1
1
0.23 m V s
;
HCh
30
Microsensors
2
1
1
0.42 m V s
MGT3: GaAs, HCh
2.7 The S
f for double- drain mosfet
NB
From (2.11) it is obtained the noise-equivalent magnetic induction spectral density:
2
B S
f
S
f
(2.14)
NB
N
NI
2
f
SA
In case of shot noise, by analogy with (2.12) it results that:
2
2
W
1
1
W
1
1
1
S
f qI
q
(2.15)
NB (
) 2
4
8
2
2
2
2
2
L G
I
L G
I
H
D
H
D
Ch
Ch
Conclusions
Although magnetotransistors have a low magnetic sensitivity, very large signal-to-noise
ratios are obtained, hence, a high magnetic induction resolution is resulting. A signal-to-
noise ratio of about
5
8 10 at a magnetic induction of 200mT has been obtained at double-
drain magnetotransistors in case GaAs.
The analysis of the characteristics of magnetotreansistors structures shows that the
W L 0.5 ratio is theoretically favourable to high performance regarding the noise-
equivalent magnetic induction.
The noise equivalent magnetic induction lowers with the increase of carriers mobility, this
increase being significant for drain currents of relatively low values.
From double-drain MOSFET magnetotransistors, in case of shot noise, the
W / L 0.5 structure provides superior SNR values, and smaller detection limit values. A detection limit of about
6
0,2 10
T at a total drain-current of 0,5 mA has been obtained at
double-drain MOSFET magnetotransistor in case GaAs.
Also substituting the silicon technology by using other materials such as GaAs or InSb with high carriers mobility enables to manufacture higher characteristics devices.
2.8 The measurement of the torque at the naval engine shaft
Efficient operation of maritime ships and prevention of some considerable damages require
supervision, measurement and adjusting of the main engine parameters together with other
equipment and installations on board ship. Of a great importance is the permanent
knowledge of the torque developed at the naval main engine shaft. The measurement of the
mechanic torque M can be made based on the twisting angle that appears between two
transversal sections of the shaft when this transmits mechanical power.
Following this purpose two disks S1, S2 are placed within those two sections which contain
along their circumference, magnetic recording of two sinusoidal signals or rectangular of
equal frequency.
Two transducers made with Hall magnetic microsensors positioned in the immediate
vicinity of those two disks, allow during the rotation of the shaft to furnish information
regarding the phase difference between those two signals, the rotation of the shaft to furnish
Magnetic Microsensors
31
information regarding the phase difference between those two signals, owing to its torque.
The result of the measurement is exposed in numerical form.
2.8.1 Transducer based on the double-drain
Figure 2.15 shows the electrical diagram of a transducer based on double-drain
magnetotransistors.
If the double-drain MOSFET works in saturation the differential output voltage is the
following :
L
V
GV B
(2.16)
D
H
R
Ch W
This voltage is applied to a comparator with hysteresis, which acts as a commutator. The
existence of the two travel thresholds ensure the immunity at noise to the circuit. The
monostable made with MMC 4093 ensures the same duration for the transducers generated
pulses.
Fig. 2.15. The electrical diagram of transducer
2.8.2 Block diagram of the instalation and description of function
The disks with magnetical registration are distributed in such a way that the free rotation of
the shaft, over the time when it is not transmitted the mechanical power, the signals
produced by those two transducers are rigorously on phase.
At the power coupling, owing to the shaft torsion between those two sections S1 and S2
(figure 2.16) a twisting angle appears to which a phase difference between those two
signals corresponds..
The work of installation may be supervised by means of the block diagram (figure 2.17) and
by the forms of wave shown in figure 2.18.
The signals from the output of those two monostable CBM1, and CBM2 are applied to the
differentiating circuits CD1 and CD2 which activate the bistable circuit CBB.
The positive impulses of the signal (b) put the flip-flop in the state 1 (high) and the positive
impulses of the signal (b’) bring it back to the state 0 (low).
32
Microsensors
(a)
(b)
T1
CBM1
CD1
(c)
CBB
P
N
T2