Simple Magnetic Encoder Counter Circuit

Thread Starter

Jase009

Joined Aug 27, 2018
8
Hello all,

I am wanting to learn more about the circuit required for a Magnetic Encoder, in particular this one : https://www.pololu.com/category/157/encoders-for-micro-metal-gearmotors

The task I want to accomplish is to be able to stop small dc motor after a set number of revolutions that will be inputted into memory before the motor starts. Can anyone point me in the direction of the components I may require?

The only design requirements are for it to be small and for it to have very low power consumption.

Thanks,
Jase
 

danadak

Joined Mar 10, 2018
4,057
Do you do embedded work ? Program in C or like kind language ? Or Basic.

Or need a turn key solution ?

Is DC motor used reversible or just one way rotation ?

Regards, Dana.
 

spinnaker

Joined Oct 29, 2009
7,835
I would also be interested in theroy of operation. Looks to me like two hall sensors? Says "12 counts per revolution". Trying to figure out how that can do that? Wouldn't it just give two square wave signals?
 

Thread Starter

Jase009

Joined Aug 27, 2018
8
Do you do embedded work ? Program in C or like kind language ? Or Basic.

Or need a turn key solution ?

Is DC motor used reversible or just one way rotation ?

Regards, Dana.

Thank you Dana,

I know my way around a few programming languages so I'm ok there.

The dc motor is reversable, so I will either briefly short the terminals or reverse polarity.

On a side note, looking at the data sheets available for this encoder, I cannot seem to find the maximum frequency the hall sensors can run at i.e. the theoretical maximum rpm the encoder disk can rotate and still provide a clear waveform.

Jase
 

danadak

Joined Mar 10, 2018
4,057

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ebeowulf17

Joined Aug 12, 2014
3,274
I would also be interested in theroy of operation. Looks to me like two hall sensors? Says "12 counts per revolution". Trying to figure out how that can do that? Wouldn't it just give two square wave signals?
You're right that it's got to just be putting out a pair of rectangle wave signals (configured as quadrature encoder) which can be resolved into 12 pulses per revolution with external hardware or a microcontroller.

As for pulse rate, it depends on their special magnet disc assemblies:

https://www.pololu.com/product/2599

The disc consists of six evenly spaced magnetic poles (three north-south pairs), which allows for an encoder system that offers 12 counts per revolution.
 

ebeowulf17

Joined Aug 12, 2014
3,274
Switching freq of sensor you are looking at seems to be limited to 15 Khz
in spec tables.....
Which is interesting, because the motors they recommend connecting these to have no-load speeds (motor shaft, not gearbox output) ranging from 12,500-30,000rpm. Multiply that times the 6 polarity changes per revolution (for each of the two sensors) and each sensor is switching at 75,000-180,000 changes/minute. Multiply by 60 to get changes/second, resulting in 4.5-10.8MHz (300-720 times the sensor's max speed.)

I may be off by a factor of two, cause I'm not sure if switching frequency is spec'd based on each state change or each compete cycle. Either way, it's quite a disparity!

I don't know - maybe it doesn't matter since you probably don't measure no-load speed very often in real world use. But I know that if I'd just gotten my first motor/encoder combo, the first thing I'd do is get it spinning and read the outputs. I'd be disappointed if they made no sense.

It's also possible I just totally messed up the math above. The numbers seem startlingly high to me.
 

Bernard

Joined Aug 7, 2008
5,524
Just thinking out loud. Drive a reversable counter with one output, output to one side of a digital magnitude comparator
other side to required number. Stop on = , reverse on greater, forward on less than.
Something like 74HC85 or 74HC521 its companion 8 bit, all stackable, or 74C85 for higher Vcc.
Motor direction from decoded quadrature signals.

.
 

Bernard

Joined Aug 7, 2008
5,524
Quadrature decoding notes can be found top of this page, " EDUCATION, Vol. IV, CH. 11,
Synchronous Counters, At bottom.
My take on mag. disc is 6 poles, 3 N & 3 S. ,if so could divide signals by 3 & use one channel , rising edge
& then have 1 pulse / rev.
 
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