measure a voltage across of a dc motor

so with that resistor i will reduce this current but i won´t be able to get 3V at the output of the AO,the maximum that i check,was with duty cyle 95% and the voltage was 2.6V,is it normal because the power resistor in series with the motor take the another 0.4V??

i think that it is the only way to get this work well with this motor a amplifier

and one think that i observed was that,if i generate the pwm signal with the cine555 the current consumed for the motor is little bit lower than when i generate the pwm signal with compactrio,why is this happen?