25/08/2004 ©
Söring
Service-Manual Arco- and RF-units
1
not valid for A-3000/ -2000
2
not valid for Arco-2000
-35-
3
not valid for MBC600
4
not valid for Arco-1000
6.7. Power limiting
The power limiting circuit consists of the power measurement and the power regulation.
The regulator works exactly the same way as for current limiting (analog switch IC3c and
PI-regulator with U1d) and acts also on the DOWN-signal. Power measurement is done
with a voltage- and current signal, retrieved from TR3 (current) and TR4 (voltage). Both
RF-signals (RF_CURR and RF_VOLT) are multiplied in real time by a broadband
multiplier U7. As the Multiplier delivers only 300mV/100W, an amplification with the
operational amplifiers U2a, U2b and U2c is necessary, before the power signal can be
passed to the PI-regulator U1d via the analog switch. Due to the large adjustment range
of power from 1 to approx. 500W, a regulation only by preset value (P_SET) is
insufficient (instability at low preset values). Therefore the amplification of the power
signal for the multiplier is switched in 8 range steps. This is done by the analog switch
IC5 and the resistors R412 to R419. By means of P2 the offset of the multiplier and the
operational amplifier U2a is adjusted to zero. Beginning of power limiting is transferred to
the microprocessor by the signal PREG via the interface board IF02.
6.8. Arc regulation
Cutting of tissue is only possible, when the RF-voltage at the active electrode is so high
that a fine arc occurs between electrode and tissue. If no arc is achieved, only
coagulation of tissue occurs. The voltage at the active electrode, necessary for an arc,
depends on kind of tissue and geometry of electrode. If the cut-voltage is increased
beyond the necessary value, the intensity of the arc rapidly increases and the tissue
burns. When operating from muscles to fatty tissue, the arc can extinguish, so that the
electrode sticks. The presence of an arc can be sensed by harmonics in the cutting-
current, due to nonlinear effects. The higher the harmonics content, the stronger is the
arc intensity. If the arc intensity is too high, the RF-voltage has to be reduced and it has
to be increased in case of a too small arc. In this module the arc intensity is retrieved
from the cut-current signal RF_CURR. The signal RF_CURR is passed through R136 to
the operational amplifier U4.
By means of a reactive current simulation, which simulates and subtracts the stray
capacitance of the handpiece cables and the inductive currents of the output
transformers (capacitors C132, C133, resistors R137 to R142 and relay for the different
conditions under monopolar and bipolar applications), the current signal (C_SIG) goes to
2-quadrant multiplier U8 for amplitude regulation. After appropriate amplification (U6a,
U6b, final stage T15, 16) the amplitude is evaluated (difference amplifier U3a, D22 and
D23 etc.), and passed to a PI-regulator (U3b). This regulates the output voltage (signal
CURR_SIG) to a value, independent of cutting current. This signal contains the
fundamental and possible harmonics, caused by the arc. It is rectified after high-pass
filtering by R377, C309 and L1. The resulting voltage is proportional to the harmonic
contents in the cut signal and corresponds to the arc intensity during cutting. This actual
signal and the reference signal are routed via the analog switches IC3a and -b to the to
the PI-regulator U2d, which regulates the power supply down at too high arc intensity
(signal DOWN). The down regulation is notified by the signal SPREG to the interface
board IF02 and thereby to the microprocessor for evaluation.
6.9 Pulse generation and control logic
The following operational modes are known to the control logic:
-Cutting
-Coagulation
-continuous operation
-pulsed operation
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