MSUB Board Theory

The trigger circuit is responsible for generating the DAS trigger rate that the Data Acquisition System (DAS) uses as a reference for sampling x-rays. The trigger circuit must function in 5 modes: Offset, Axial, Helical, Scout, and Static. With the exception of offsets, these trigger modes correspond with their respective scan types. Offset triggers are issued before and after all scans to collect DAS/detector offset data for offset correction. Offset data is taken without x-ray exposure.

Axial and helical scans are taken as the gantry rotates. This requires that the axial and helical trigger signal be based off of an axial encoder. Scout and static scans use a fixed 3 MHz time base for their reference.

Axial and helical scan modes must be capable of using a variable encoder frequency input that varies with the prescribed scan speed (0.35, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1,2, or 4 sec/rev gantry period). The MSUB provides the TGP three synchronization signals for exposure command start: the gantry, the table, and firmware. The TGP will use these signals to produce the exposure command signal to the LSCOM section of the TGP

DAS triggers are quadrature decoded pulses from the axial encoder which are passed through a phase locked loop (PLL; see below) frequency scalar and then divided directly down to the desired trigger frequency by a divide by N counter. The signal is then passed through a 64 bit digital filter before being sent to the DAS as triggers.

The phase locked loop circuit is used to divide and multiply the gantry encoder pulse rate to generate an integer output trigger rate @ 984 Hz for axial scanning. The phase locked loop is necessary because the DAS geometry (n*984 samples per gantry revolution) is not coincident with the encoder resolution (2048*13*4).

Signals from the axial encoder and azimuth detection board measure the gantry position. The gantry position is used to monitor axial drive positioning, velocity performance and to determine starting x-ray and view angles.

Gantry Position is measured with an incremental encoder mounted on the gantry's motor drive shaft. The motor and encoder turn 13 times for every one gantry revolution. The encoder has two channels, A and B, in quadrature with a resolution of 2,048 pulses per channel per encoder revolution. There are (2,048 X 13) = 26,624 Ch. A and Ch. B encoder pulses per gantry revolution. Channel A and B taken in quadrature provide 106,496 pulses per gantry revolution resolution. Encoder accuracy of 0.0135 degrees per channel pulse, or 0.00338 degrees per quadrature pulse.

The home position is the gantry position with the x-ray tube positioned at 12 o'clock. This position is sensed using an optical home flag and home flag sensor board. The home flag is a 1 inch wide piece of metal attached to the rotating bearing. The MSUB board supplies +5v to the home flag board. The home flag is used on the MSUB board in conjunction with the axial encoder “C” pulse to determine gantry home position for firmware.

The status of the axial drive enable switch can be read back by the TGP via registers on the MSUB. While the axial drive switch is in the disabled position, the axial drive enable output is forced inactive and the axial holding brake output is forced to release the brake.

The axial holding brake, which is normally applied, is released by applying 120VAC to the brake leads. The 120 VAC is controlled by a solid state relay that in turn is controlled by the MSUB. Note that the MSUB axial brake output is forced to release the brake when the axial drive enable switch is in the disabled position.

The axial brake is meant as a static brake to hold the gantry still once it has been positioned by the axial drive. The brake's friction is not sufficient to hold the gantry still against the full accelerating force of the motor and amplifier. Should the brake fail while the gantry is in motion, the gantry will continue to rotate until halted by firmware. The brake is also not sufficient to hold the gantry while the tube or inverters are being changed. Whenever servicing the rotating base, the gantry should be locked out using the locking pin mechanism.

The axial brake is released when the Gantry Loop Contactor Service Switch is in the “disable” position. This allows the gantry to be rotated by hand without fighting the friction of the brake.

The axial motor drive is powered by two sources: 480VAC, 3 phase, provided from the PDU, and controlled by the axial power loop contactor. The second source of power is 380VDC, transformed from 120VAC, which is provided via the drive’s DC bus by the axial Dynamic Brake assembly. The MSUB receives a isolated 12Vdc signal from the drive that is monitored and indicates to firmware through the FPGA, that either of the two mentioned power sources is available or both are not present.

The 480VAC axial power contactor status will have a maskable interrupt signal that can be read back on the MSUB when a change of state occurs.

The axial drive is commanded by the firmware via CAN network. There are seven signals that are used on the CAN network: three control signals (Start, Stop and Enable) and four feedback signals (Fault, At Speed, At Freq. and At Pos.).

An isolated power and ground bus on the MSUB is powered by a 12V supply from the axial drive. The axial CAN and control signals to the axial drive are optically isolated from rest of the MSUB and use the +5V isolated power for the output stages of the opto-isolators. A detection circuit with read back via TGP processor is provided to determine if the isolated 12V power is being supplied to the MSUB.

An 82527 CAN controller interface IC, accessible via the TGP processor is used to provide CAN communications to the axial drive. The interrupt signal, “AX_DR_COMM_INT*”, for the 82527 is sent to the TGP.

To activate the control signals to the axial drive, the appropriate terminals on the axial drive need to be shorted together. Since the signals are polarized, opto-couplers with transistor output stages are used to short the terminals together.

Note that the axial drive enable signal is forced inactive by any one of these signals: an overspeed condition, the watchdog circuit being inactive, a disabled axial drive enable switch, an open loop contactor or an axial drive fault.

The feedback signals provided by the axial drive are contact closures which must be detected. The input of a Schmidt Trigger is tied by resistor to either power or ground such that the output is inactive. When the contact is closed the input to the Schmidt Trigger is pulled such that the output is active.

A +24Vdc input signal from the PDU that indicates the customer room door interlock is open. This signal dynamically tracks the status of the interlock circuit on the TGP.

This signal is used to sense the Exposure Enable Loop functionality. This sense circuit is located on the MSUB between the Exp En Loop firmware controlled solid state relay and the watchdog timeout solid state relay on the TGP.

A phase under voltage / open fuse detection circuit on the PDU Control board monitors the status of the three fuses described above. If the voltage w.r.t. ground on any one or more phases drops below 185Vac (~70% of nominal) the PDU Control board detects and latches a PH_LOSS_UV condition and issues a “PHASE_LOSS” fault signal to the system. If contactors Kxg and/or Kss (described above) are closed, the control board opens them.

A Bus Fault detection circuit on the PDU Control board monitors the status of the HVDC. If the differential bus voltage does not exceed 200Vdc within the first 200mS after turn-on the PDU Control board detects and latches a BS_FLT condition and issues a “BUSS_FLT” fault signal to the system. If contactors Kxg and/or Kss (described above) are closed, the control board opens them.

This signal is sent from the PDU when there is a phase loss detected in PDU. The signal is latched in FPGA and an interrupt is sent to the microprocessor on TGP.

Steady-state +24Vdc input signal from the gantry to indicate the HVDC Bus is enabled. Used by service to bypass firmware control and allow manual control of the HVDC Bus within the NGPDU.

This is a steady-state +24Vdc input signal from the gantry to close the “X-Ray ON” Light relay. The x-ray interlock is passed through a relay on the MSUB. To close the relay, the watchdog circuit must be active and the relay must be commanded to close via TGP processor.

The MSUB board provides a set of 24v/40ma contacts to control a 24v relay in the PDU. The 24v PDU relay closes a set of 110VAC contacts that drive the hospital X-ray On Light. The 24v relay contacts are closed (x-ray light turned on) whenever the TGP board commands x-rays to the ORP. A register written to by firmware commands the x-ray on light’s relay. The bits of the register are cleared in the event of a power failure, CPU fault, or watchdog / board fault condition.

Steady-state +24Vdc input signal from the gantry to close the “System ON” Light relay. NGPDU provides isolated terminals for customer Room Light connections.

Steady-state +24Vdc input signal from the gantry to close the “Generator Ready” Light relay. NGPDU provides isolated terminals for customer Room Light connections.

An 82527 CAN controller interface IC, accessible via the TGP processor is used to provide CAN communications to the Table. The interrupt for the 82527 is sent to the TGP. The table drive is commanded by the firmware via CAN network. A filtered 12V power and ground bus on the MSUB are necessary for the Table CAN communication. The Table CAN is optically isolated from rest of the MSUB and uses the 12V filtered power for the output stages of the opto-isolators.

This signal is for communication from the MTCB to the MSUB quickly with a differential signal that is received on the MSUB which creates a maskable interrupt to the TGP processor.

The MSUB Table Sync signal is provided to allow the MSUB to send a signal to the MTCB quickly and asynchronously. The control of this output is available via registers on the MSUB.

This is a differential signal that is sent to the MTCB from the MSUB, which is used to provide for an electrical hard line from the Gantry Control Push Buttons, Remote Tilt Buttons, Smart View foot switches, and Service Tilt switches. This differential signal must be met in order for motion to be available in the table.

This is a differential signal that is used to indicate to the MTCB that E-Stop has been pressed and within 1.5 seconds, the 120VAC to the table will be turned off. This provides the MTCB enough time to stop the cradle and prepare for the loss of power.

This is a differential signal generated by the MTCB that indicates that there is a fault condition on the MTCB. This signal is activated by firmware only.

The MSUB will interface with the MTCB and function as a jumper from the console to the table assembly for the Console to Table Intercom System. These two signals are marked as speaker + and speaker -.

The table foot switches are used to raise and lower the table. There are two foot switches on either side of the table. One for raising the table and one for lowering the table. The switches have an inverse logic dual path input. A read back and maskable change of state interrupt of each signal is available on the TGP processor. When the up pedal is active the up signal will be high into the processor. When the down pedal is active the down signal will be high into the processor.

The MSUB can measure gantry balance using 2 separate sensing devices and indicates a difference in voltage due to vibrations it senses. These voltages are amplified on the MSUB and then sent to the A/D component on the TGP.

The MSUB has two remote thermal sensors connections. The input signal from the thermal sensor is sent to the TGP where it digitizes through an analog to digital converter. The digital signal will then be read by firmware every 800 ms.

(This input is considered a spare sensor input at this time. The input signal will be amplified on the MSUB and then sent to the A/D component on the TGP.)

Tilt position feedback is provided by a 5-turn potentiometer.

Sensors are mounted on the front and rear of the gantry. When the sensors are not pressed the impedance is 20 kOhms. When the sensor is pressed, the resistance becomes near 0 Ohms. This signal is decoded on the board, first to indicate that the maximum impedance of the sensor is 40 kOhms (fault condition). Second that the sensor is not a short circuit (interference condition). This is done for both the front and rear sensors. If there is interference or a fault condition both tilt and table elevation will be disabled.

The MSUB board intercepts the signals that currently go to the tilt elevation amp from the MSUB board. The signals required for generating tilt (forward and backward) are decoded and sent to the tilt relay board. Also the signal enabling elevation is also intercepted and another enable is created so that the MSUB board can disable elevation if the interference sensor is in fault or interference is detected.

The remote tilt switch is located at the console SCIM keyboard. The function is Prescribed Remote Tilt, which means that these switches will only tilt the gantry to the prescribed RX position. This feature enhances the productivity of the technologist as they can more easily position the gantry between groups or series prescriptions. For patient safety, the Gantry Mounted Interference Touch Panels will disable gantry motion if any contact is sensed on the surface of these panels.

The Prescribed Tilt board (2269601) detects the status of the push button, either pressed or released, on the SCIM keyboard by the use of two independent paths. The MSUB receives one differential hardwire signal line and one serial differential RS232 line to initiate remote tilt. When both signal lines are active, a signal is sent to the TGP microprocessor. The MSUB sends the remote tilt indication to the TGP board via a register in the MSUB FPGA. The microprocessor must check for a stuck button condition. There is also a line that indicates fault status of the prescribed tilt input to the board.

Tape switches have been added to the tilting base of the gantry to detect if cables on the rotating base are contacting the tilting base. The tape switches provide an interrupt to the TGP when the switches are actuated.

Two signals are provided for use with defined cardiac features: Exposure Command and R-Pulse. R-Pulse is a signal received from the Gantry Options board, indicating when the heart is in a particular part of the cardiac cycle. The status of the R-Pulse signal and a maskable positive edge sensitive interrupt can be read back by the TGP via registers on the MSUB.

Two signals are provided for use with defined pulmonary features: Resp_mon and Resp-Pulse. Resp-Pulse is a signal that is received from the Gantry Option board, indicating when the lungs are in a particular part of the volumetric capacity. Resp-mon is a signal that is sent back to the monitor to indicate when exposure is on. These signals are optically isolated from the pulmonary monitor by the circuitry on the Gantry Options Board. Firmware can read the status of the latched Resp-Pulse interrupt input signal. Firmware has the ability to mask and clear the interrupt condition that can be read back by the TGP via registers on the MSUB.