Brake pressure sensor based LOAD CELL

[Belatti]

I was looking to buy sensors to measure aero downforce and found out that load cells cost arround $2000 each. I have seen in the past years some that are difficult to calibrate and mount if you want to use them in different type of cars (touring or formula) with different type of dampers .

I also have seen some home made cells that consist in an Aluminum ring between the damper nut and the spring. Inside there is damper (or brake) fluid and a brake pressure sensor (100Hz is the max rate you can use with that, at least in AIM systems)

With that you can measure compression but not expansion Im afraid, but it may be enough to measure aero loads in straing line testing or in a skidpad. The cost of each sensor is $150 and the hidraulic device is very cheap, only 4 pieces of Al7075, a couple of orings and some fun time in the mill.

I would like to hear opinions or if there is any other drawback Im not seeing.

[Lycoming]

Are strain gauges really that expensive? I would think that a foil gage on the wing mount would be reasonably affordable, but admittedly I haven't checked the prices myself.

[Jersey Tom]

Cheap easy solution for straight line testing... why not just use linear damper pots. Especially if you have coilover spring/dampers. Could probably get you reasonably close.

Calibrate damper displacement to wheel travel with a height gauge and use use a rated spring rate to back out wheel load versus travel... or if you're not going to change springs, just pile on weights and calibrate wheel load to damper travel on scale pads.

[Caito]

http://www.omega.com/guides/straingages.html

Strain gages are not that expensive. But it would require a little bit more work. You'd need a bridge, and amplification. That's a bit of work, but for the price difference..

If you just need to measure differences(downforce increased/decreased with such part, rather than the exact value), it can be done relatively cheap. You could even buy the perpendicular ones and have temperature compensation.

Bye,
Caito!

[Tommy Cookers]

this type of cell might be suitable ?
http://www.omega.co.uk/pptst/LC8200.html
and this ?
http://www.omega.co.uk/pptst/DMD4059.html

[andylaurence]

This is an interesting topic. Please keep it on the forum if you can! I'm about to do the same, but I have chosen the linear position sensor route, rather than a strain gauge/load sensor for easier interfacing to my data logger (vref, gnd, signal).

[Greg Locock]

advantages of hydaulic load cell -robust, no glue, simple electronics

disadvantages -interrupts load path, may not be very stiff in all 6 directions.

unknown-frequency response (probably doesn't matter)

[flynfrog]

You could do what JT suggest but use oring on the shock shafts. Strait line run to known speed measure travel.

[Belatti]

Cheap easy solution for straight line testing... why not just use linear damper pots. Especially if you have coilover spring/dampers. Could probably get you reasonably close.

Calibrate damper displacement to wheel travel with a height gauge and use use a rated spring rate to back out wheel load versus travel... or if you're not going to change springs, just pile on weights and calibrate wheel load to damper travel on scale pads.

Thats what I have been using since I started back in 2010.
With stiff cars, the values you expect to change are in the order of magnitude of the noise level of linear pots.
Beside, with linear pots you can calculate load from spring displacement but not load from damper speed.
Im looking to do a test with both sensors per wheel and work with the results

[Tim.Wright]

Damper loads at the ground are tiny, and can be safely disregarded if you are doing straightline aero tests.

I have seen the same thing with noise in the linear pots. What you need to do is use soft springs for the aero tests to increase your signal/noise ratio. You need some experimentation to set the static ride height to the correct height such that at the test speed you are running at the target height but its possible.

Another way to clean up the signal (which I haven't tried yet) is to measure vertical and pitch accelerations and use them to cancel out the oscillations in the damper signals.

[Greg Locock]

The trouble with trying to set your car up as an inertial base is that you are multiplying all your errors together, and if you start looking for small differences in large numbers then it all gets very ugly. So I think measuring the force directly has a lot of upside.

[DaveW]

Loads are difficult to measure.

As Greg suggested, using position is simple, but requires much thought. As I'm sure JT knows, dampers are used quite deliberately to move running ride heights. Here is an example. Two damper set-ups, same springs, no D/F, trajectories start at the left at 1Hz, and sweep up in frequency to 40Hz. I have removed static load, and the position measurements are scaled in inches.

We tried pressure measurements for load when developing active suspension, but failed. Largely because they are the wrong side of friction, and any change in load was accompanied by large transients related to "water hammer" we thought. Let us know your experiences.

In the end, We designed our own load cell structure, bridging strategy, and signal conditioning. We hired a sub-contractor to mount the shear gauges and temperature compensate the result to our design. Not cheap, but it worked....

[Tommy Cookers]

Loads are difficult to measure. .....
In the end, We designed our own load cell structure, bridging strategy, and signal conditioning. We hired a sub-contractor to mount the shear gauges and temperature compensate the result to our design. Not cheap, but it worked....

loads can be quite easy to measure
the cost of 4 suitable load cells and a conditioning unit seems to be around $2000

or the amateur can design 'cells' around his own application's structural situation and make them, for a trivial expenditure
(this was my job for 20 years)
there is a lot of false mystique in this area, colleges are 50 years behind in their coverage of this technology
they seem unaware that gauges are essentially self-temperature-compensating (STC)
so eg for most amateur purposes including this one, temperature compensation needs no attention beyond the design stage
and anyway temperature compensation services will only compensate for (temporal) variations of spatially uniform temperatures
they will run a mile from anything else

if anyone is seriously interested in making these 'cells' I can contribute to the design process etc at no charge

btw
DWs need for load measurement following the choice of force as the nominal controlled variable in the active ride system ....
would have been alleviated if eg position or velocity had been chosen as the nominal controlled variable ?
which would be possible with some alteration to the control signals computed ?
eg if starting again today ? this would have practical advantages ?

[DaveW]

or the amateur can design 'cells' around his own application's structural situation and make them, for a trivial expenditure (this was my job for 20 years) there is a lot of false mystique in this area, colleges are 50 years behind in their coverage of this technology they seem unaware that gauges are essentially self-temperature-compensating (STC) so eg for most amateur purposes including this one, temperature compensation needs no attention beyond the design stage and anyway temperature compensation services will only compensate for (temporal) variations of spatially uniform temperatures they will run a mile from anything else .. if anyone is seriously interested in making these 'cells' I can contribute to the design process etc at no charge

The eternal amateur. That would describe me exactly.

I was drawn to design of figure 6c of this reference because of its high stiffness/output ratio. In detail however, the strain gauges are mounted in a region of relatively high strain gradients, implying that the quality of the device would depend in the accurate placement of the gauges. Further, the output of a simple shear bridge would be sensitive to offset loads (bending moments) and/or torsional loads. Minimizing cross axis sensitively required each of the structural shear arms to be loaded with four separate gauges, all wired up as a single bridge. In addition, the sixteen gauges were all manufactured using the same foil to minimize temperature sensitively of the gauges, but the length of the links still had to be adjusted to minimize temperature sensitivity of the connections. Not a complete stroll in the park, I think.

btw
DWs need for load measurement following the choice of force as the nominal controlled variable in the active ride system .... would have been alleviated if eg position or velocity had been chosen as the nominal controlled variable ? which would be possible with some alteration to the control signals computed ? eg if starting again today ? this would have practical advantages ?

Not really, Tommy. The whole shebang was designed to control loads. Why would you choose anything else?

Also btw, load cells weren't the only measurements used in the system.... others included strut position (LVDT's), lat & long acceleration, yaw rate, road speed, airspeed, steer angle, and hub vertical accelerations. So if you wished to play with alternative control laws, the system could easily have accommodated it....

[Greg Locock]

One practical approach to strain gauges is to mount lots of then around the item under test, and then calibrate them, in a SIMO fashion, in response to carefully applied loads. If you've guessed lucky then you'll get a nice response matrix that covers all the loads. There are pitfalls, and error analysis becomes much more difficult, but in practice it probably gives a better load set that the more typical approach of cutting the structure apart and adding perfect little strain gauge rosettes.