Okay guys, this one isn't for me; it's for my mechanical brethren. I'm a motion control engineer and I was assigned to a project to control an axis with high-dynamicism using a low-inertia, high-torque motor. The ratio of the motor inertia to the reflected load inertia is greater than 1:400. Now, to me, it's obvious that there is no shaft coupling in existence that is stiff enough to make this work. I have shown the mechanical group bode plots, demonstrated regions of stability compared to their terrible step-response and oscillating step-responses with regions of instability. I've tried to explain the dynamics of PID-controllers and the closed loop phase margin. But NOTHING is getting through to these mechanical engineers. How can I explain to them that their design JUST... WON'T... WORK?! I show them graphs, draw them diagrams, write down equations... but at the end of the day, this is still some how my problem and I'm supposed to make it work. They keep telling me that the motor has plenty of torque (as if that had something to do with the problem). I calculate a needed bandwidth of 100Hz or more with less than 90 degrees of phase lag, but the open-loop response has 90 degrees of lag before 1Hz! What words can use to show these guys what I mean? What pictures can I draw?!