Brushed DC Motor Hall Effect Sensor

Thread Starter

Carloco0306

Joined Nov 13, 2018
47
Hi all,

I am attempting to read the RPM of a brushed DC motor. Whenever I look up anything about hall effect sensors, it is for Brushless motors.

Is it possible to use hall-effect sensors for brushed motors?
 

cmartinez

Joined Jan 17, 2007
6,880
I don't see why not. The important thing lies in the hardware. What specific sensor are you planning to use? What size of motor do you have in mind?

On the other hand, remember that you have to mount a magnet somewhere on the rotating part of the motor for the hall effect sensor to work. Make sure that there's space for that too.
 

shortbus

Joined Sep 30, 2009
7,599
Hi all,

I am attempting to read the RPM of a brushed DC motor. Whenever I look up anything about hall effect sensors, it is for Brushless motors.

Is it possible to use hall-effect sensors for brushed motors?
I'd say a resounding maybe. Would your motor withstand the off balance that putting a permanent magnet on it? Hall sensors detect a magnet. Is the shaft of a big enough diameter to even put a magnet on? How fast does your proposed Hall sensor react? Is it fast enough for the motors RPMs?

Not knowing much about your motor doesn't give much to give better suggestions on how to do this. Encoders, and opto-interupters are other ways of doing it.

EDIT C types faster than me.
 

BillB3857

Joined Feb 28, 2009
2,532
Hall devices ARE frequently used in brushless DC motors to control the commutation of the signals from the motor controller. The slotted commutator of a brushed DC motor controls which armature winding is active at any given point of rotation. To answer the question of reading the RPM of a brushed DC motor... Any device that would provide a pulse type output can be used. One common device is an optical sensor with a white line painted on the shaft. A Hall device would require a magnetic pole that would activate the sensor once per revolution, which, in my opinion, is much harder to accomplish, but, anything is possible, which was the original question.
 

MaxHeadRoom

Joined Jul 18, 2013
20,412
What I often use where a encoder is not warranted is a slot sensor or the retro-reflective as already mentioned.
For this it is useful to use a motor that also has a rear shaft.
If no rear shaft, you could fit a slim sized small gear wheel on the drive end and use a small/miniature proximity switch.
If you use any kind of transducer on any rotating part of the motor, remember a single sensing point may (will) cause unbalance.
Max.
 

Alec_t

Joined Sep 17, 2013
11,137
If space permits inside the motor housing you could probably find a position for a Hall sensor close to the passing armature poles. An analogue output type of sensor would likely be better than a digital (on/off) type of sensor, since the latter type might be kept permanently 'on' by the permanent field magnets.
 

schmitt trigger

Joined Jul 12, 2010
198
I have done both magnets and photo-reflective.

Why do I like photo-reflective best? Because as others have mentioned, the pitfall is that the magnet's weight will unbalance the rotating elements. You have to put a complementary magnet 180 degrees away, to reduce unbalance. But achieving a full vibration-free setup, may prove challenging for the novice.

Also, if the magnets are not VERY SECURELY attached to the rotating element, they can fly away with higher RPMs. Ask me how I know.

For that reason I nowadays prefer photo-reflective.
 

MaxHeadRoom

Joined Jul 18, 2013
20,412
With the motors such as T.M. motors, they have a rear shaft with slotted disc, for a slot opto device, I prefers to make my own disc up from acrylic, there are ebay sources that will laser cut to various diameters.
I use a slot opto with the schmitt trigger and pull up resistor all built in such a the OPB615.
This way I can make the acrylic disc what resolution I need.
Max.
 

samples69

Joined Feb 2, 2019
7
In the automotive world, they use enhanced hall effect sensors with a built in current shunt. Placed in series, sized for the current the brushed motor uses, but senses the high current when the brushes line up on the commutator plates on the armature. This allows counting once the total number of plates the brushes ride on, are counted for one complete revolution. This signal can be used for speed or position. HVAC actuators operate with only two wire and no feedback circuit. Power windows or Blend door motors run from open to close after replacement to relearn. If a Corvette with an automatic retracting roof is opened or closed, logic is used to roll down the window 10mm to clear the seal, then rolls back up to closed position. Same with certain HVAC actuators that are either open or shut. Using Hall effect sensors with a built in current shunt simply creates a usable signal for movement or position and no extra parts have to be added to the motors space & area. Just an option.
I apologize in advance, if the post is incorrectly added. I am on many forums and most functions are different, easier or harder to figure out.
 
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