View Full Version : Hall sensors ? ? ?
04-07-2007, 08:01 PM
Model 503 is a complete pwm servoamplifier for applications using DC
brushless motors in torque ( current ) mode. It provides six-step commutation
of three-phase DC brushless motors using 60° or 120° Hall
sensors on the motor, and provides a full complement of features for
ˇˇAbove is part of the description of a servoˇmotor amp that I'd like to control with my BS2. It calls for digital Hall sensors, not an encoder. What exactly are they reffering to and would this still be available ? Could an arrangement be made using the Hall switches that Parallax sells ? Looks like most motors come with encoders, not Halls. Is the Hall "Old School" ? Should I look for something else ?
Encoder is a generic term for any device (in this context) that produces regular, machine readable, pulses relative in frequency to the rotation speed of a motor. Some use light, some magnetic pulses (as yours), some a physical switch-- I'm sure there are other means as well. If the rest of the package meets your need, and the encoder pulse frequency is adequate... Hall technology is well understood, comon, reliable, and cheap; I'd say very usable.
04-07-2007, 08:51 PM
How does 60 or 120 degree hall compare to a quadrature encoder ?
04-07-2007, 09:30 PM
Being a three-phase brushless motor, it works more like a stepper. The Hall sensors are used to detect when to commutate (or switch) from coil to coil to keep the rotor turning.
The 60 deg or 120 degree sensing is like full-step or half step with a smaller stepper.
The Hall signals may or may not be brought out for your use, but I expect they are set up in quadrature so the motor controller can do reversing. The fact they advertise "full complement of features" makes me want to believe that. You may have to call the manufacturer or do some reading on some of the r/c airplane sites as thats where a lot of the current high-power 3Ph DC motors are being used.
04-07-2007, 11:55 PM
VERY VERY GOOD explanation. I can pick up a couple of these amps. pretty cheap.
Are the 3 ph brushless motors common items ?
04-08-2007, 01:35 AM
With a BLDC motor there are several ways to get the phase out, one of the most common is the hall sensor, the newest method is called "sensorless", so there are some brushless motors now that do not have the Hall wires on the motor(+5, GND, Sense1,2,3), but they would be clearly described as "sensorless". Make sure when you look for the motors that they have the Hall sense connector or wires. Brushless motors have adavantages over brushed motors, they have higher efficiency and do not have the brushes that will wear out after so many spins. The usual parts to wear out are the bearings, as there are no other moving parts creating friction, but the usual life of the BLDC is many times that of brushed.
If you are needing motors, here is a very good source for inexpensive brushless motors with excellent tech support and sales staff. Click the button "BLDC motors"
The motors are more complicated than a brushed DC motor. You cannot just hook up a voltage to the motor and make it spin There are 3 wires(3 phases) and they run in sequences, similar in theory to a stepper, in that each each phase is controlled externally as to when it is it's turn to be on. I just happen to be designing a BLDC driver and Propeller controller board right now, and find it to be a lot of fun, much more learning and challenge than other motors/drivers. With a BLDC, you have to output a PWM and direction in a typical setup. I am not sure on your drivers you have found, but make sure that you have determined how you are going to provide the PWM from the Stamp if in fact you drivers require a constant PWM.
In a servo or closed loop system, there needs to be an encoder mounted on the shaft, the quadrature encoder output is counted by the processor, and the processor moves the motor precisely to where it is told based on the encoder count. I no not think you can manage a closed loop system from within the Stamp, so external hardware may be needed whereas the Stamp just manages the system. You use and SX or Propeller to be the PWM engine and encoder counting engine. If can be more specific on what you are tryng to accomplish, I am sure some suggestions will come that can help clarify what some options are.
Post Edited (TChapman) : 4/7/2007 8:43:52 PM GMT
04-08-2007, 02:51 AM
TC, Anaheim has very good illustrations and along with your help I understand much more now. Thanks.....
1 thing I dont see, on their driver, there the Hall connections, power, motor,fwd/rev,enable, brake and optional external speed pot but where and how would I hook-up the pulse train or PWM in to the driver.
It's probably obvious, I just dont see it anywhere in the lit.
Thanks again for the excellent advise.
04-08-2007, 03:30 AM
Curious1 Their drivers are more designed for on and off, with a pot for setting speed. In a servo(closed loop) system, you could possibly PWM the enable, you could verify that with the tech support, although I am not leaning towards that being the optimal solution. Certainly a digitally controlled pot could control speed on their driver, but the typical scenario is a processor outputs PWM for controlling speed. There are I2C digital pots that you coulld run from a Stamp. You haven't been specific on your project, so without further info, it is just guesswork as to what would be the best method to run the motor.
If you are DIY and can do SMT boards, there is a 3phase motor driver IC that I am designing some boards around called the Allegro 3938. It accepts PWM for speed, or you can force the PWM high and set the speed with a pot or DAC, it just needs a reference voltage to set the current limiting, the current sets the speed. It will operate up to 50 volts.
Keep in mind, it may not be possible to send the PWM and DIR from a Stamp due to speed limitations, let us know exactly what you are wanting to do with more details.
Post Edited (TChapman) : 4/7/2007 8:44:46 PM GMT
04-08-2007, 03:53 AM
Thats what it looked like to me.
All that I am familiar with and comparing this to is a stepper driver with the step and dir inputs that I am running now with the pulsout on BS2. I want to eventually progress to precisely controlled, simultaneous, 2 axis, servomotor moves, point to point. I understand that the BS2 cannot do this but the Propeller will.
I will try to keep up with your project, which forum is best to follow it in ?
I guess I was assuming that what I was looking at was a servo controller.
Thanks for clearing all of this up,
04-08-2007, 04:10 AM
For what I need, speed is not really an issue.
Any idea of the 3938 $ ?
These look similar and there are several offered that may be of interest.
04-08-2007, 04:23 AM
What you linked is a similar IC, I don't like the 30v max limit, it does show a DIP package type though. You can do the same with either one(I didn't study your link in detail), I really like Allegro products, and the 3938 allows 50v max. The 3938 is less than 5$ on digikey, but only available in fine pitch TSSOP. It is not a chip you are going to solder with an iron(well, you could but...), a heat gun would be easy if you want to do the learning curve.