# Speed measurement

Discussion in 'Electronics Homework Help' started by Fenrir, Mar 1, 2014.

1. ### Fenrir

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Mar 1, 2014
hello everyone,

I'm new here and was wondering if you could help me.

I would like to make an electronic speed indicator to 1km / h accurate for my kart.

I would of course like to keep this as simple as possible
but i'd like to make it in loose electronic parts... no bicycle speed indicators and such

any help is greatly appreciated

Mvg
Fenrir

2. ### jpanhalt

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Nov 12, 2013
Do you want speed or acceleration? Two different things.

Assuming you want speed, what is wrong with bicycle indicators and "such?"

John

3. ### Fenrir

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Mar 1, 2014
sorry , i mean speed

it's an assignment for school

thanks,
Fenrir

4. ### jpanhalt

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Nov 12, 2013
So, I assume using an old inertial navigation system is not in the offering.

I suggest looking at measuring a wheel's rpm or the rpm of the motor that drives the wheel. There are a variety of encoders you can use. If the drive is electric, it is also possible to get your RPM from the back emf generated.

Since this project is for school, why don't we begin with your thoughts on the problem?

John

5. ### Fenrir

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Mar 1, 2014
i'm thinking of something with a small magnet attached to the wheel, so i'll probably need a Hall sensor, and some LED displays to schow the result.
But i can't figure out how i can change the analoge input to a digital input to the displays.

thanks
Fenrir

5,165
1,087
Dec 18, 2013
A Simple read switch would probably work for the triggering. The interesting bit is going to be to convert the RPM of the wheel into speed. Do you want an indication of speed on a digital readout, a dial gauge or say a bar graph display? Also if you can your wheel size you will have to calibrate it somehow. I'll have a look and see if I can come up with something for you.

7. ### Fenrir

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Mar 1, 2014

A digital readout would be really nice

would this be a suitable reed switch ?
http://www.gotron.be/reedcontact-1xno-0-5a-aw20-25-10-0x2-0mm.html

8. ### jpanhalt

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Nov 12, 2013
First, I believe the output from the Hall sensor will be digital, not analog, as you describe it. (There are analog Hall sensors.)

Second, the job for you is to learn how to program a microchip to convert a PWM or pulsed signal into revolutions and then convert that into speed.

Third, is the matter of accuracy. Have you heard of "pitch diameter." Are the wheels on your cart hard or compressible? How much slippage against the road is there? If those variables and other secondary variables are minor, then you may be able to use a simple equation based on the circumference of a circle to calculate the speed. Otherwise, I would suggest that you make a calibration table and then use a look-up table. In fact, depending on the microprocessor that you use and your programming skill, a look-up table may be the best way to go regardless of the accuracy needed.

Have you studied programming? What language(s)?

John

9. ### Fenrir

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Mar 1, 2014
Hi John,

is there a possibility to do this without a microcontroller?

i've had a few programming lessons ( visual basic) and i have an arduino uno but i know only the basics

thanks

10. ### jpanhalt

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Nov 12, 2013
Of course, it can be done without a microprocessor. Just hook a DC motor to the shaft and an analog voltmeter to its output. Then print a scale showing the km/h and paste that to the meter's face.

John

11. ### Fenrir

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Mar 1, 2014
it's about making a speed measurement for a go kart .
1km/h accurate and it can't be something from a shop
thanks

12. ### jpanhalt

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Nov 12, 2013
You have a whole class on measuring the speed of a kart?

I understood the assignment. My question was whether this is a physics class, math class, mechanical engineering class, computer engineering class, etc. class?

John

13. ### Fenrir

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Mar 1, 2014
it's not really a class, it's an assignment
they just give an assignment and i have to figure it out, at the end i have to show it and give a presentation

thanks

5,165
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Dec 18, 2013
15. ### davennModerator

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Sep 5, 2009
Note guys

this is homework ... I have moved it to that section

help the poster to work the problem out without giving all the answers outright

cheers
Dave

16. ### Fenrir

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Mar 1, 2014
thanks dave that's all i need, a little help to get me in the right direction

Fenrir

17. ### jpanhalt

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Nov 12, 2013
OK. You can do what you want with discrete logic, like counters. But since you will need to measure a time-dependent variable (speed), you will need a clock.

The easiest way is with a microcontroller to count the pulses or measure the duty cycle from your Hall sensor, which depends on the type of Hall detector you chose. However, a simple edge triggered flip-flop, clock and counter can be made to count pulses per unit time, as Arouse 1973 implied (I think). You will still need to convert the count to a display value and display it in the latter instance.

Have you searched the Internet for rpm measurements?

John

18. ### Laplace

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Apr 4, 2010
Is there any requirement pertaining to how often the display must be updated. If it must be real time, then there is a design approach that will show the speed for every revolution of the wheel (or fractional revolution if necessary). But if the display only needs to be updated once per hour, then a different design approach can be taken for obtaining the average speed for that hour.

19. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
You could also have an analg display by converting the pulses to a fixed length and then integrating them with a capacitor. An analog meter across the capacitor would give a reading proportional to speed.

20. ### Fenrir

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Mar 1, 2014
thanks for all the suggestions

i think i am going to use an arduino uno and a hall sensor.

i can read the output of the hall into the arduino and convert it to the display , sound good ?

thanks
Fenrir