Section 4
4-13
4.2.2. 6675A Servo Circuit
See Figure 4-5 for a block diagram of the 6675A Servo circuit.
The key feature of the 9975 servo that has been kept is the "Tracking" or rough ampere-turn
"rough A-T" balance feature, except that in this case there is no mechanical or electrical link
to the "X-side". We know what turns and current have been set on the "X-side" and this is
fed to the "S-side" via the fibre optic link to set the rough A-T balance.
The controller has been changed from a Voltage (Proportional), P-controller (9975) to a
Current, I-controller (6675A). This was done to increase the noise immunity of the servo
and to increase the open loop gain.
The "smart" feature of the 6675A servo is the 8 channel 12-bit A/D converter, the Tracking
dac and the variable speed of the I-controller. During all phases of the measurement cycle
(see Figure 4-7) the servo value is looked at and its value is added to the tracking dac the
next time through that phase.
The result is that after a couple of measurement cycles the servo output stays within a few
mV of ground S even though it has a dynamic range of ±14 V. Also during all transitions
the response of the servo is slowed down by a factor of 10 for 60 mS. The net effect of the
Tracking dac and slowed down servo is that a very deep null is kept on the flux detector at
all times, even during transitions.
4.2.3. 6675A Primary Current I
x
See Figure 4-6 for a block diagram of the Primary current module.
The main features of the "I" side are a stable 19-bit (including sign) current dac and a 7-bit
(including sign) optically isolated high voltage dac.
This section includes all mode configuration relays .
4.2.3.1. Low-
Measurement Mode
This mode allows a 4 terminal resistance measurement to be made down to 0.1 m.
Full accuracy can be maintained when a model 6623 Range Extender and an
external high current supply are used. The program out terminals provide a ±5 mA
current source with 30 volt compliance to drive the 6623 reversing switch. (The
sync output on the 6675A rear panel will also drive the Guildline model 9606
reversing switch). The Extender input is used for current sensing.
To Next Page
Loading...