tachometer calibration

imagine a setup where in a commercial tachometer and our tachometer are kept side by side pointed towards a spinning propeller connected to electric motor (let us consider that the motor can spin the prop from 0-60000 rpm)

now start the motor and measure the rpms of both the tachometers with increase in motor speed (suppose we collected 100 sample readings). now calculate the difference from the commercial model and our model and then calculate average value which should give us the error value!

from now on when ever a reading is taken add or sub the error value  depending on the out come of the error sign (+,-)

note: please also include these assumptions in the above example (for both commercial and our models)

number of blades set to 2 
Pitch of the blade  set to 2 inch

for the exact formula used in the code please refer to the site, its given there i don't remember !

dear rao

before i give a detailed example for the rest of the stuff, i would like to know whether you know what a CRO is or how is used what type of signal a servo accepts n stuff like that ? and how far or deep a example would you like me to give ??

sahil

My Dear Sahilkit,
so the tachometer part does not show true values and could not be calibrated  to give true values but you have to add or subtract an error value to the reading displayed by the device. Then what is the purpose of having an instrument to measure the r.p.m  at which a propeller is spinning when that instrument could not be made to give true values.
And I think I should know what a C.R.O is and how it is used and stuff like that because I received my B.E(Electronics) degree from Bangalore university in 1982. Since it was so long ago you can teach me afresh and you may go and give as far or deep an example you could give.
Rao.

no one can do this in the code or rather change the result by adding or subtracting and recompile the assembler code and reprogram the IC then one will get a tachometer reading calibrated to commercial unit we used in the example ! plus this does not purely depend upon the code etc the analog circuit also plays an important role, noise, type of photo sensitive element used, sensitivity or speed at which the circuit can measure change in light intensity   ......all these factors will play a role in measurements of rpm !


since i and you are in the same field plus to keep things simple. 1st to refresh yourself about CRO please see this link

http://www.ee.usyd.edu.au/tutorials_online/topics/labintro/crogw1.html

now coming back to out discussion,there are three leads to a servo. Two are for power, + 4 to 6 volts and ground. The third lead feeds a position control signal to the motor, and the control signal is a variable-width pulse. A neutral, mid range positional pulse is a 1.5-ms (millisecond) pulse, which is sent 50 times (20 ms) a second to the motor.

This pulse signal will cause the shaft to locate itself at the midway position +/-90 degrees. The shaft rotation on a servo motor is limited to approximately 180 degrees (+/-90 degrees from center position). A 1-ms pulse will rotate the shaft all the way to the left, while a 2-ms pulse will turn the shaft all the way to the right. By varying the pulse width between 1 and 2 ms, the servo motor shaft can be rotated to any degree position within its range.

let me group the tester features into 3 parts

1. servo tester and electronic speed controller tester do the same thing
2. receiver tester
3. voltmeter

1. the servo tester and ESC tester generate the position signal from 1msec to 2msec as explained above!

2. receiver tester measure this position signal value and display it on the LCD

3. voltmeter : A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit ,i our case its used to measure battery voltages! it can be calibrate in two way on with the help of multimeter or with CRO for true value!

voltmeter calibration

please refer to the circuit diagram. The input voltage for the tester is given at the pin 1 via voltage divider network which sets the ADC scaling value !

  voltage input to 39kohms(1%) to pin 1 of the IC
                                       .
                                       .
                                       .
                                   2kohms(1%) to ground

by changing these values according to once need, should be able to calibrate the voltmeter to true reading depending on which instrument is used as reference!

note: the ADC can read a maximum of 0-5v and has 10bit resolution !(ADC upper and lower limit are internally set, unless and external reference value is given)

sahil

more to come later, if possible will post video on how to check with a scope 

bye

sahil

Thank you. :)Please make the video as clear as your explanation above
Rao

Great effort Sahil!!..

However, I should say one comment. You have put so much effort to breadboard the circuit but I wish you had put a little more effort and made a nice enclosure for this circuit so that it is more presentable and usable at the feild. I am pretty sure that after a few months you will be robbing components from this board to go on with another new project... This happens to me always!. Now, I always plan for the enclosure first before starting with the breadboard or PCB.

I know, we hobbiest are anxious to get the stuff working at the earliest.. but... its better to have a neat product that keeps you proud for years.

-Ismail

While this Tester would be great to have in the hobby room I don't think this tester is meant for use in the field. Nevertheless a neat cabinet is a desirable thing.At the same time a cabinet would make access to the switches difficult.

Rao

  are you pulling my leg 

yes i have to find a suitable box, or i might use it like that until my micro star 2000 radio is up and then i might give it away or salvage it for parts 

receiver calibration setup picture

receiver tester mode calibration

we need the following parts to complete our setup!

1. transmitter
2. corresponding receiver
3. receiver battery pack
4. y-harness
5. R/C multi tester.
6. battery pack for the tester
7.CRO along with a probe

now connect all these stuff according to the figure in the previous post! make sure ever thing is connected properly and power up. once everything is powered up, calibrate and set up the CRO and assume that the y-harness is connected to the rubber channel!

channel A to DC mode (we will not be using channel 2 or B)
set timing to .5msec/division and
   voltage to 1volt/division 

steps (by now the transmitter,receiver and R/C multi tester are on and tester displaying some position value plus the other end of y-harness is left out)

1. connect the CRO probe to channel A
2. now connect the probes ground wire to the negative(black) of the left out Y servo cable and position signal wire(white) to the probes positive lead (some signal should be displayed)
3. now position the rubber trim on the transmitter to center and leave the stick at center! (1.5msec)
4. from the above step assume that the transmitter is sending 1.5msec position pulse and the receiver is giving out 1.5msec position pulse. Now, measure this value on the CRO if everything is correct one should be able to see a high pulse up till 3 division on the timing line or horizontal line and low signal up till 37 divisions !
5. see whether the R/C tester is showing this value or not, if its is then one has zero error or calculate the difference and modify the code to get a accurate value

Repeat steps 1-5 for various positions of the joystick ! please note that the micro controller is being run at 4mhz clock speed so the minimum theoretical value the device can calculate or measure is 10usec. 

component list along with prices(to the best of my knowledge), if anyone finds that there is a mistake or something missing please correct it or let me know i update the list with correction 

bye

sahil

Dear Sahil,
I am not pulling your leg. I mean what I say.
best wishes.
Rao.

thank you Mr.Rao, by the way was the receiver tester calb. mode thing any useful ?

regards

sahil

okay i finally decided to add covering and mark this build finished 

dear Sahil,
my interest is in tachometer and servo tester sections only. The optional wind speed measurement interface would be interesting though.

Rao

Great !

I am blushing seeing RCINDIA.ORG stamped on the case     Thanks !

finished