PID controller based on DC motor

I suspect that the time constant of the motor can be found using the tachometer you have. You will need to convert the tachometer frequency to a dc voltage, if it isn't already. Then, run the motor at a low dc voltage and "step" the applied voltage to a higher level quickly. Trigger a scope on that step, then watch the tachometer output (converted to dc) on the scope. That waveform should tell you the time constant. You may want to try stepping both low-to-high and high-to-low, and see if they are the same.

If the signal coming off the tachometer is a 50-50 square wave, then use one edge to fire a narrow-pulse one-shot of constant width. Then integrate that to create the frequency-to-voltage converter. Good luck.

time constant for finiding the dc m/c is very simple.

1.apply dc voltade to the motor. then u can find the R vlauve by

E=V-iR

2.apply ac voltage to only for the winding terminals.

then we will find the Z=V/I

3.we know that

Z=sqrt (R^2+(wL)^2) here w=2*3.141*50

by using 1,2 we will find out the L value easily.

4.time constant of motor is t=L/R

Speed control under a uniform load can be obtained.

Exact position control is much more difficult. a motor will overshoot the desired position and negative movement must be introduced to return to the correct location.

But since a motor cannot return its position an external encoder must be used.

Do you intend using encoders?

Evo,

I cut and pasted the info below from the Emoteq Megaflux catalog (emoteq.com). It supports Guests' definition from above, and also includes a mechanical definition of a motors time constant.

Electrical Time Constant (TE)

is the ratio of inductance

LM in Henries, to the resistance RM in Ohms. This is the

inductance and resistance as measured across any two

phases in a delta or wye configuration.

TE = LM/RM

Note: This definition is from a brushless motor catalog, You would measure through the armature leads with a brush motor.

Mechanical Time Constant (TM)

is the time required to

reach 63.2% of the motors maximum speed after the

application of constant DC voltage through the commutation

electronics, ignoring friction, windage, and core

losses.

TM = JM * RM/ KT*KB

I hope this helps.

Dear Mr Elvin

I require Mechanical Time Constant (TM) i.e time required to

reach 63.2% of the motors maximum speed after the application of

constant DC voltage

TM = JM * RM/ KT*KB

please tell about the formula u have given what are these parameters JM , KT , KB

and how these parameters can be calculated practically

waiting to hear from you

thanks

Evo,

All of that is in the Megaflux catalog at www.emoteq.com

http://www.emoteq.com/downloads/Megaflux%20V2.zip