Electronics > Beginners

How to make an analog circuit to proportionally scale a sensor output voltage?

<< < (3/9) > >>

vk6zgo:
As an analog person from way back, it pains me to say that you would be better off doing it digitally.
Use an ADC, then in software, modify the result to your requirement.
Obviously, it can be done in analog, but it gets fairly messy quickly.

GerryR:

--- Quote from: sokoloff on July 04, 2019, 11:30:23 am ---Could you feed the GM MAP sensor with a 6V reference voltage and dispense with the op amp?

--- End quote ---

That's a very clever scaling solution;  I like clever!!

David Hess:
Scaling and offset with a single operational amplifier is simple enough but if that non-linearity has to be corrected, then a break-point in the transfer function would be required.  That can be done with diodes or transistors within the feedback or input network.  Analog Devices has a lot of application notes on this subject.

Today a tiny and inexpensive microcontroller with ADC and DAC could also do it depending on the resolution requirements.

james_s:

--- Quote from: SparkyFX on July 04, 2019, 09:21:41 am ---
--- Quote from: rs20 on July 04, 2019, 05:48:55 am ---Here's the important question: do we actually WANT to emulate this crappiness? Does this application require such precision that modelling that slight inflection point at 0.5 Bar is really necessary?
--- End quote ---
In a turbo engine you rarely see negative pressure that far down. (it´s a manifold absolute pressure sensor)

--- End quote ---

I've had cars with turbo engines for ~20 years now, and when at low throttle and especially idle the MAP is the same as on a naturally aspirated engine. The turbo only really does anything once you are producing enough exhaust gas to spool it up and start boosting the pressure. During normal driving the boost gauge is in the vacuum region.

HwAoRrDk:

--- Quote from: Dundarave on July 04, 2019, 05:34:53 am ---But since there is an inflection point at 0.5 Bar, it makes things a bit more complicated.  However, I noticed that at 0.5 Bar, the output of the original looks to be 0.7ish volts...

--- End quote ---

Yes, I screwed up the original graph which made this crucial attribute not obvious. The figure for 0 Bar on the original sensor actually comes out to -0.2V according to the transfer function, but seeing as the sensor can't actually output a negative voltage, I manually entered zero, inadvertently making the graph slightly misleading. :palm: Correct graph below.



Which leads to...


--- Quote from: GerryR on July 04, 2019, 11:09:00 am ---Upon thinking about this a little more, the slopes of the output curves are different, so just a single scale factor won't work.  At 2.5 Bar, Original at 4.75 V and GM at 4 V; at 1 Bar, original at ~1.8V and GM at 1.55 V.  Therefore if you scale for 2.5 Bar, multiplier is 1.1875; at 1 Bar multiplier is 1.16.  That's about a 2.5% error.

--- End quote ---

The multipliers across the pressure range are specifically as follows:

Bar:0.51.01.52.02.5Multiplier:1.0291.1451.1831.2021.213
So yes, quite a large difference in gain between 0.5 Bar and 3.0, and a single scaling factor (e.g. 1.1875) wouldn't work properly. This is what I was trying, but failed, to put across in my original post - I really shouldn't do these things late when I'm tired ^-^.

Now, the question of whether I need to handle the case of values beyond the inflection point, below 0.5 Bar, is a good one. The honest answer right now is that I don't really know. I do know that some vacuum (i.e. < 1.0) will be seen, but I don't know by how much.

If it turns out that vacuum figures will not matter, then maybe, just maybe I can fudge around and find a good compromise single gain figure that works across the range and only introduces as little percentage error as possible.


--- Quote from: sokoloff on July 04, 2019, 11:30:23 am ---Could you feed the GM MAP sensor with a 6V reference voltage and dispense with the op amp?

--- End quote ---

No, I only have the 5V reference supply to work with.


--- Quote from: Dundarave on July 04, 2019, 05:34:53 am ---...which brings to mind the voltage drop of a diode...  Perhaps by arranging the feedback gain resistance in the op-amp circuit so that below 0.7 v, a fixed resistance (calibrated for the initial slope) is used, with a parallel second resister + diode (which would thus conduct at 0.7 v) to handle the slope (i.e. gain) difference for the rest of the curve.

--- End quote ---

I'm having trouble visualising what you're talking about here. Could you possibly illustrate it?

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod