Question about strain gauges

[karthikshanmugam]

Hello everyone,
We're planning to use half bridge strain gauges configuration to measure the load(stress) that is being applied on the A arms of our car and log data using a data logger (during testing in a close environment and remotely when the car is on track). We're planning to install nearly 25 strain gauges on our car. My questions are:
1)Do the race capture pro 2 have enough room for all these stain gauges?
2)Apart from the strain gauges we plan to install few more of the sensors for DAQ. What device/tool do we need to expand the input methods for the Racecapture Pro 2?
P.S.- I heard there's an equipment called VIMs ( Versatile Input Module ) which works in sync with the MoTec data loggers, that would work with differential sensors as well as increase the room space for the number of channels of the Central Logger. Do we have any similar kinda setup for the Racecapture Pro 2 as well?

Any help would be really appreciated. Thanks in advance. [/b]

[brentp]

Hi -

RaceCapture/Pro has 7 direct analog inputs. If you need many input channels, we recommend using an analog to CAN bus expansion, or custom build hardware for your strain gauges to output data directly in CAN bus. Yes, you have a very custom need with a very custom solution required. However, RaceCapture can accept the CAN data and map them to virtual channels.

Once you find your analog to CAN solution, see the CAN integration how-to: https://wiki.autosportlabs.com/CAN_Bus_Integration

Hope this helps, let us know what you find out.

[karthikshanmugam]

[quote="brentp"]Hi -

RaceCapture/Pro has 7 direct analog inputs. If you need many input channels, we recommend using an analog to CAN bus expansion, or custom build hardware for your strain gauges to output data directly in CAN bus. Yes, you have a very custom need with a very custom solution required. However, RaceCapture can accept the CAN data and map them to virtual channels.

Once you find your analog to CAN solution, see the CAN integration how-to: https://wiki.autosportlabs.com/CAN_Bus_Integration

Hope this helps, let us know what you find out.[/quote]

Thanks a lot for the reply, brent.
Recently, we came up with an idea of mutiplexing the input sources, in order to expand the number of input channels. Will this help us solve the issue?

[brentp]

Can you explain in more detail how it would be multiplexed?

[toga94m]

If you don't need really fast data, you could use 4 digital output bits to drive a 16:1 mux, which would feed one of 16 analog channels at a time into one RCP analog input. Or, you could use two 16:1 mux chips to bring 16 pairs (32 analog channels) into two RCP analog inputs in parallel. Or 3 digital output bits commanding four 8:1 chips routing 32 signals into four analog inputs in parallel. Then do some fancy scripting to sort it all out into virtual channels. Update rate depends on script loop rate. More parallel channels/simultaneous analog sampling means a faster pass through all 32 discrete analog inputs.

Disclosure : I make strain gage amplifiers at my day job. You won't get good results just hooking the half-bridge directly to the RCP input, or thru a mux. You'll need amplification per channel first.

[brentp]

Interesting, but a muxing solution like that actually exceeds the complexity of using off the shelf analog to CAN converters.

Leveraging the CAN bus for I/O expansion is *exactly* the purpose it was suited for, and will yield much higher sample rates.

Good point about strain gauge amplification.

[karthikshanmugam]

[quote="toga94m"]If you don't need really fast data, you could use 4 digital output bits to drive a 16:1 mux, which would feed one of 16 analog channels at a time into one RCP analog input. Or, you could use two 16:1 mux chips to bring 16 pairs (32 analog channels) into two RCP analog inputs in parallel. Or 3 digital output bits commanding four 8:1 chips routing 32 signals into four analog inputs in parallel. Then do some fancy scripting to sort it all out into virtual channels. Update rate depends on script loop rate. More parallel channels/simultaneous analog sampling means a faster pass through all 32 discrete analog inputs.

Disclosure : I make strain gage amplifiers at my day job. You won't get good results just hooking the half-bridge directly to the RCP input, or thru a mux. You'll need amplification per channel first.[/quote]

That sounds like a great idea! But, I couldn't understand the later part about the virtual channels? What and why "scripting" needs to be done for this kinda method?
Yes, we're planning to buy some instrumentation amplifiers with programmable offset gain.

[karthikshanmugam]

[quote="brentp"]Interesting, but a muxing solution like that actually exceeds the complexity of using off the shelf analog to CAN converters.

Leveraging the CAN bus for I/O expansion is *exactly* the purpose it was suited for, and will yield much higher sample rates.

Good point about strain gauge amplification.[/quote]

I agree with you brent. multiplexing would result in low sampling rates. But do you think analog to CAN would result in a better way than these?

[brentp]

Yes, the way I described would be a superior solution for all of the reasons stated.

[karthikshanmugam]

Does the RCP come with a built in excitation voltage, so that we can use this for exciting our bridge circuit? If yes, do we have to make use of it by any scripting methods?

[brentp]

Hi,

RaceCapture/Pro has a fixed 5v reference output that can power external sensors - see the specifications here: http://wiki.autosportlabs.com/RaceCapturePro

If this is insufficient, you will need to create an external circuit to amplify and condition the signal from the strain gauges.

[karthikshanmugam]

Thanks for the quick reply brent
I came across NI-DAQMX, in which we can design and execute daq systems (with or without real hardware ) in LabVIEW.
In a similar fashion, does RCP works on any platform?
We're trying to validate our results with the simulation results that can be acquired by modeling the system on some VIs. If our results match almost precisely, from there we're planning to design our own signal conditioning pcb.

[brentp]

RaceCapture/Pro lets you map any 0-5v analog signal to a real-world value. You'd just need to build your circuit to provide that kind of output.