Aspen Return Monitor (A.r.m.) - ConMed Sabre 2400 Operators & Service Manual

3.1.8 Aspen Return Monitor (A.R.M.)

Circuitry and Software
The A.R.M. Circuit converts the electrical resis-
tance appearing in the return electrode circuit
into a digital value which can be processed by the
microprocessor. Software processes use this value
to determine when a RETURN FAULT condi-
tion exists. The Resistance Indicator is also driven
by software to indicate the value of the measured
DUAL FOIL resistance in the 10 to 150 ohm
range. Portions of this function are implemented
on the A2 Micro/Control PWB, Figure 4.4, and
on the A1 Power PWB, Figure 4.6.
The A.R.M. Circuitry on the A1 PWB comprises
an oscillator section and an isolation section.
The isolation section employs a shielded toroi-
dal transformer, T5, to couple the impedance
presented at return electrode plate plug, A1J9 to
the A.R.M. oscillator, while isolating that circuit
from the effects of applied RF electrosurgical cur-
rent and voltage. Capacitors C55 and 56 split the
return current evenly between the two legs, thus
minimizing the RF voltage appearing across T5
windings. T5 also acts to step up the return cir-
cuit impedance by about 10:1. The shield serves
to prevent the RF stray magnetic field generated
by the monopolar output transformer, T3, from
interfering with the A.R.M. circuitry during RF
activation.
The A.R.M. oscillator generates a low-power sin-
ewave voltage of about 36 KHz. This frequency
is determined by the inductance of T4 in paral-
lel with the capacitance presented by C52-54,
and that of C55 and C56 reflected through T5.
Transistors Q14 and Q15 are cross-coupled via
R49 and R50, so that when one transistor is con-
ducting, the other is fully turned off due to lack
of base drive. The conducting transistor turns off
at the next zero-crossing of the sinusoidal voltage
on the primary of T4. This allows its collector
voltage to rise and thus provide base current to
the other transistor to turn it on. In operation,
the collector voltages appear like half-wave recti-
fied ac, with each collector 180 degrees out of
phase with the other.
The A.R.M. oscillator is powered by a constant
0.5 mA dc current driven from the A2 PWB via
the VARM signal line. This current feeds into
the center tap of T4 primary. The voltage on the
center tap is the average of the two collector volt-
ages, so it appears as a full-wave rectified sine
wave with a peak amplitude of one-half that on
either collector. Inductor L1 helps hold the cur-
rent fed to T4 constant regardless of these voltage
variations, while C65 serves as a bypass to limit
the noise conducted from the A1 PWB up the
VARM line to the A2 PWB.
The A.R.M. oscillator is a dc-to-ac power con-
verter, with its major losses appearing as resis-
tors in parallel with the resistance of the return
electrode circuit, R , transformed up through
LOAD
T4 and T5. In effect, the A.R.M. oscillator trans-
forms R into an equivalent dc resistance,
LOAD
RIN, appearing at the VARM input to the cir-
cuit. Thus when R is very high, as when no
LOAD
connection is made to the Return Electrode jacks,
R is maximum, allowing the VARM voltage to
IN
rise to +2.3 - 3.0 Vdc.
When R falls into the 10 to 150 ohm range
LOAD
normally encountered with a properly applied
dual-foil electrode, R also drops and VARM
IN
falls into the 1.0 to 2.5 V range. If R
low, as when a single foil electrode is connected,
VARM drops to about +0.8 Vdc. Resistors R51
and R52 serve to set a lower limit to the resis-
tance applied across T4's secondary. Without this
lower limit, the effective short circuit presented by
a single foil return electrode would reduce the Q
of the 37 KHz tuned circuit to the point that the
oscillator would behave erratically. Thus VARM
varies directly with the resistance appearing in
the return electrode circuit. The relationship is
essentially logarithmic, with increases in VARM
becoming vanishingly small as R
1000 ohms. This means that VARM will change
by a nearly constant voltage for a given percent-
age change in R anywhere in the 10 to 150
LOAD
ohm range. The balance of the A.R.M. Circuitry
resides on the A2 Controller PWB, Figure 4.4.
Diode D11 is a +1.235V regulator whose output
voltage appears across the 2.49K resistor R21,
thus driving a constant current of 0.5 mA to the
VARM line. R23 and C23 act as a low-pass noise
filter. U15:B is connected as a noninverting ampli-
fier with a gain of 2 to amplify the filtered VARM
voltage to 2VARM and is an input to the analog to
digital converter U16.
R
is very
IN
rises above
LOAD
3-9