# Valentine's Day Quarrel

[9.19.15]

{
[Martin Taylor 2015.02.14.13.35]

``````How does this contradict what Bruce said? Why is what Bruce said
``````

“odd”. It just describes exactly what is shown in the figure.

``````They would enter as another force Fd in parallel with the force F at
``````

the summing junction. The output force from the control system and
the disturbing force from the environment both have to be in the
integral along with the two negative feedback forces from the
friction that opposes the velocity and the spring compression that
opposes the change in position.

···

[From Rick Marken
(2015.02.14.0950)]

``````                Bruce Abbott (2015.02.13.1910
``````

EST)–

``````                        BA:  The
``````

right box in Figure 5-1 is not the
comparator of a control system. It
represents the summing junction where
disturbance (F) and feedback (the two arrows
with negative sign) act on the
mass/spring/damper system.

``````          RM: This strikes me as a very odd way of looking at F
``````

since F is the FORCE output of a control system (Figure
5-2) that is controlling the position, P, and velocity, V,
of the mass. F is the output that is used to compensate
for actual disturbances to the velocity and
position of the mass that are independent of the outputs
of the control system.

``````          RM: These actual disturbances are not shown in Figure
``````

5-1 or 5-2 but they would enter the MASS ON SPRING WITH
DAMPING box of of Figure 5-2 from the environment side of
the diagram (below the grey line). And they would enter
the box that computes V in the mass-spring model in Figure
5.1 from somewhere other than above, which is where the
control system inputs and outputs go. A true disturbance
to this system would be a force applied to the mass
independent of the output, F, of the control system, such
as someone other than the person controlling the mass
pushing up or down on the mass.