Ohmeda 3000 - Page 11

1/Functional Description
Heat Control Routine
Proportional control of the heater power is obtained
by varying the number of full heat cycles of ac cur-
rent delivered to the heater. To allow for line vol-
tage compensation and stili have at least 20 dis-
creet levels of heat, a proportioning range of 0 to 60
full heat cycles was selected. In other words, at
very low line voltages, 100% heat will be output by
having the heat “On” for 60 full cycles out of a pos-
sible maximum of 60. Similarly, at this low line vol-
tage 90% heat is obtained by having the heat "On"
for 54 out of 60 cycles.
In the “manual” mode of operation, the heat output
is determined by the bargraph setting selected by
the operator. There are 20 steps on the bargraph so
each step represents a 5% heat increment. To
accomplish the desired compensation for line vol-
tage variations, the maximum number of heat cycles
is calculated based on the last measurement of the
power line voltage. For 115v nominal units, at 106
volts or less, the maximum number of heat cycles is
60. At greater than 125 volts the maximum number
of heat cycles is 40 cycles out of a possible 60.
Therefore, the number of cycles of current furnished
to the heater in the manual mode is determined by
multiplying the maximum for the line voltage pre-
sent by.the bargraph setting. For example: assume
the line voltage is 115v (maximum number of cycles
“On" = 50) and the bargraph setting is 30%; the
number of heat cycles to be output will be 0.3 times
50 = 15 cycles. Under these conditions the heat will
be “On” for 15 cycles and “Off” for 45 cycles, this
sequence will continue until the line voltage
changes or the setting is changed on the bargraph.
In the “servo” mode, the heater power is controlled
by comparing the patient's skin temperature to the
selected value of control temperature. The differ-
ence between the control temperature and the pa-
tient temperature is referred to as"PTG" (patient
temperature gradient). A positive PTG indicates a
patient is cooler than the control temperature and a
negative PTG occurs when the patient temperature
is higher than the control temperature. Based on
the magnitude and sign of the PTG, a software look-
up table is used to find the percent heat required.
The percent heat is then converted to the appropri-
ate number of bargraph steps and then the selected
amount of heat is output by the same process used
in the manual mode.
A hardware circuit is used to interrupt the micro-
controller once every cycle of the ac power line.
During the interrupt routine, two registers are de-
cremented to keep track of the heater "On" and
“Off” cycles. One register is used for counting the
total interval (60) and another register is loaded on
every sixtieth count with the number of heat cycles
to be output. A flag is set whenever this register is
not zero, the heat is “On” only when this flag is set.
The operation of the heat control software and the
heat output hardware are repeatedly tested during
operation of the warmer. An opto-isolator connected
with a series resistor directly actoss the heater ter-
minals is used to monitor heater power. The output
of the opto-isolator is fed into a Schmitt trigger,
which outputs directly to an input port of the micro-
controller. Therefore, the microcontroller can verify if
the heat is actually on when it is supposed to be on.
If not, a system fail alarm will be activated. Approxi-
mately every three minutes, an external hardware
network (safety circuit) signals the microcontroller
to switch “Off" the heat. This hardware also moni-
tors the output of the Schmitt trigger (heater status
line). If the heater power is not switched “Off” after
a short delay, the hardware circuit will de-energize
the “safety” relay to switch “Off” heater power and
algo initiate an alarm which cannot be silenced
without switching the power “Off".
Service Features
The electronic controller assembly is easily removed
for servicing or calibration. This controller contains
all the circuitry and components except for the hea-
ter, alarm lamps, and observation lamps.
All indicators and the audio alarm are activated on
power-up for operator verification of proper display
operation. These can also be activated by depres-
sing the alarm silence switch for 2 seconds. In addi-
tion the software revision number and the line fre-
quency are displayed.
Test points on the printed circuit boards are acces-
sible for troubleshooting and calibration without re-
moval of the boards. In addition integrated circuits
with 24 pins or more have sockets to aid in troub-
leshooting and repair.
Software routines are built into the warmer to pro-
vide test functions, to aid in troubleshooting, calib-
ration, and operation verification. These test
routines are activated using à DIP switch located on
the control board. Some of the test routines can be
activated using the display panel.
Calibration may be verified on the controller display
without disassembly. A high calibration point and a
low calibration point are displayed when the service
test switch is pressed for 2 seconds.
Line voltage is monitored by the warmer and fluctu-
ations of + 10% from nominal voltage are compen-
sated for so that heat output is held constant. If the
voltage exceeds + 17.5% from nominal an alarm is
activated and the heater switches off.
Self Test Functions
The following text is a description of the self test
functions performed by the infant warmer. If an
error results on any of the power-up or on-line tests
then the error number will be displayed on the
elapsed time display in the format E ##. The high
priority alarm (System Failure LED) will be “On"
and cannot be silenced. Power must be switched
“Off” to reset this alarm.
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