Some help required.

jmbertoncelli@USA · 2009-01-10 23:00

Hello, I am using the Prop for a robotics platform and I would like to feedback the current battery voltage (leftover) to another computer but I do not have any idea on how to mesure the battery voltage using the prop.· if somebody would like to put me on the right direction?· Thanks a lot.

jm.

Leon · 2009-01-10 23:10

Use an ADC like the 12-bit Microchip MCP3201. It's got an SPI interface so it's easy to interface it to the Propeller.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

jmbertoncelli@USA · 2009-01-10 23:15

I am not very experimented from electronic point of view and I have a question about ADC,· I think that an ADC need some kind of reference voltage to work then how could I provide a reference voltage if the robotic platform is using the same battery source that I want to measure?

jm

grasshopper · 2009-01-10 23:19

Yea that would work. However if you want to be real accurate a precision voltage regulator would do the trick. Make sure that the battery voltage is not greater than the Voltage max on the ADC.

**Edited

You could probably get away with using some diodes to measure the voltage. Wire them up as a switch and limit the current into the propeller.

***Edited again

Just thought of a great simple idea:

Using a Varicap diode one could measure battery voltage if configured as a RC Circuit. I think there already is a RC measuring object. Since the Varicap diodes capacitance changes with respect to voltage then this could work. This is a theory however, but seems possible.

Post Edited (grasshopper) : 1/10/2009 11:30:44 PM GMT

Leon · 2009-01-10 23:38

The capacitance is very small with most diodes, that idea wouldn't work. Even with "hyper-abrupt" diodes, the change is only 100 pF or so over 5V, and they are very expensive.

Leon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

grasshopper · 2009-01-10 23:40

Eh it was a thought. I was about to look this all up. Thanks leon for saving me some time [noparse]:)[/noparse]

jmbertoncelli@USA · 2009-01-10 23:52

The voltage that I need to mesure is in a range of 7.5V to 14.5V.

Paul Baker · 2009-01-11 00:06

Use precision resistors to divide the voltage into a measurable range then use an ADC to measure it.

Many ADCs have internal references that use Vdd to generate it, since you have a regulator putting out a stable voltage everything will work.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Paul Baker

kwinn · 2009-01-11 00:08

Sounds like a job for the 2 pin sigma delta adc circuit.

Paul Rowntree · 2009-01-11 00:31

@jm : you are right to be concerned about using a reference derived from the same battery that you want to characterize ... The trick is to use a stabilized reference for the ADC, which can be a seaparate reference chip output or just use the 3.3 V that is driving the Prop. It will be kept at 3.3 V even if the battery source falls significantly, so then the divided-down battery voltage will be easily measurablke relative to this stabilized 3.3 V.

Cheers!
Paul Rowntree

jmbertoncelli@USA · 2009-01-11 01:04

Thanks! I shall give it a try tomorrow. Have a great evening all!

Ariba · 2009-01-11 16:23

It should be possible to use a simple R-C ADC to measure the Battery voltage.
Attached is a schematic, and here is a Spin code: (only the ADC methode, and not tested)

CON
  batt = 7               'I/O Pin for Battery ADC

PUB getBatt : time
  dira[noparse][[/noparse]batt] := 1        'discharge C
  outa[noparse][[/noparse]batt] := 0
  time := 0
  dira[noparse][[/noparse]batt] := 0        'Pin to input
  repeat
    time++               'measure time
  until ina[noparse][[/noparse]batt] > 0    'until C > 1.4V threshold
  dira[noparse][[/noparse]batt] := 1

You get not a value in Volt, but a value which is higher for low Battery voltage, and lower for higher Battery voltage.
(because the C is charged faster if the current is higher, and the current is higher if the battery voltage is higher).

Andy

Some help required.

Comments