Lake Shore Model 475 Gaussmeter User’s Manual
Background
2-7
2.3.3
Orientation
Hall generators come in two main configurations, axial and transverse. Transverse devices are generally thin and
rectangular in shape. They are applied successfully in magnetic circuit gaps, surface measurements and general open
field measurements. Axial sensors are mostly cylindrical in shape. Their applications include ring magnet center bore
measurements, solenoids, surface field detection and general field sensing. Figure 2-7 shows these two orientations.
Figure 2-7. Axial and Transverse Configurations
2.4
FIELD CONTROL
The Model 475 includes the necessary electronics and firmware to perform closed-loop field control in an electromagnet
system. The system must be equipped with a voltage controlled power supply. The Model 475 uses Analog Output 3 to
program the power supply. A Lake Shore probe is used to measure the field and completes the closed-loop configuration.
The instrument, probe, power supply, and magnet play important roles in establishing stable field control.
2.4.1
Closed Loop PI Control
Closed loop PI control, often called feedback control, is the control mode most often associated with field controllers. In
this mode, the controller attempts to keep the magnetic field at exactly the user entered setpoint that can be entered in
units of flux density, from the front panel. To do this, it uses feedback from the probe to calculate and actively adjust the
control (analog) output. The Model 475 uses a control algorithm called PI that refers to the two terms used to tune the
controller for each unique system.
The PI control equation has two variable terms: proportional (P) and integral (I). See Figure 2-8. Changing these
variables for best control of a system is called tuning. The PI equation in the Model 475 is:
+
=
∫
dt
e
I
e
P
)
(
1
Output
where the error (e) is defined as: e = Setpoint – Feedback Reading.
