3.9 .2 Parameter 2 (Hysteresis)
3-14
Operation
The second parameter controls setpoint hysteresis. In many cases, it is
undesirable to have a function tum on and off at the same point
regardless of system history. For example, imagine a vacuum system
in which an electron gun is operated only below a given pressure.
Using the previous example, the setpoint is programmed to tum on at 5
x 10-
7
and thus activate the gun.
A typical operating sequence for this system may be:
l. System pumps down to below 5 x 10-
7
.
2. The gun turns on, outgassing, and raises the pressure to 6 x 10-
7
.
3. The gun turns off, outgassing decreases, and the system drops to
below 5 x 10-
7
.
4. The gun turns on, outgassing, and raises the pressure to 6 x 10-
7
.
By not having any hysteresis on the setpoint, a large oscillator has
been built out of the vacuum system and the electron gun.
If parameter 2 is non-zero, it determines the point at which the setpoint
turns off, while parameter 1 still determines the point at which the
setpoint turns on. For example, with parameter 1 at 5 x 10-
7
and
parameter 2 at 7 x 1 o-7, a typical operating sequence may now be:
1. System pumps down to below 5 x 10-
7
_
2. The gun turns on, and the system pressure rises to 6 x 10-
7
.
Since
the pressure has not reached the value of parameter 2, the setpoint
remains on and the gun remains energized.
