INFICON MDC-370 Thin Film Deposition Controller - Manual (Page 91)

MDC-370 DEPOSITION CONTROLLER
6. TUNING THE MDC-370 CONTROL LOOP
6.1 Control Loop Basics
If evaporation rate were a function of source power alone, a rate controller would
not be necessary. One would establish the power required to achieve the desired
rate, set the power at that point and that would be that. In control system
parlance, this is called “Open Loop” control.
Unfortunately, evaporation rate is a function of many variables. With E-gun
sources, rate is affected by material level, water-cooling temperature, beam
position, sweep pattern, etc. With filaments and boats, rate is affected by material
level, boat or filament condition, power line voltage, power losses in cables,
connections, transformers, switches, etc. Even when sputtering under the
conditions of constant power and constant pressure, rate is affected by target
condition.
So, if we want to achieve a known and constant rate, we need a rate controller.
The rate controller compares the measured rate with the desired rate and attempts
to keep them equal by adjusting the command signal to the power supply. This is
called “Closed Loop” or feedback control.
The most common example of feedback control is a car and driver. The car is the
“Plant”. It is controlled by pedal pressure and steering wheel angle. Its output is
direction and speed. The driver is the “Controller”. The driver monitors the
direction and speed and adjusts pedal pressure and steering wheel angle to achieve
the direction and speed he/she desires. If we hold the controls steady and close
our eyes, no feedback, then our control is open loop. If the road is very straight
and there is no wind, “no disturbances to the plant”, we can sometimes stay on the
road for a pretty good distance. If the road is rolling or we have a good
crosswind, the time we can stay on the road in open loop control can be pretty
short indeed.
If the controller is slow and sluggish, i.e. a drunk driver, the difference between
the desired speed and direction can be very different from the speed and direction
desired. The driver can be all over the road, speeding up, slowing down, etc.
If the controller’s gain is too high, typical of a young person’s first driving
experience, the response to an error is both slow and too great and the car careens
from one side of the road to the other. This control “System” would normally go
completely unstable and crash if control were not assumed by a different
controller.
In the case of a young driver with a little more experience under his/her belt, the
response speed has improved but the gain is still a little high. The vehicle stays
pretty well in control but there is a lot of steering wheel action. We say this
controller is “oversteering”.
When we go from one vehicle to another, especially if the vehicles are very
different in size or weight, we find that we must really concentrate on our driving
at first. That is because we are learning the characteristics of the “Plant”. As
Tuning the MDC-370 Control Loop 6-1