CPI CMP 200 Series Service Manual page 292

CPI Canada Inc.
7.2.12 AEC (MD-0936)
Refer to MD-0936, sheet 1:
Sheet 1 shows the circuits on the generator control board that provide the interface between the
generator CPU and the AEC board. Eight lines supply the AEC chamber select signals, the start signal,
and the field select signals to the AEC board via U90 and driver U91. The AEC ramp (PT RAMP) from the
AEC board is voltage-amplified by a factor of approximately 20 by U66B and compared to the AEC
reference voltage by U73. The output of U73 is normally high, switching low when the magnitude of the
AEC ramp is approximately 5% of the AEC reference voltage. This step change at the output of U73 is
detected by the CPU via U71, and is used by the CPU to generate an error message if the trajectory of
the AEC ramp is such that the AEC exposure will exceed the AEC backup time.
The AEC reference voltage is generated by D/A converter U44, buffered by U38B, and fed to comparator
U73 and to the AEC board via J7-10. The magnitude of the AEC reference voltage is determined by the
CPU (based on the upcoming exposure), and will be a value between 0 and 10 volts.
The PT stop signal is generated by the AEC board when the magnitude of the AEC ramp is equal to the
AEC reference voltage, and indicates that the desired AEC exposure should be terminated. The PT stop
signal is routed via U71 to the CPU, where it will terminate the AEC exposure immediately when this
signal is received.
Refer to MD-0936, sheet 2:
This is the functional schematic of AEC assembly 734614, used with various ionization type AEC
chambers. The AEC chambers are connected to J1/J11, J2/J12, J3/J13, and / or J4/J14. The AEC board
will either be fitted with 12 pin in-line connectors, or 15 pin 'D' connectors, depending on the application.
The AEC signal from the AEC chamber(s) is routed to the input of analog switches S1A to S1D on the
AEC board. These analog switches are controlled by the chamber-select signals.
The chamber-select signals originate on the generator control board as per sheet 1. Only one chamber
may be active at one time, and DS1 to DS4 on the AEC board indicate which channel is active. The
chamber-select signals are inverted by U3B, U3C, U3D, and U3F; and connected to the control inputs of
the analog switches described in the previous paragraph. The analog switch corresponding to the
selected AEC input channel will be closed when that channel is selected, thus connecting the AEC signal
to preamplifier U1A, which provides voltage gain. The input of U1A will be configured such that U1A is a
non-inverting amplifier for use with AEC chambers that have a positive going output, and as an inverting
amplifier for use with chambers with a negative voltage output.
The start signal also originates on the generator control board. This signal, when active, will light DS5 on
the AEC board. The start signal is buffered and inverted by U3E and U3A, and will be logic low at the
output of U3A when the start signal is active. This opens the analog switch that is part of the sample and
hold circuit at the input of U2A during an AEC exposure. This analog switch is closed at all other times.
The sample and hold circuit will sample any electrical noise at the output of U1A during standby
operation, and subtract this noise from the AEC signal during an AEC exposure. This ensures that the
output of U2A is proportional to the AEC chamber output voltage only, and is not influenced by noise.
The output of U2A connects to the common inputs of analog switches S2A to S2D. These analog
switches are controlled by the chamber-select signal, and the switch corresponding to the active channel
will be closed. This connects the output of U2A to the AEC gain adjustment potentiometer R1 to R4
corresponding to the active channel. The AEC signal, which will be either a ramp or a DC voltage
depending on AEC chamber type, is now fed to the input of U2B. This will be factory configured as an
integrating amplifier by connecting C4 into the circuit for use with AEC chambers that output a DC output
voltage, or U2B will be configured as a linear amplifier by connecting R32 into the circuit for use with AEC
chambers that provide a ramp voltage. The start signal also connects to analog switch S4. This switch
opens when an AEC exposure starts, allowing U2B to start integrating or amplifying the AEC signal.
Use and disclosure is subject to the restrictions on the title page of this CPI document.
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CMP 200 X-Ray Generator Service Manual Ch # 901476-09 Rev. C
7
Theory of Operation
Page 7-11