Adrenalynn

01-10-2009, 03:57 PM

I'm working with the SpectraSymbol flex resistors that are in the Trossen Catalog (http://www.trossenrobotics.com/sparkfun-flex-sensor-4-5-inch.aspx)and have some interesting results testing them in the simplest possible voltage divider setup...

They are horrendously noisy, and I tried to keep the test consistent by strapping the bottom to a stiff wire and then bending them each time until they touched the edge of my breadboard. Not exactly scientific, but fairly consistent. I had two sensors to test, and they were pretty close together in their readings (thankfully!).

Here's the results of using different resistors in the divider [Vin = 5.026v with +- .00350v drift:] [All resistors tested are 10%]

22k : 3.60 - 2.63v = 0.97v

27k : 3.84 - 2.94v = 0.90v

33k : 4.03 - 3.23 = 0.80v

15k : 3.15 - 2.2v = 0.95v

12k : 2.87 - 1.9v = 0.97v

10k : 2.64 - 1.7v = 0.94v

4.7k: 1.67 - 0.89v = 0.78v

Now, since this is a pure resistors thing with everything else being equal, we should be able to consider it a special case "resistive divider":

Vout = [R2 / (R1+R2)] * Vin right?

The flex resistor is approx. 9k - 22k

If we say our R2 = 22k

22k / (9k+22k) * 5 = 3.55v (Tested 3.60v)

22k / (22k+22k) * 5 = 2.5v (Tested 2.63v)

1.05v spread predicted vs 0.97v measured +- 10% = 0.097 <= 1.067

Sample another:

R2 = 12k

12k / (9k+12k) * 5 = 2.85v (Tested 2.87)

12k / (22k+12k) * 5 = 1.76v (Tested 1.9)

1.09v spread predicted, 0.97v measured +- 10% = 0.097 >= 1.067

R2 = 4.7k

4.7k / (9k+4.7k) * 5 = 1.715v (Tested 1.67v)

4.7k / (22k+4.7k) * 5 = 0.88v (Tested 0.89v)

0.835v spread predicted, 0.78v spread measured +- 0.078 <= 0.858

Wow. I love electronics and math. I'm glad I started writing this (maybe it should have been a blog, but...) The 10% resistors are as guilty of killing the math as the flex resistors are.

Be that as it may - the voltage swing is really tiny so the resolution is low, and the sensors are rather very noisy. I'll need to come up with some kind of smoothing.

Hmm. Program to predict the best resistor combination? That sounds like a good college programming class question. :)

[edit] Given the thousands of resistors I have on hand, R2 should be optimized at 15k Ohm (3.125 - 2.027 = 1.099) I think my measurements of 3.15 - 2.2 = 0.95 were effected adversely by a 10% resistor (0.855 to 1.045)

They are horrendously noisy, and I tried to keep the test consistent by strapping the bottom to a stiff wire and then bending them each time until they touched the edge of my breadboard. Not exactly scientific, but fairly consistent. I had two sensors to test, and they were pretty close together in their readings (thankfully!).

Here's the results of using different resistors in the divider [Vin = 5.026v with +- .00350v drift:] [All resistors tested are 10%]

22k : 3.60 - 2.63v = 0.97v

27k : 3.84 - 2.94v = 0.90v

33k : 4.03 - 3.23 = 0.80v

15k : 3.15 - 2.2v = 0.95v

12k : 2.87 - 1.9v = 0.97v

10k : 2.64 - 1.7v = 0.94v

4.7k: 1.67 - 0.89v = 0.78v

Now, since this is a pure resistors thing with everything else being equal, we should be able to consider it a special case "resistive divider":

Vout = [R2 / (R1+R2)] * Vin right?

The flex resistor is approx. 9k - 22k

If we say our R2 = 22k

22k / (9k+22k) * 5 = 3.55v (Tested 3.60v)

22k / (22k+22k) * 5 = 2.5v (Tested 2.63v)

1.05v spread predicted vs 0.97v measured +- 10% = 0.097 <= 1.067

Sample another:

R2 = 12k

12k / (9k+12k) * 5 = 2.85v (Tested 2.87)

12k / (22k+12k) * 5 = 1.76v (Tested 1.9)

1.09v spread predicted, 0.97v measured +- 10% = 0.097 >= 1.067

R2 = 4.7k

4.7k / (9k+4.7k) * 5 = 1.715v (Tested 1.67v)

4.7k / (22k+4.7k) * 5 = 0.88v (Tested 0.89v)

0.835v spread predicted, 0.78v spread measured +- 0.078 <= 0.858

Wow. I love electronics and math. I'm glad I started writing this (maybe it should have been a blog, but...) The 10% resistors are as guilty of killing the math as the flex resistors are.

Be that as it may - the voltage swing is really tiny so the resolution is low, and the sensors are rather very noisy. I'll need to come up with some kind of smoothing.

Hmm. Program to predict the best resistor combination? That sounds like a good college programming class question. :)

[edit] Given the thousands of resistors I have on hand, R2 should be optimized at 15k Ohm (3.125 - 2.027 = 1.099) I think my measurements of 3.15 - 2.2 = 0.95 were effected adversely by a 10% resistor (0.855 to 1.045)