# Digital Control of dc motor question advice

#### Saviour Muscat

Joined Sep 19, 2014
166
Dear helpers,

This is one of the last units of my part-time engineering course, hope to conclude it well with your kind help.
Currently, I have a digital control scenario question of a cascaded control of a position/velocity of a dc motor. I'm trying to tackle questions one by one and advise me if I'm in the right way. Attached in fig1 the coursework question, fig2 the tentative solution block diagram of B3 a).
For GP1 and GP2,

For GP2=1/s

For GP1, I did this derivation illustrated on fig3 referred to block diagram fig 2, kindly advice

GP2=0.01/(5.00625*10^-3s^2+0.060075s+0.100125)

Thank you

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#### MrAl

Joined Jun 17, 2014
9,179
Dear helpers,

This is one of the last units of my part-time engineering course, hope to conclude it well with your kind help.
Currently, I have a digital control scenario question of a cascaded control of a position/velocity of a dc motor. I'm trying to tackle questions one by one and advise me if I'm in the right way. Attached in fig1 the coursework question, fig2 the tentative solution block diagram of B3 a).
For GP1 and GP2,

For GP2=1/s

For GP1, I did this derivation illustrated on fig3 referred to block diagram fig 2, kindly advice

GP2=0.01/(5.00625*10^-3s^2+0.060075s+0.100125)

Thank you
Hello,

After plugging in the given constants i find only minor difference in the solution you got and mine.
Mine is:
0.01/(0.00500625*s^2+0.060075015625*s+0.100125)
Yours was:
0.01/(5.00625*10^-3s^2+0.060075s+0.100125)

and so i am assuming that you rounded that central constant, the one multiplied by 's'.
I am also assuming you do not have to include the disturbance torque.

#### Saviour Muscat

Joined Sep 19, 2014
166
Hello,

After plugging in the given constants i find only minor difference in the solution you got and mine.
Mine is:
0.01/(0.00500625*s^2+0.060075015625*s+0.100125)
Yours was:
0.01/(5.00625*10^-3s^2+0.060075s+0.100125)

and so i am assuming that you rounded that central constant, the one multiplied by 's'.
I am also assuming you do not have to include the disturbance torque.
Dear MrAl,

I truly appreciate your help, just saw I had some rounding in the equation.

Thank you very much,

#### MrAl

Joined Jun 17, 2014
9,179
Dear MrAl,

I truly appreciate your help, just saw I had some rounding in the equation.

Thank you very much,
Hi,

You are welcome.
Also, it may be ok to round, but when i see that they used higher precision to quote the physical constants i hesitated to round because it may be the tiny portions they wanted to see correct. In real life we would probably round like you did though, but then gain i doubt we would be able to make the measurements of the physical constants as well as they seem to have done More than likely they are made up constants just for the exercise (note the extreme precision of the friction and inductance values which would probably be impossible to measure that accurately).