Calibrating sensors to your Traqmate

#13

jlevie wrote:

[quote]Ranger wrote:

[quote]jlevie wrote:

Hence my surprise that it took successive iterations of comparing session notes with data to ultimately succeed. Each calibration change I made took the values farther away from what they should have been. At endstate I had calibration values that made no sense, but I had good data.[/quote]
I don’t know what you did, but things to consider are that a computed zero (or max depending on the slope of the sensor) may not be zero volts at the logger input. You have to take zero bias into account for the other data points. And understand that when using a current limiting resistor the Vout is:

Vout = Rs*Vref/(Rs+Rr)

where:
Vout is the input to the logger
Rs is sensor resistance at that cal point
Rr is current limit resistor
Vref is the reference voltage[/quote]
No offense, but I’m an engineer, not a poet. Maybe you’re explaining the details for those non-technical types that might stumble across this thread in years hence?

#14

[quote]Ranger wrote:
No offense, but I’m an engineer, not a poet. Maybe you’re explaining the details for those non-technical types that might stumble across this thread in years hence?[/quote]
No offense, but if I had a dollar for every engineering mistake I’ve encountered I could retire and investigate the affect of enforced idleness on the human condition.

What you need to straighten things out is a physicist/mathematician.

#15

I don’t like bumping old threads but I figure it’s better to keep my post grouped with the other relevant posts.

So I think I’m going to follow Ranger’s suggestion of buying a DC-DC converter. I’m thinking of going with the 12 VDC just because 15 VDC is too much for my DAQ (0-5V).

I did some maths and decided to use a 500 ohm resistor for the voltage divider. Doing some more maths shows me that when the sensor is at 240 ohms the signal voltage will be 3.89V and when the sensor is at 33 ohms the signal voltage will be 0.74V. This is a reasonable range since my DAQ is 0-5V with an ADC resolution of 1024 divisions which results in a resolution of around 0.12 psi which is very reasonable for measuring things like oil pressure where a resolution of around 0.25-0.50 psi would be sufficient.

My next problem and this is where I need help. Calculating V^2/R for the resistor at both extremes (240 and 33 ohms for the sensor) and I find that the 500 ohm resistor is drawing 0.13 W and 0.25 W but calculating it from the DC-DC converter datasheet which shows an output current of 1250 mA I*R results in 62 W being absorbed which is quite a lot.

Which one is correct? Also, I can’t send a lot of current into my DAQ. I’m not sure what the limit is but it’s pretty low.

#16

Your an engineer. Why did you get a traqmate? Traqmate is for the nontechnical people. I thought you were going to build an arduino logger dash and I was very interested in your progress.

#17

I am. I’m trying to figure out how to wire the sensors up to it without destroying it. The wiring of the sensors is pretty much universal regardless of whether it’s a Motec datalogger, a Traqmate, or my homebrewed setup.

#18

[quote=“MrDomino” post=64429]I don’t like bumping old threads but I figure it’s better to keep my post grouped with the other relevant posts.

So I think I’m going to follow Ranger’s suggestion of buying a DC-DC converter. I’m thinking of going with the 12 VDC just because 15 VDC is too much for my DAQ (0-5V).

I did some maths and decided to use a 500 ohm resistor for the voltage divider. Doing some more maths shows me that when the sensor is at 240 ohms the signal voltage will be 3.89V and when the sensor is at 33 ohms the signal voltage will be 0.74V. This is a reasonable range since my DAQ is 0-5V with an ADC resolution of 1024 divisions which results in a resolution of around 0.12 psi which is very reasonable for measuring things like oil pressure where a resolution of around 0.25-0.50 psi would be sufficient.

My next problem and this is where I need help. Calculating V^2/R for the resistor at both extremes (240 and 33 ohms for the sensor) and I find that the 500 ohm resistor is drawing 0.13 W and 0.25 W but calculating it from the DC-DC converter datasheet which shows an output current of 1250 mA I*R results in 62 W being absorbed which is quite a lot.

Which one is correct? Also, I can’t send a lot of current into my DAQ. I’m not sure what the limit is but it’s pretty low.[/quote]

The idea of “sending current into the DAQ” is wrong. There’s a misunderstanding of electricity there. The DAQ senses voltage, that’s it.

The most current your bridge will pass is 15V/(500+33) so ~ 30mA. 30mA thru 500 Ohms is 15W so I’d get a 25W resister and fasten it to a piece of AL as a heatsink.

Get a chip that is rated at 500mA@15V so it will be nice and stable. Then fasten it to a piece of AL also.

#19

I’m just learning about this sh%% this semester. I’m a 27yo born again freshman. Your the only one who can figure it out. If your still in college ask your professor. If not just PM me and I’ll ask one of mine. I’m at Milwaukee School of Engineering right now. I think I can find someone who knows the answer.

#20

As to the power through the current limit resistor, Scott has the right answer.

I’d sacrifice resolution and use a larger limiting resistance. I can’t think of any reason that you need better than 1-2psi resolution for oil or fuel pressure. The allows for the use of a lower power resistor.

Since the DAC input is limited to 5v, I would drive the measurements with a 5v regulated source. That would eliminate any risk to the DAC should the resistor get shorted. That is how the analog inputs to my IQ3 are set up.

#21

[quote=“jlevie” post=64444]

Since the DAC input is limited to 5v, I would drive the measurements with a 5v regulated source. That would eliminate any risk to the DAC should the resistor get shorted. That is how the analog inputs to my IQ3 are set up.[/quote]

I respectfully disagree with the 5V source. You’re on the right track Chris, a 5V source would give you some tiny measurement input range of 1.5V or so. Something that is designed to work with each other and the sensor has it’s own ground can work ok with a 5V reference source but to be reliably accurate and keep the calibration ya gotta start with a higher reference V.

IMO the power resister shorting out isn’t the most likely way to hit the DAC with too much V. If the input gets hit by too much V some day it will probably be a wire to the car’s battery brushed it or it got killed by welding.

#22

[quote=“Ranger” post=64434]

Get a chip that is rated at 500mA@15V so it will be nice and stable. Then fasten it to a piece of AL also.[/quote]

When you say chip do you mean the DC-DC converter, the resistor, or something else?

#23

So what are you using anyway. I would guess some kind of usb powered system that has built in 5v outs. I would go the 5v route. I don’t think the supply voltage matters as much as the consistency of it, as long as you don’t burn it out.

#24

The 5V out doesn’t give me that great of resolution and based on Ranger’s experience it sounds like it doesn’t exactly provide stable measurement. Ranger’s solution allows me to keep the sensor power in the engine bay and then just run the signal wires back to the DAQ while likely achieving a better signal quality from them. I’d prefer to not waste time reinventing the wheel and Ranger has spent considerable time working this out already.

#25

[quote=“MrDomino” post=64464][quote=“Ranger” post=64434]

Get a chip that is rated at 500mA@15V so it will be nice and stable. Then fasten it to a piece of AL also.[/quote]

When you say chip do you mean the DC-DC converter, the resistor, or something else?[/quote]
Well, I suppose you could call it a DC-DC converter, but what it’s really called is a regulated power supply. Something like 15V out and 12-24V in. That is to say that it will tolerate a wide range if input V but can be depended upon to maintain output V at 15V.

#26

I think it depends on the ohm range of the sensor your using. I can’t be bothered to do any math right now. you want 0 to 5v in the signal wire. If the minimum ohms of the sensor gives you 5v or less at 15v input then go for it. I don’t think that in any situation increasing the supply voltage will improve the resolution of your sensor.

Ranger had a very specific reason and application for the 15v input which I can’t remember now.

#27

The datasheet of the part number you listed earlier calls it a DC-DC converter which I agree is misleading since it gives the impression that it’ll take a fluctuating DC voltage input and turn it into a fluctuating DC output just scaled.

#28

[quote=“turbo329is” post=64469]I think it depends on the ohm range of the sensor your using. I can’t be bothered to do any math right now. you want 0 to 5v in the signal wire. If the minimum ohms of the sensor gives you 5v or less at 15v input then go for it. I don’t think that in any situation increasing the supply voltage will improve the resolution of your sensor.

Ranger had a very specific reason and application for the 15v input which I can’t remember now.[/quote]

The “very specific reason” was “did the math”.

#29

Yep. The only way to get good resolution is to input 12 or 15 VDC.

#30

Is there any reason in particular that you said to not buy 10-180 ohm sensors? I want to buy ones with floating grounds since I won’t be able to properly ground the water temperature sensor since I think I’m going to use something like this I think http://www.frsport.com/Greddy-16401636-Radiator-Hose-Water-Temp-Sensor-Adapter-36mm_p_15545.html

#31

There’s 2 common sensor resistance ranges, 10-180 Ohm and 240-33 Ohm, or near enough. The first is often called the “VDO range” and the other the “US Range”. I might have those reversed, but I think it’s right.

Note that the two are reversed so full sweep VDO range is high resistance whereas US range it’s low resistance. The US range is easier because the resistance doesn’t drop so low, and also generally the sensor won’t go full scale so you’ll likely never drop below 50 Ohms. In contrast the VDO range sensor is going to be at 10 Ohms every time you start the car. Consider what this means in the context of current limiting. Every time you start the car the VDO sensor is going to pull a lot of current so your power supply and power resister are going to get hot and stressed.

The floating ground business is a non-issue. Just buy a sensor with a ground prong and wire it to a ground. That’s a separate issue from VDO or US range. Alternately imagineer a way to connect a ground wire to the coolant hose adapter.

#32

The problem is that there aren’t that many 240-33 ohm temp sensors with a floating ground. I figure running a dedicated ground from the DAQ to the sensors would help cut down on the amount of noise and eliminate any offset issues with the senors. I’ll have to calibrate each sensor individually based on the exact resistance of the current limiting resistor and all should be good.