Measuring the RPM of a motor without using microcontrollers

Avikaar Sonlall

Joined Mar 14, 2017
11
Hi all

I have a design project where our group needs to build an AC motor speed controller, the only rule is no microcontrollers allowed. My part is to measure the rpm of the motor using and IR LED and phototransistor pair. So far I have done research and found a circuit that everytime a specific blade of the motor passes the IR sensor, it will flash the LED below R3 (Refer to attached circuit).

The only problem I have now is I need to create an additional part that measures how many times per minute that LED flashes and displays it on a few multiplexed SSDs.

I am fairly new to digital circuit design and any help at all would be appreciated.
Thank you

Ramussons

Joined May 3, 2013
1,370
If you are willing for a +- 10% tolerance, you can make a simple frequency meter using CMOS decade counters and a 555 timer. The complexity of the circuit will depend on what the maximum speed of the motor is. Can you give a little more information?

Kermit2

Joined Feb 5, 2010
4,162
Is the design already laid out by the instructor?

This sounds like it could be better accomplished with a simple triac and a speed control pot. along with a few caps and a diode.

Kjeldgaard

Joined Apr 7, 2016
476
A digital tachometer without microcontroller can be difficult to get useful, especially at low speeds.

But to get started:
What's the wish for indication range and resolution, and how many impulses can be achieved per motor revolution.

Sensacell

Joined Jun 19, 2012
3,327
Now that you have a pulse-per-revolution, you can design an RPM counter.

Typically, you would have a series of cascading decade counters that are incremented by every motor revolution pulse.
The binary output of the counters goes to an equal number of latches, the latch output feeds a digit driver to drive an LED digit.

A clock then latches the count and resets the counters, the current RPM is held in the latches and displayed, while the next count is accumulated.

MrAl

Joined Jun 17, 2014
10,887
Hi,

Not sure what parts you are allowed to use other than not a uC chip.

For the control part in theory you could use a frequency to voltage circuit, but for the recording part you'd have to either create a frequency meter (like with the decade counters) or you'd have to incorporate a voltage meter into the circuit so that the readings from the output of the frequency to voltage circuit can be converted into digital form, and after calibration, stored in digital form.

Alec_t

Joined Sep 17, 2013
14,005
What are the minimum and maximum rpm you are required to measure?

Kermit2

Joined Feb 5, 2010
4,162
Reading comprehension is low today. I thought they needed to control RPM. I didn't catch that word "count".

Joined Jul 18, 2013
28,023
What is the nature of the AC motor? The type will often decide the control method.
Max.

djsfantasi

Joined Apr 11, 2010
9,128
What is the nature of the AC motor? The type will often decide the control method.
Max.
That is definitely the next issue that must be attackef to help the TS. However, he seems to be stuck on measuring the RPMs.

Avikaar Sonlall

Joined Mar 14, 2017
11
Thanks for all the replies so far guys. Here is the project description. For the actual speed control of the motor, we did go for a triac and pot circuit. My part is just to measure the rpm of the motor and display it digitally

Attachments

• 281.4 KB Views: 38

Avikaar Sonlall

Joined Mar 14, 2017
11
In other words from what i already found, that LED in the attached circuit will flash every time a revolution is detected(We use a 1 blade fan for testing). All I need is to find out how many times per minute that LED will blink.

Thanks again

Joined Jul 18, 2013
28,023
That the 'Motor should operate at the referenced speed' implies some kind of feedback loop control?
LM2917 maybe could be employed?
Max.

Avikaar Sonlall

Joined Mar 14, 2017
11
That the 'Motor should operate at the referenced speed' implies some kind of feedback loop control?
LM2917 maybe could be employed?
Max.
Yes basically what our mentor said was if you set a reference speed, say 600 rpm but the motor is running slower or faster than that, it has to adjust on its own.

Sorry if I seem new to this but this is our first design course to introduce us to circuit design

Joined Jul 18, 2013
28,023
You need some kind of PID loop set up, which is not that simple to employ with discrete logic.
Max.

djsfantasi

Joined Apr 11, 2010
9,128
How are you setting the reference speed of the motor? Do you have to display it also? Your answer can be used in the motor matching the RPMs.

How many counters will you need?Which is why someone asked for the max and min RPMs.

Then, you use a clock that sends a signal every minute. That signal can be used to get RPM. Warning: There may be considerations in how you implement this.

You need to control the speed. You have how fast the motor is going in RPMs. You know how fast/slow that you want to go,

So what do you need to do! I'd compare the values. There is a chip which is a binary/BCD comparator (74HC688). There are others, but read this one's datasheet.

Note that I wouldn't use all digits for comparison. Why?

Sorry, there's a lot of questions. Please try to answer them and we can continue to help.

Avikaar Sonlall

Joined Mar 14, 2017
11
How are you setting the reference speed of the motor? Do you have to display it also? Your answer can be used in the motor matching the RPMs.

How many counters will you need?Which is why someone asked for the max and min RPMs.

Then, you use a clock that sends a signal every minute. That signal can be used to get RPM. Warning: There may be considerations in how you implement this.

You need to control the speed. You have how fast the motor is going in RPMs. You know how fast/slow that you want to go,

So what do you need to do! I'd compare the values. There is a chip which is a binary/BCD comparator (74HC688). There are others, but read this one's datasheet.

Note that I wouldn't use all digits for comparison. Why?

Sorry, there's a lot of questions. Please try to answer them and we can continue to help.

Here is the circuit we are using to control the speed of the motor. As for the motor itself our university has not given us the motor yet but all we know that it is a 230V 60W single phase AC motor. Sorry I am also not too sure about the expected RPMs as yet. The speed is controlled by pot P1 and this circuit works as a type of PWM.

Vigneshwaran12

Joined Jun 6, 2019
1
If you are willing for a +- 10% tolerance, you can make a simple frequency meter using CMOS decade counters and a 555 timer. The complexity of the circuit will depend on what the maximum speed of the motor is. Can you give a little more information?
Hi bro,
Can u please send the circuit diagram for measuring rpm and the components used.