> But is this true?