T-SM47j P13/54
T20-a
OPERATION TABLE
3.0 TECHNICAL DESCRIPTION
See Fig. 3.1 and Appendix 3 for schematic diagrams.
3.1 Power supply assembly
This assembly (Part number 111765) which is located in
the table base, has no user serviceable parts and must be
changed as a complete unit. The assembly includes, a
Battery charger loom, a Battery charger module, a Battery
management PCA, a Battery connector loom, a Chassis
plate, two Battery support brackets, a Switch, two Fuses
(32 x 6.35mm 20A ‘T’ type), two Fuse holders, two battery
pairs and all connections, cable ties and screws etc.
3.2 Control electronics
3.2.1 Overview
The control electronics for the T20-a is distributed between
two PCA’s (the ‘4 actuator PCA’ and the ‘3 actuator PCA’).
These two PCA’s contain three distinct microprocessor
daughter PCAs (two on the ‘4 actuator PCA’ and one on
the ‘3 actuator PCA’).
The ‘4 actuator PCA’ (111524) is located on the column
and includes the ‘main controller’ and the ‘4 actuator
controller’. Each of these ‘controllers’ consists of a
‘microprocessor daughter PCA’ (111526) which have their
control electronics on the ‘4 actuator PCA’.
The ‘Three actuator PCA’ (111525) is located in the long
trunk section of the table and contains the ‘3 actuator
controller’, another ‘microprocessor daughter PCA’
(111526) with its control electronics on the ‘Three actuator
PCA’.
3.2.1.1 Microprocessor daughter PCAs
The daughter PCAs consist of a Motorola MC68HC912B32
micro controller, a watchdog circuit, an RS232 line driver
and an SPI UART and RS485 line driver. All the micro
controllers on the T20-a require these components. The
RS232 communications can be used to provide a telemetry
link to custom software running on a PC. Once fitted to
their respective controller boards the micro controllers are
programmed with the appropriate software application.
There is a ‘main controller application’ (110442) and an
‘actuator controller application’ (110443). Both the 4-
actuator and 3-actuator are programmed with the same
‘actuator controller application’ (110443). A configuration
file is then downloaded to each micro controller that
specifies which controllers are present on which board. It
is possible to configure any actuator to any board, however
the configuration of the electronics is specific to each
actuator.
3.2.1.2 The ‘main controller’
The ‘main controller’ provides the control and supervisory
functions for the table. This consists of the interface to the
infrared handset, corded handset (via RS232), footswitch
and standby panel. It also controls the table power LED.
The main controller communicates with the two actuator
controllers (one of which is co-located on the 4 actuator
PCA) by means of an RS485 multi-drop bus. This system
enables multiple slave controllers to be attached at any
time. The main controller issues a health check poll
message to each of the actuators (up to four of which can
be controlled by the single PCA) to ensure that they are
functioning correctly.
When a command is received from any of the user-input
devices the main controller issues the appropriate
movement commands to the required actuator (or actuator
groups in the case of combined movements).
3.2.1.3 The ‘4 actuator PCA’
The ‘4 actuator PCA’ is responsible for movement of four
actuators on or within the table’s column. These are the
‘top and bottom height actuators’ inside the column
assembly, the ‘Trendelenburg actuator’ and the ‘Tilt actuator’
mounted on the column. A number of signals control the
movement of each actuator.
A PWM signal is used to set the speed of the motor; digital
signals control direction (forward/reverse), a digital signal
enables the drive circuit and another digital signal effects
a brake on the motor by shorting the windings together.
The electronics provide a digital fault signal to indicate when
there is a problem with the control circuit, or an over current
situation has occurred. An analogue signal is also provided
on all but the ‘bottom height actuator’. The analogue signal
on the top height actuator is the output from a potentiometer
that indicates the combined position of the height actuators.
Although the column contains two motors, there is only a
single potentiometer to indicate the height of the column.
This analogue signal is fed to the ‘top height actuator’; the’
bottom height actuator’ is effectively slaved to the top.
3.2.1.4 The ‘Three actuator PCA’
The ‘Three actuator PCA’ is responsible for movement of
the three actuators within the tables long and short trunk
sections. These are the ‘left break actuator’, ‘right break
actuator’ and the ‘traverse actuator’. These actuators are
controlled by the same signals as on the ‘4 actuator PCA’.
In terms of feedback, both the ‘left and right break actuators’
have potentiometers and the ‘traverse actuator’ has limit
switches which open when the long trunk reaches either
end of its movement. The ‘left and right break actuators’
are independently controlled to move at a set speed and
to maintain a minimum deviation between them.
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