# Confusion in block diagram of open loop and close loop control system?

I am studying Modern Control Systems, 11th edition, by Dorf and Bishop. On page 221 it first shows in fig4.7 open loop control system(the writer names it as open loop) and on page 222 close loop control system is shown in fig 4.9

Snapshots of both images are attached

I wonder why the writer is calling fig4.7 as open loop despite the fact that it contains feedback path with gain block Kb

• There is something off about how the author draws the "back electromotive force" coming off the speed, but I'm pretty sure it's open loop since it doesn't use a direct measurement of what it is controlling in the feedback loop. The second one uses a tach which provides a measured speed, the first one does not. Sep 26, 2019 at 13:01
• what are the parameters"Td","Tm" and"Tl" ??what do they denote?
– engr
Sep 29, 2020 at 6:36

This system:

Actually uses the Motor's back EMF as a feedback signal, it does not use the speed information directly.

From a back EMF viewpoint, this is a closed system (the back EMF is directly controlled).

From a motor speed viewpoint, this is an open system (the speed is not directly controlled).

However, this system:

does use the speed of the motor as a feedback signal so the speed is directly controlled.

There is still the back EMF feedback but that does not control the motor speed directly (assuming "sane" choices for $$\K_b\$$ and $$\K_t\$$)

The back EMF is an inherent characteristic that is a factor in determining the motor's torque vs. speed curve. Without feedback, the motor speed is not indeterminate. It varies as load change as determined by the slope of the torque vs. speed curve. The back EMF gives the motor a certain amount of built-in feedback, but the effect is limited. The gain can not be adjusted.

Your first diagram shows the behavior of the motor alone. The feedback (back EMF) shown does exist, but it does nothing to stablize the motor speed against load changes. For a given voltage in, and a fixed load torque (bearing friction, load friction, air resistance, etc) you can set the motor speed by varying the input voltage. Some systems do this quite handily - electric fans, for instance. However, if you vary ("disturb") the load torque, the motor speed will change. This is not an issue for some applications (electric drills, electric golf carts, etc). For these applications, the operator supplies the feedback when the output speed needs to be constant.

Because there is no built-in stabilizing loop, these systems are called "open loop", although if you include the effects of a human operator this description is not accurate.

When you add external control paths, as shown in your second figure, the effects of the added tachometer allow the system to maintain speed without operator intervention, and these are called closed-loop systems.