No, the ones I am familiar with use speed feedback from the motor. In some cases, the back emf is sensed to determine speed; if the motor slows down, the back emf decreases. In other cases a hall effect sensor is embedded in the motor winding to sense rotor speed. In others, a DC tachometer is used to sense speed, etc. In others, a photoelectric pick-up is used.Thanks for the response Mike, but a speed controller just sends out a specified voltage right? A motor under the same voltage but lighter load would move faster right?
Can you see the shaft as it goes into the gearbox? Mount a mirror on the shaft and use an LED and a phototransistor to sense the rotation rate.The gear motor I have is just two wires, can the motor still give feedback to the speed controller?
PLL's really good. I once synchronised two floppy disk drives together (without a PC at all), using a PLL, phase locked using the 1 pulse per rev datum signal produced by the "index pulse" is it?. The master FDD set it's own rpm according to its XTAL and I had the slave FDD motor controlled by the PLL's VCO, comparing the master and slave FDD datum sensors. All this... so I could defeat copy protected diskette. Worked really well.Use a 32768Hz crystal for the oscillator and you should be able to set your watch to it.
Cassette tape drive motor is quite stable, is this the way they do it?Hi,
A relatively simple method is to measure the back emf of the motor and use
that as a feedback signal to regulate the speed to some set value. This can
be done using an analog technique too so doesnt require a microcontroller.
Using a voltage regulator alone unfortunately does not work because the
motor speed varies even with constant terminal voltage when the load
That's for a two-wire PM DC motor right? How does measuring the motor current determine how much voltage to apply?Some measure the motor current and increase the supply voltage as the current gets larger. There are special ICs that do that for cassette players.
A DC motor doesn't draw a constant DC current.That's for a two-wire PM DC motor right? How does measuring the motor current determine how much voltage to apply?
That's for a two-wire PM DC motor right? How does measuring the motor current determine how much voltage to apply?
I always thought that circular shaped circuit board inside the motor was a voltage regulator, with an adjustment pot below the hole to trim the speed.
Just as a sanity check, I connected an ordinary DC PM motor to a variable-voltage supply here, the RPM is proportional to set voltage. The RPM doesn't change by much when loading the spindle. When the loading on the spindle makes the current exceed the power supply limit, then the revs drop. I suspect the voltage is dropping.