Guildline 6622T Series, Руководство по эксплуатации

Страница: 13 / 116

Section 1
OM6622T-A-00
30 January 2018
1-3
1.4. PRINCIPLE OF OPERATION
A simplified diagram of the automatic direct-current-comparator is shown in Figure 1-1.
Two sets of four terminal connections hardware are provided, one set for the standard
resistor and one set for the unknown resistance. A current (Ix), set under control of the
microprocessor, is passed through the variable turns of winding (Nx) of the comparator and
through the unknown resistance (Rx). The isolated slave current (Is) is generated under
microprocessor control, as a linear function of the number of turns (Nx) and is forced
through the fixed slave windings (Ns) and the reference resistor (Rs). The currents (Is) and
(Ix) flowing in the windings (Ns) and (Nx) respectively generate a net magnetic flux
imbalance in the flux detector windings (Nd). This flux imbalance is proportional to the net
ampere-turn error between the two windings [(Is) x (Ns)] and [(Ix) x (Nx)]. By sensing this
imbalance with the sensitive flux detector winding and an electronic peak detector, a voltage
signal is generated proportional to the sign and magnitude of the ampere-turn error. This
voltage is used in a simple closed loop control configuration to adjust the slave current (Is)
to bring the (ampere-turn) error signal to zero. At balance, the ratio of the two currents (Ix
and Is) flowing in the two windings is the same as the ratio of the turns set for each winding
(Nx and Ns).
Complete electrical isolation between circuit currents is maintained throughout the 6622T
by making full use of fiber optic technology. A fiber optic serial data communication link is
used to transfer data between each functional component and the embedded microcontroller.
A link joining the two potential leads of the unknown resistance (Rx) and the reference
resistance (Rs) and the nanovoltmeter detector provides the only connection between the
two halves of the bridge. Power supply isolation techniques as recommended by Kibble
2
are used throughout to further isolate the bridge halves. The nanovolt detector is a high
input impedance (>1 G

) amplifier arrangement capable of resolving voltages at the
nanovolt level with a typical linearity accuracy of 14 bits (to a maximum of 22 bits). In the
automatic direct-current-comparator bridge arrangement this amplifier is used to measure
the potential across the reference and unknown resistances.
Knowledge of the reference resistance value (Rs), the effective resistance potential
imbalance and confirmation that ampere-turn balance is maintained, with the turns (Nx, Ns)
and current (Ix, Is) settings, allows the unknown resistance value (Rx) to be calculated.
2
Kibble, B.P. and Robinson, I.A., "An Isolated Mains Driven Power Supply", NPL
Memorandum DES 54, September 1985.