Jaguar current sensing technical details?

[Kevin Sevcik]

Actually, I'd look move to pages 8 and 9 of the data sheet, on "Accuracy variations as a result of Vsense and common mode voltage".

This amp is apparently most accurate at sense voltages >20mv. Which equates to 20A for us. There's 3 operating cases in this range, and we're not in the worst... So it could be worse? I've attached a snapshot of an example graph. What it boils down to is that the amp goes non-linear below 20mv, and eventually lands on some positive offset voltage at the output, instead of zero. They state the worst case zero offset at the output is 300mv. It looks less linear the closer you get to 0 Vsense, so that's exciting too.

Oh yeah. And both Jags record a zero offset for the current ADC on power up, and then subtract that from current readings forever after. So if you end up with the worst case 300mv offset, then your readings above 20A will read 15A low. We're probably not getting this worst-case scenario, but that's obviously going to affect the accuracy of ALL the current readings. Better might be to just assume/hope that you have little absolute error in the ADC, and just use the zero offset as a linear correction below 20mv Vsense.

Maybe FIRST will let us modify the Jag firmware this year?

Yeah, you're right. That's never gonna happen.

[EricVanWyk]

Originally Posted by Kevin Sevcik

Maybe FIRST will let us modify the Jag firmware this year?

Yeah, you're right. That's never gonna happen.

But TI can modify it. Perhaps you could submit the change to them. Bonus points for sending code with the request.

[Kevin Sevcik]

Originally Posted by EricVanWyk

But TI can modify it. Perhaps you could submit the change to them. Bonus points for sending code with the request.

Oh certainly. I'll probably try playing with some of our jags sometime this season or after and see how trustworthy the current sense is and how good a fix my proposal is. My only concern would be that I'll be modifying the reference firmware and this all might not apply to the FRC firmware. I'll have to see if they're willing to release that source code to interested parties.

[kamocat]

Originally Posted by Kevin Sevcik

Actually, I'd look move to pages 8 and 9 of the data sheet, on "Accuracy variations as a result of Vsense and common mode voltage".

Where did you find this datasheet? It's certainly not the INA193AID one I attached.

[Al Skierkiewicz]

Kevin,
The reason for the non linearity is the device's inability to drive the output to zero volts. The specification is for a minimum of 3 mV typical but can be as high as 50 mV. The TI datasheet on page 4 has a very descriptive curve showing the errors. In applications where minimum errors are needed, designers would usually add a biplolar power supply to allow driving to zero volts.
http://pdf.chinaicmart.com/3AI/INA193AIDBVR.pdf

[Ether]

Originally Posted by Ether

See attached images. For simplicity, I showed path through only one FET of each pair, and only for + currents. Green path is during ON portion. Blue path is during OFF portion (for high-side switching). Red path is during OFF portion (for low-side switching).

I don't know if the Jags use high-side, or low-side, or some other switching method. Does anyone know, definitively, what switching method the Jags use? I've heard many different claims.

Kevin Sevick has posted information about the Tan and Black Jag switching.

I could not edit my earlier post in this thread to reflect this new information, so I am attaching an updated image here.

Also, there's an error in the Tan Jag image. The blue line should be shown going through the Q10,11,&12 FETs, not their Zener diodes.

[Kevin Sevcik]

First, here's the datasheet I'm working from. The latest and greatest from TI itself:INA193 datasheet

I see the note about the swing range of the output, but there's additional notes just above that and on page 8 and 9 that declare the output bottoms out at 300mV typ. Perhaps the 300mV thing isn't the dominant issue and it really will swing down to 3mV with increased error. It's kinda hard to decide, which all the more inclines me to just empirically test the thing.

[kamocat]

Originally Posted by Kevin Sevcik

Actually, I'd look move to pages 8 and 9 of the data sheet, on "Accuracy variations as a result of Vsense and common mode voltage".

This amp is apparently most accurate at sense voltages >20mv. Which equates to 20A for us. There's 3 operating cases in this range, and we're not in the worst... So it could be worse? I've attached a snapshot of an example graph. What it boils down to is that the amp goes non-linear below 20mv, and eventually lands on some positive offset voltage at the output, instead of zero. They state the worst case zero offset at the output is 300mv. It looks less linear the closer you get to 0 Vsense, so that's exciting too.

The accuracy of the INA193−INA198 current shunt monitors is a function of two main variables: VSENSE (VIN+ − VIN−) and common-mode voltage, VCM, relative to the supply voltage, VS. VCM is expressed as (VIN+ + VIN−)/2; however, in practice, VCM is seen as the voltage at VIN+ because the voltage drop across VSENSE is usually small.
This section addresses the accuracy of these specific operating regions:

• Normal Case 1: VSENSE ≥ 20mV, VCM ≥ VS
• Normal Case 2: VSENSE ≥ 20mV, VCM < VS
• Low VSENSE Case 1: VSENSE < 20mV, −16V ≤ VCM <0
• Low VSENSE Case 2: VSENSE < 20mV, 0V ≤ VCM ≤ VS
• Low VSENSE Case 3: VSENSE < 20mV, VS < VCM ≤ 80V

So we're in Low VSENSE Case 3, where VCM is greater than the 5v supply voltage? That must be why they measure current on the high side.

Above 20A, then, this current measurement is incredibly accurate; within %0.001 if your battery is at 11.5v or greater.

Originally Posted by Kevin Sevcik

Oh yeah. And both Jags record a zero offset for the current ADC on power up, and then subtract that from current readings forever after. So if you end up with the worst case 300mv offset, then your readings above 20A will read 15A low. We're probably not getting this worst-case scenario, but that's obviously going to affect the accuracy of ALL the current readings. Better might be to just assume/hope that you have little absolute error in the ADC, and just use the zero offset as a linear correction below 20mv Vsense.

Except for that. They just made it measure low for the WHOLE range. So that means, if you get a reading of 5A or greater, you need to add 15A to make it accurate. XP

Wait a minute. Where did you get the 15A? I see 0.3v on the graph, which equates to 30A.

[Kevin Sevcik]

Originally Posted by kamocat

Wait a minute. Where did you get the 15A? I see 0.3v on the graph, which equates to 30A.

300mV on the output / 20V/V = 15mV = 15A

Again, this is all kinda theoretical. I'd really like to check it all with an actual amp meter. After all, they note the 300mV on the output as a worst case.

[kamocat]

Originally Posted by Kevin Sevcik

300mV on the output / 20V/V = 15mV = 15A

Again, this is all kinda theoretical. I'd really like to check it all with an actual amp meter. After all, they note the 300mV on the output as a worst case.

I messed up again! (I'm starting to get used to this) I mean 3A, not 30A.

You're right according to the 20V/V spec on the datasheet, but the graph shows 100V/V.
(At 20mV input, it says there's a 2V output)

It appears this datasheet conflicts with itself.

EDIT:
Would you say a Fluke 179 would work for measuring current? Or would you prefer a 0.01 ohm current shunt with analog meter?

[jhersh]

Originally Posted by Kevin Sevcik

My only concern would be that I'll be modifying the reference firmware and this all might not apply to the FRC firmware.

The FRC firmware is directly based on the reference firmware. The only goal of the separate version is to ensure safety (DS / FMS in control). As such you can assume that other aspects of the firmware are the same as the RDK.

Originally Posted by Kevin Sevcik

I'll have to see if they're willing to release that source code to interested parties.

In my experience, that's very unlikely.

[Al Skierkiewicz]

[quote=kamocat;992566]
You're right according to the 20V/V spec on the datasheet, but the graph shows 100V/V.
(At 20mV input, it says there's a 2V output)

[quote]

The graph(s) is data for three different devices. You have to be careful to look at the 193 (20v/V) graph only.