Wideband Logging PSA

[MasterG]

I'm using the MVPI Pro interface, and I have an Innovate LC-2 tied into it using its 5V analog output. The innovate controller is configured to output 4V while the sensor warms up and before it starts outputting the typical 0V-5V output. Today I noticed that VCM scanner was showing ~4.1V during warm up before the normal readings started showing up.

VCM scanner can actually pull serial (digital) data direct from the wideband controller as well. I figured out today that the response rate and resolution is much higher from the digital output vs the analog output, and I also noticed that the transformed analog data was showing about 15.0 AFR at idle while the digital data was hovering around 14.7 AFR.

The transform equation is (voltage / 0.3324) + 7.35 = AFR. That 0.1V corresponds to a 0.3 AFR error (0.1V / 0.3324 = 0.3) which I was also seeing comparing analog data vs digital data side by side. I'm not sure if the error is on the MVPI interface side or if it's on the wideband controller side (maybe tech support can offer some insight?), but the point is that you should double-check your numbers. In this case, the wideband controller is calculating AFR and then generating a voltage based on that value. The MVPI interface measures that voltage and converts it back to AFR. It's like a game of telephone. The digital interface gets rid of that variability and error that can often plague analog signals.

Hope this is helpful for someone else too.

[Keith@HPTuners]

The error is in voltage offset. It's a common problem, and discussed often.

[MasterG]

Don't mind me then. Just a newbie making obvious statements.

[MasterG]

I guess as a follow-up to this... Are the offsets typically even across the range or do they follow a curve? In other words, is my 0.1V offset likely to be constant across the full range (0V = 0.1V and 5V = 5.1V) or would it vary? Thanks, Keith.

[Jay@HAP]

The Tuning School has an Excel spreadsheet to help calculate voltage offset. It's quite nice and works well. I can post the file if anyone wants it.

[MasterG]

I'm interested. I added a custom math parameter to contantly calculate "error", but I have a feeling this may not be the way to go as I'm guessing there's some level of latency between the digital signal and the analog one. Therefore, if the AFR is changing, the math parameter is always going to show an error.

[Clutch1973]

I would like the file please

[dr.mike]

Unfortunately, this will only appear to work The voltage offset depends, largely, on the current that the controller draws to run the wideband sensor's heater. This runs anywhere from 0 to 3 Amps, depending on the EGT and exhaust gas volume/velocity. So, you may have one offset value at idle; and a completely different value at WOT. Assuming that the controller is properly managing the sensor's heater. The heater may be completely off at high-EGT and high flow. Or, it could be maxed out at low-EGT and high flow. At low flow, they tend to hoover around 1 amp.

The common practice of "making it read 14.7 at idle" only guarantees just that. To get reliable readings, when it counts, either use digital data transfer ( CAN or serial ) or, use an wideband that has active voltage offset compensation ( differential output ) which zeros out the error, automatically.

Here is a VERY mild example that just happened to be sitting on my desktop.

heater-load.JPG

You can see the heater driver current ( Yellow line ) reacting to RPM (green ) and Manifold Pressure ( red ). The error/offset voltage would tend to follow the heater drive current ( yellow ) line. It was around 50% at idle. But, would hit 80% under full load. If you set the offset value at idle, the readings would be off under load. But, would appear OK when checked again at idle / low-load.

[ElecTech]

Make sure you are also wiring a ground input to the MPVI that is referenced to your wideband ground. It seems obvious, but it is often overlooked and can contribute to your offset. I've also found that a capacitor at the input terminal helps greatly.

[MasterG]

Make sure you are also wiring a ground input to the MPVI that is referenced to your wideband ground. It seems obvious, but it is often overlooked and can contribute to your offset. I've also found that a capacitor at the input terminal helps greatly.

What size cap are we talking about? I've noticed during periods of rapid transition (say dumping the throttle after a WOT pull), the analog signal lags and looks more like a staircase than a smooth arc. Wouldn't a capacitor further decrease response time or does it actually help smooth out that "staircase" signal?

transition.png

See image above, the yellow signal is analog and red is digital. The transition shows a change from 13 AFR to full lean in about 500ms.

[MasterG]

Here's my first crack at a correcting for the voltage offset. I have a custom math that calculates percent difference between analog and digital. This graph shows digital AFR along the x axis.

AFRerror.png

I don't understand why the error increases so much as it leans out, but maybe that has something to do with the heater current like Dr. Mike said? Either way, I'm really happy with these results in the area that I'll be tuning. I had to subtract about 0.1V or 0.3 AFR from the analog signal to get this result. I just copied the nominal transform equation and subtracted 0.3 from the y-intercept.

[dr.mike]

See image above, the yellow signal is analog and red is digital. The transition shows a change from 13 AFR to full lean in about 500ms.

It looks like the digital/serial signal is being interpolated/smoothed. The Innovate wideband updates its serial and analog data at the same time ( ~ 12 per second ) . They should both have the "staircase" form with a period of 82ms per step. The data ( red trace ), between the updates, doesn't really exist.

[Pector55]

I also ran into the voltage offset issue. Mine had me off terribly and I only discovered it when they showed AFR at a dyno. My NA 5.7 was running 11.5 AFRs and I was running 12.3 in the 1/4 at the time. I went home and leaned the car out to the mid 12's and the next weekend I ran a 12.0. That's a significant difference. I used the voltage offsets to correct things but after I did, I found out about the serial input and found that to be spot on.

[MeentSS02]

I fought this exact same thing for years with the LC-1 I had in my old Camaro...for my current ride, I went this route:

https://www.hptuners.com/forum/showt...d-beta-testers

It is the only way to go IMO...no voltage offset worries, and super fast data rates.

[bkg06gto]

So 2 things,

1. Sorry to thread jack
2. Even more sorry for the stupid question that is to follow

I have been using my LC2 as well analog output with the pro interface. This particular LC2 was given to me by a Friend who upgraded. I have noticed the same fluctuations and voltage issues which also show my AFR not being accurate. How would I go about setting this up to the digital readout?

[MasterG]

Open up your list of parameters. There will be an option for Serial and LC-2. Add that to your list of recorded parameters and the software does the rest. I've noticed that you need to plug in your LC-2 and your MPVI before starting the scanner software otherwise it won't find it. I have to do this every time I shut the car down, close the scanner, unplug both USB cables, plug both back in, and restart scanner or it won't find the right port when you start logging.