2020/11/17 19:13:30
bonedoc
Hello! It’s been a long time since I’ve posted here! I’ve got a weird question. I’ve inherited a high dollar massage chair, and it has a huge flaw on the sensor side, sort of like the old Land Rover wheel sensors. It has 3 motors that have hall sensors to measure rpm. The system has wiring issues and such sloppy hall sensor design, that the machine continuously throws errors and shuts off. I may just have to write new code for it and make a new remote, but I wanted to try something else first. I thought I would try to override the hall sensor lines to make it think it’s was getting the correct measurements. I tried to simply use a pull down or pull up resistor on the sensor lines to the chip. This didn’t hack it.


Does anyone know how RPM is usually measured on the chip side? I didn’t know if the hall sensor output a voltage the ADC was measuring, or what it was doing

Any creative ideas on fooling the inputs that the motors are still spinning?

If I can’t figure out a trick I’ll have to write some code!
2020/11/17 21:03:45
davea
Does anyone know how RPM is usually measured on the chip side? I didn’t know if the hall sensor output a voltage the ADC was measuring, or what it was doing

normally it would send a pulse or 2 at every rotation (or have a counterbalance) 
check the gap between magnet and sensor, and the strength of of the magnet
otherwise you would measure the voltage on the motor and convert it to pulses
and the possibly of that working is slim..
and if it did it would most likely oscillate (not a bad thing for a massage chair ??)
  
2020/11/17 23:57:29
Mysil
Hi,
There are many variants of Hall sensors around,
you may try to find out what make and type is used, and search for a Datasheet.
 
Some sensors may be just the Hall sensing element, and deliver a low level analog signal,
This will need analog comparator (in microcontroller) and maybe also a operation amplifier.
Some sensors have amplification and detection circuits integrated, and deliver a digital signal as output.
Output may be open collector, alias open drain circuit, and require a pull-up resistor somewhere.
 
You may try redoing wiring to existing sensors, with twisted pair wiring separately for each sensor.
Signal & Vss(aka. ground) in a twisted pair for each sensor,
and Vss and Vdd in another pair with decoupling 0.1 uF between Vss and Vdd at the sensor end.
Or screened wiring separately for each sensor.
 
If you have access to a oscilloscope, study the signal quality reaching the controller.
If there are ringing and reflections,
a series resistor between 20 and 100 Ohm, in series in the signal line, may help. 
 
It is possible to detect the state of a three phase BLCD or PMSM motor without using hall sensors in the motor.
It is done by sensing voltage on each motor wire in the time drive to the respective winding connection is switched off. Also current sensing may be involved.
Microchip have made many Application Notes on: Sensorless Motor Control...
They do this to promote use and sale of dsPIC  microcontrollers with builtin ADC and OP-amps and analog comparators, and motor control PWM peripherals.
It is a big subject with some learning curve.
 
    Mysil
 
 
2020/11/18 07:05:18
oliverb
Silly question but if it isn't exactly brand new have the magnets that trigger the sensors decayed? If something used to work and doesn't now then the question on my mind is what's changed.
 
Also FWIW what do the sensors look like and is there any hope of getting a part number? I realise the sensor may be unmarked or embedded in resin. Also is it definitely magnetic not optical?
 
I would expect the sensor to be three-lead digital output, kind of like the ones used in many DC fans.
2020/11/18 07:13:07
bonedoc
oliverb
Silly question but if it isn't exactly brand new have the magnets that trigger the sensors decayed? If something used to work and doesn't now then the question on my mind is what's changed. Also FWIW what do the sensors look like and is there any hope of getting a part number? I realise the sensor may be unmarked or embedded in resin. Also is it definitely magnetic not optical? I would expect the sensor to be three-lead digital output, kind of like the ones used in many DC fans.


Hey! It’s had little use and less than a year old. We found out they discontinued their line because of issues. The sensor board has five to-92 packages labeled “4103”. Under this, they all say 544 and one say 522.
2020/11/18 09:44:13
Mysil
Hi,
Search the web, Google give a number of finds for "4103 hall sensor".
Also read the Wikipedia article on the general subject.
 
Such sensors are made in several versions with different sensivity to magnetic strength, 
that may distinguish between 0 and magnetic North,  or between 0  and magnetic South,
or between North and South.
 
    Mysil
2020/11/18 13:59:23
NKurzman
how are the Hall switches set up mechanically?
magnets in front of the sensor? If so putting a small piece of metal behind the switch will improve it performance by concentrating the magnetic field.
Also look at the gap between the Halls and the target.
If the magnet is behind the sensor, then it may need to be stronger (or weaker)
 
There is a lot of ways this could be done and a lot that can go wrong.
Try a scope on the Motor signals and see how they look.
 
 
2020/11/18 14:49:22
davea
It has 3 motors that have hall sensors to measure rpm

wouldn't you think that sensor's would be in the motors or attached to the end shaft (or gear box)
whats the wire count from that assembly ?? 2 big 3 small ??
Hey! It’s had little use and less than a year old. We found out they discontinued their line because of issues. The sensor board has five to-92 packages labeled “4103”. Under this, they all say 544 and one say 522.

what makes you think board is actually a sensor board (5 for 3 motors)
are there moving arms that pass by it (limit sensors)
4103 is not a good part number
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