Planmeca Proline CC, Technical Manual

Page: 12 / 207

Chapter A - GENERAL & TECHNICAL DATA
A-6 Planmeca Proline CC panoramic x-ray
Technical Manual
5 GENERAL FEATURES
5.1 Fault protection
In order to protect the x-ray tube high voltage generator circuitry as well as the tube head and the
x-ray tube itself against various kinds of overload conditions certain actions are taken, explained
in the following.
Amplifier section voltage check
The generator operational amplifiers have operating voltages of about +14V and -14V. If one of
them is below about 12.5V the generator is disabled with corresponding error indication in order
to prevent erroneous operation.
Tube head temperature
The tube head temperature is measured continuously and exposures are not allowed if the tem-
perature rises to 60°C or above. Should the temperature sensor be inoperative (open circuit or
short circuit) the exposures are as well not allowed.
Earthing check
The temperature measurement is done by using the tube head protective earth wire as signal
return to the generator processor board. This will create a less than 2 mA current to the wire and
allows to check that both the tube head and the electronics ground are connected to the protec-
tive earth. If not, a temperature sensor open error will be indicated and generator is disabled.
Feedback check
Before the exposure is started both the kV- and mA-feedback inputs are measured. If the cables
are not connected properly or for some other reason the results are not normal the system does
not allow exposures and an error is indicated.
Focal spot temperature
During each exposure the x-ray tube focal spot is strongly heated up. Therefore a certain time
must be allowed between exposures in order to allow the heat to spread in the tube head. The
cooling characteristics depend on the tube type and therefore the cooling times are calculated
using different formulas based on the tube manufacturer data. Before each exposure the tube
thermal conditions are checked and based on the requested exposure parameters (kV, mA and
time) it is calculated whether an exposure can be started immediately or only after a waiting
period, that is visible to the user.
Pre-heating and filament check
When the kilovolts are raised across the x-ray tube the tube filament must be in such a tempera-
ture which will generate the desired current through the tube. To achieve this the filament must
be pre-heated. During this two second long pre-heat period the filament voltage is adjusted by
the microprocessor to match the mA-value requested. (The actual filament voltages have been
measured by the system at each mA-value during so called ‘filament initializing’ procedure that
has to be run through once when the system is first taken into use. If this is not done, normal
exposures will not be allowed by the system.) At the end of the pre-heat period the filament volt-
age is checked. If it differs more than 0.1 volts from the desired one the exposure is not started
and an error indication is given. The filament power supply has a current limit of about 4 A and it
is fused with a 3.15 A fuse. The two second time is short enough not to create overheating of any
kind or blow the fuse even if the filament was shorted. If the current limit is defective and the
adjustment circuits are inoperative the fuse will blow.