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i need a cheap long lasting life DC motor!

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eltonjohn

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74hct4046 pll howto

i have been on the phone for hours looking for a cheap BRUSHLESS DC motor
so far they want too much money ..the specs are 15k rpm 12 volts and an life of at least 20,000 hours .. Okm this morning it occured to me that the computer fans run close tho those specs and their life is typically 40,000 hours at least .!
why no use the basic design of those MOTORS .. but how to control it precisely in a close loop circuit??

By fortune the nature of my project provides me with a free 8 pulses per revolution encoder .. Now i need just a simple design to control the speed accuratly on those MOTORS .. Any body knows if there is a project that has done this ?
 

HiQ

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I believe i invented this,but i'll share it anyway. If you have power on the rotor you can place an infrared diode pointing downwards (toward the stator). it is powered by a sawtooth signal generator (or in my case a PIC witha trensistor & capacitor) The frequency should be equal as the desired RPSecond. On the stator,facing up towards the diode is placed a IR transistor (used as a photoresistor) It controls (trough some aditional parts) the power level of a computer impulse power supply (with disabled internal safety's to allow more output variations) That in turn powers the motor.
Each time the rotor makes a revolution the IR tr. "meets" the IR diode and "sees" a particular light level coming from it. That light level determines how much voltage the impulse power sends back to the motor (for example 4 volts) At the next revolution the diode & tr. meet again & if the signal level is the same that means that the RPSec. is in tune with the sawtooth signal thet "feeds" the diode, and the feedback voltage remains the same. If the rotor speed has exeeded thet of the sawtooth freq. gener. the IR tr. sees the diode light at a higher level and automatically decreaces the voltage to the motor. If the speed has decreased it sees a lower light level & sends more voltage to the motor. It can also be done with all electronics on the stator ,the IR diode & the IR transistor facing a small mirror glued to the rotor.
 

eltonjohn

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well what you describe is the same thing as a PLL but insted of a square signal you use a sawtoth and do a phase comparaison .. I started doing that ,by using a 74HCT4046 with a regular BRUSH motor .. now i want to port that to a BRUSHLESS DC motor .. i would like to use some kind of PID ..i have 2 ideas already in my mind
One is to convert the pulses to DC voltage and apply it to a analog PID ..
i found a nice and cheap tachometer chip fron national instruments LM2907,..but no SPICE models .This morning i found a rusian guy that have done it!
 

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