Subject:
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Re: Voltage Sensor
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 17 Sep 2002 12:33:14 GMT
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Viewed:
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616 times
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In lugnet.robotics, Horst Lehner writes:
> Hello,
>
> > If so you can use the RCX it'self to monitor the battery.
>
> This idea, while in itself interesting, will not work when the 12V battery
> gets regulated down to 9V. This is because these 9V will barely vary across
> the regulator's permitted input range. If of course the regulator is
> specifically designed to not be ideal with respect to line regulation [1],
> then the idea would work and is indeed very elegant, because it saves one
> sensor input. What I mean is this: When a 10V input voltage causes the
> regulator to drop its output level to whatever the RCX assumes to be
> BatteryLevel() 0%, and a 14,4V input voltage translates to whatever the RCX
> recognizes as BatteryLevel() 100%, then the regulator itself can elegantly
> be used as the measurement circuitry for the 12V battery's charging level.
It's amazing when you get a different perspective on a problem! While not
really applicable for this application the idea is indeed very elegant and
saves on a precious input port. I have a spare regulator which I will modify
to test this out.
Thanks.
> On the other hand, Tom seems to be ready to give up one sensor input for
> that, so a voltage sensor seems a straightforward way to do this. Here is
> one idea for that: It has been stated [2] that temperature sensor inputs
> convert a voltage of 0..5V to a raw reading of 0..1023 quite linearly. Now,
> for the purpose of reading a 12v battery's voltage, there is three
> possibilities (ordered by increasing accuracy):
> A) Make up a resistive network from the internal 10k pullup resistor and
> 2 external resistors (one pulldown to ground, one to the battery to be
> measured), and calculate output voltage as a function of input voltage
> B) Extend the range from 0..5V to 0..15V by a resistive divider followed
> by an Operational Amplifier
> C) Offset the range from 0..5V to 10..15V by an Operational Amplifier
>
> A) is the simplest of these, but in addition to not using the full 0..5V
> swing of the sensor input (which limit accuracy) it will draw some current
> out of the battery through the resistive network.
> B) and C) both need an operational amplifier in order to force some voltage
> in the range 0..5V into the RCX sensor input against the 10k pullup
> resistor.
>
> I can certainly elaborate more on one or the other solution, but not right
> now, and not on all of them ;-), so if you have more specific questions ...
>
> Greetings
>
> Horst
>
> [1] line regulation characterizes a voltage regulator's ability to
> retain its output voltage even when the input voltage changes.
>
> [2] <http://www.plazaearth.com/usr/gasperi/lego.htm>
Thanks for the reply Horst.
Given an option, I would rather pursue option C (there is never a substitute
for accuracy). I assume that option C is a development of option B with the
inclusion of the voltage off-set components.
With the setup you describe (option C) does the output of the op-amp go
directly to the RCX, or does it still need to be sent through a general
analog interface?
How does the voltage off-set (0..5V to 10..15V) improve the accuracy of the
sensor? And in this setup does the RCX still only receive a 0 - 5V input
range?(I am also assuming that the entire 0 - 12V battery range is being
monitored)
Typically, what components and their corrisponding values and type would be
required to build the option C configuration?
I live in Melbourne, Australia, and have access to a few different component
suppliers so general values and types would be OK.
I am rather new at electronics, so 'pull up resister' and 'resistive
networks' sound great but are new to me - but, I am always willing to listen
and learn from those who know.
Thanks again for the feed back
Cheers
Tom Beattie
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Message has 1 Reply:
 | | Re: Voltage Sensor
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| Hello Tom, (...) Yes. You need a low impedance output driving 0..5V into the RCX, thus overriding its internal pullup. (...) Actually, I should have written "resolution" instead of "accuracy". The 10..15V range spans 5V with 256 steps, giving you a (...) (22 years ago, 17-Sep-02, to lugnet.robotics)
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Message is in Reply To:
| | Re: Voltage Sensor
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| Hello, (...) This idea, while in itself interesting, will not work when the 12V battery gets regulated down to 9V. This is because these 9V will barely vary across the regulator's permitted input range. If of course the regulator is specifically (...) (22 years ago, 16-Sep-02, to lugnet.robotics)
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