Hall Effect Sensors vs Magnetic Encoders

[alphaai]

Hello,

Im just trying to understand this hall effect sensors.
Ive been looking into the maxon motors and realise that some double shafted models have hall effect sensors inside the motor.

Q1) What are the hall sensors used for? Why are they used in specialty motors.

Q2) How do the Hall Sensor relate to position and velocity control?

Q3) If a Double Shafted motor or a single shafted motor could be connected to a joint, which would provide more torque?
I believe it would be the double shafted. Not sure though.

Q4) How does Hall Effect Sensors differ from Magnetic Encoders??

Q5) What if I were to add two gearheads to either end of the shaft, if the hall sensors could measure rpm and somehow work out the position, then
maybe that could be possible but would not get the high resolution that can be provided by a magnetic encoder.

Thanks,

Ben

 

[koolguy]

Hall effect is used to determine type of semiconductorr wheter p-type or n-type this is easy to determine, then other costly experiment..................



for more detail search this you tube!!

 

[misterT]

A1) Brushless motors have three hall effect sensors for detecting the angle of the (permanent magnet) rotor. This information is used for the electrical commutation needed to drive the motor properly. Here is a picture of the rotor angle vs sensor outputs:
**broken link removed**

A2) You can think of hall effect sensors as a low resolution encoders. You can calculate position and velocity from the hall sensor signals.

A3) Two shafts don't double the torque. Usually the other end of the shaft is connected to sensors and the other end is connected to load. Double shaft motor delivers the same torque as single shaft motor.

A4) Hall effect sensors are magnetic sensors and some magnetic encoders are based on the hall effect. Read this for starters: **broken link removed**

A5) Yes, you can calculate the rpm and position information, but the resolution is low.. you can easily increase the resolution with high ratio gearhead.

 

[alphaai]

Thanks for responding MisterT, you seem to know your stuff.

A1) Brushless motors have three hall effect sensors for detecting the angle of the (permanent magnet) rotor. This information is used for the electrical commutation needed to drive the motor properly. Here is a picture of the rotor angle vs sensor outputs:
**broken link removed**

Q1.2) Do all brushless motors have hall effect sensors? Howabout the RC brushless motors that you can find on hobby RC sites?

A2) You can think of hall effect sensors as a low resolution encoders. You can calculate position and velocity from the hall sensor signals.

Q2.2) Could you show me a link where I could calculate position and velocity for the from the hall sensors signals for maxon motors or maybe other motors?

I know maxon motor sell EPOS 1 and 2 and yeah paying for the motors itself is expensive enough, I wouldnt mind trying to hook it up to some kind of microcontroller or something. I believe I would need a motor circuit built as well.

A3) Two shafts don't double the torque. Usually the other end of the shaft is connected to sensors and the other end is connected to load. Double shaft motor delivers the same torque as single shaft motor.

Q3.2) What if the both shafts had a gearhead each with the same gearhead ratio. Would that double the torque?

A5) Yes, you can calculate the rpm and position information, but the resolution is low.. you can easily increase the resolution with high ratio gearhead.

Q5.2) Now when you mean high ratio gearhead, you talking about a gearhead with more than 4 stages? So something that would produce more torque, how does that relate to resolution?

Q5.3) What kind of resolution am I looking at with a hall Sensor?
I know that with a magnetic encoder, you can get about 4096 positions per revolutions or even 16384 positions per revolution.

Thanks heaps for your responses so far,

Ben

 

[misterT]

Q1.2) Do all brushless motors have hall effect sensors? Howabout the RC brushless motors that you can find on hobby RC sites?

No. There are three kinds of commutation: Sensorless, Hall sensors and high resolution encoders.
RC brushless motors use sensorless commutation. This means that the motor does not have any sensors in it. The controller uses a clever system that reads the induced voltage (back emf voltage) from the motor coils to get the position and speed information.
Sensorless commutation is not very accurate, but is good enough for RC car motors. If you need a very smooth and precise operation then a high resolution encoder must be used.

Q2.2) Could you show me a link where I could calculate position and velocity for the from the hall sensors signals for maxon motors or maybe other motors?

It is pretty much the same technique as with high resolution encoders. I couldn't find any good examples for hall sensors for now, but here is one application note that describes the overall control system:
https://www.electro-tech-online.com/custompdfs/2011/06/AN10661.pdf
https://www.electro-tech-online.com...larC4B1n2020BLDC2020Temel20C396zellikleri.pdf

Commutation using hall sensors or no sensors at all is called "block commutation". That might help you with google searches.
Commutation with high resolution encoder is called "sinusoidal commutation".

Q3.2) What if the both shafts had a gearhead each with the same gearhead ratio. Would that double the torque?

No, the torque comes from the rotor magnet attracting the stator coil. The shaft is just a metal rod through the rotor.. for a two-shaft motors they just make the metal rod longer so it comes out both ends.

Q5.2) Now when you mean high ratio gearhead, you talking about a gearhead with more than 4 stages? So something that would produce more torque, how does that relate to resolution?

Gear ratio means the ratio of speed reduction. With 0.5 ratio the motor turns 1 turn each time the output turns only 0.5 turns.

With higher gear ratio you get higher output torque and slower output speed. Slower speed means more accuracy for positioning tasks.

Q5.3) What kind of resolution am I looking at with a hall Sensor?
I know that with a magnetic encoder, you can get about 4096 positions per revolutions or even 16384 positions per revolution.

With hall sensors you get 6 positions per revolution.

There is a very good book from Maxon Motors called "The selection of high-precision microdrives". It is a must have: https://www.maxonmotor.com/media_releases_The-design-of-high-precision-microdrives.html

 

[alphaai]

Hey misterT,

Thanks for your responses. They have been enlightening.

Just a few more extra questions please:

1) I bought some A-Max 110117 and EC-Max 16 283833 off ebay and they came with no gears. I spoke to a maxon representative who said
"I can sell you the gearhead X and the mounting hardware X for the motor X but you will find it hard mounting the gearhead to the motor because it needs to be laser cut

Q1) Is this true? Have you had any experiences with this?
If this is true, then maybe I ought to stop buying these motors and maybe I ought
to look for a pinion gear that would fit the shaft and connect it to some other planetary gearhead.

Q2) Is that book "High precision microdrives" more in-depth than the powerpoint animations on maxon academy?
Is there a ebook of it? would buy it if there was.

Q3) Would u know what motors they used in Nao and which gearhead and encoder?

Q4) Is it possible to connect a gearhead with a larger diameter to a motor with a smaller diameter?
If yes, what problems might I come across.

Thanks big time,

Ben

 

[misterT]

"I can sell you the gearhead X and the mounting hardware X for the motor X but you will find it hard mounting the gearhead to the motor because it needs to be laser cut

Q1) Is this true? Have you had any experiences with this?
If this is true, then maybe I ought to stop buying these motors and maybe I ought
to look for a pinion gear that would fit the shaft and connect it to some other planetary gearhead.

I have only worked with already assembled Maxon motors. The thing is that they aim for very high quality. I don't know what you would need to laser cut when you attach a gearhead to a motor, but I trust the representative.

Probably the best option for you is to look for other "general purpose" gearing. A pinion gear or a belt/pulley drive. Just make sure that the solution fits the motor shaft. The maxon gears are also very expensive.

Q2) Is that book "High precision microdrives" more in-depth than the powerpoint animations on maxon academy?
Is there a ebook of it? would buy it if there was.

It is a very good book. In depth enough and practical with design examples etc. I haven't seen the powerpoint animations on maxon academy.

I haven't seen it for sale as an ebook. I bought it straight from maxon (via email) and they sent the book with a 40 EUR bill.

Q3) Would u know what motors they used in Nao and which gearhead and encoder?

No.. Would like to know all the details of their system though.

Q4) Is it possible to connect a gearhead with a larger diameter to a motor with a smaller diameter?
If yes, what problems might I come across.

Usually the gearhead is attached (screwed) straight to the motor. You can support the gearing and motor separatelly on a rigid frame. The important thing is that the motor and gear is aligned properly.. so that they are straight. And of course the motor shaft must fit the gearing. Maybe some kind of flexible coupler between is necessary: https://www.google.com/search?q=mec...t=mode&cd=2&ved=0CCgQ_AUoAQ&biw=1920&bih=1109

Belt and pulley system might be the easiest and cheapest to construct.. it is also a very quiet system. But just use your common sense and you will figure out how to get the required gearing.

 

[misterT]

More about the book "Selection of high-precision microdrives". It is a very good book for designing servo systems and robotic drives. But, it does not go into electrical designs of H-bridges or detailed algorithms to calculate speed and position from sensor signals.

It does cover topics like motion of the load, mechanical drive design, selection of gear/motor/sensor/controller etc. Would you like for me to post the table of contents of the book? I would have to write it down manually

 

[alphaai]

Hey there misterT,

Yeah that would be awesome if you could post the table of contents if that is no trouble.
I do have some other questions though:

Maxon Motors are the best when it comes to effiency (electrical power to mechanical power). One of my projects is to build a robotic arm to sit on my desk
and to be plugged in the wall. Maybe Maxon Motors are a bad idea for that goal because now I think that Maxon Motors are more ideally suited for
for mobile robots.

Q1) If my robotic arm is plugged into the mains then efficiency wouldnt matter as much would it?
If however my robotic arm was to be connected to a moving robot like johnny five lol then yeah maxon motors would be the way to go.

The other application I can see with maxon motors besides mobile robots
is lightweight power generators.
Are Maxon Motors ideal for Power Generators??

3) Would u agree with me than Maxon Motors are ideally suited for Mobile Robots and maybe Power Generators
and should not be used for a desktop Robotic Arm.
if yes, then what motors could you recommend for a desktop Robotic Arm?

4) Whos pricier, whos cheaper? Wheres value for money?
There is Faulhaber, Maxon, Portscap

Again look forwards to your response

Thanks

Ben

 

[misterT]

Maxon Motors are the best when it comes to effiency (electrical power to mechanical power). One of my projects is to build a robotic arm to sit on my desk
and to be plugged in the wall. Maybe Maxon Motors are a bad idea for that goal because now I think that Maxon Motors are more ideally suited for
for mobile robots.

No, no. Good servo motor is great for all precision tasks and dynamic control. A good efficiency also means that a lightweight motor can deliver lots of mechanical power and that is really important in robotic arms. With all electric motors the output power is proportional to the weight of the motor and good efficiency helps reducing the weight and size of the motor.

Q1) If my robotic arm is plugged into the mains then efficiency wouldnt matter as much would it?
If however my robotic arm was to be connected to a moving robot like johnny five lol then yeah maxon motors would be the way to go.

This is true also. Battery powered robots benefit from high efficiency and small weight, but the light weight is also important in robotic arms and other dynamic systems.

Are Maxon Motors ideal for Power Generators??

I don't think so. Too expensive. Maybe if you get them very cheaply. They might be ideal in theory, but definitely not in practice. They do provide good efficiency for regenerative braking, which is basically using the motor as a generator when the robot slows down. But I wouldn't use them solely as a generator.

3) Would u agree with me than Maxon Motors are ideally suited for Mobile Robots and maybe Power Generators
and should not be used for a desktop Robotic Arm.
if yes, then what motors could you recommend for a desktop Robotic Arm?

Maxon motors and other high quality motors are perfect for (small) robotic arms. Other competitive option (for big robotic arms) would be using pneumatics or hydraulics.

4) Whos pricier, whos cheaper? Wheres value for money?
There is Faulhaber, Maxon, Portscap

Don't know. You have to research that on your own. It depends on the type and size of the motor also. I like Maxon, but I believe that they are at the high end of motors and are probably not the cheapest one. The models they sell the most are the most cheapest naturally.. and maxon sells a lot of some of the models.