Hill-Rom P870, Manual

Page: 104 / 231

Electrical Theory of Operation
Chapter 3: Theory of Operation
Resident
TM
LTC Bed Service Manual (man136rc) Page 3 - 7
3
The motor actuator circuit consists of the following:
• The “normally open” contacts of the six relays in the switching circuit (two
relays for each motor actuator)
• The windings of the three motor actuators
• Two limit switches in each motor actuator
When one of the momentary contact switches in the switching circuit is
activated, the circuit of that specific function is completed, and DC voltage
flows to the isolation switch in the caregiver control lockout box. If this
isolation switch is in the locked position, the circuit remains “open” and no
motor actuator operation occurs. If the isolation switch is in the unlocked
position, current flows to the relay coil corresponding to the motor actuator
function activated. Current flow to the relay coil closes the “normally open”
contacts and allows current flow through the relay to the corresponding motor
actuator limit switches and then to the motor windings to operate the motor
actuator. As long as the momentary contact switch is activated, the
corresponding motor actuator continues to operate until its physical travel limit
is reached. When the motor actuator reaches the physical limit of its travel (in
either direction), an integral limit switch corresponding to the travel position is
activated and “opens” the circuit to stop current flow to the motor actuator.
With this integral limit switch “open”, the motor actuator can only operate in
the opposite direction until this limit switch is “closed”, at which time normal
operation in either direction is possible.
When the automatic contour isolation switch (on the control lockout box) is in
the unlocked position, and the relay corresponding to the head up function is
activated, current also flows to the coil of the relay corresponding to the knee
up function. This current flows from the knee up coil through the “up” limit
switch in the automatic contour module to the knee motor actuator. When the
knee section reaches an angle of 15
o
(± 2
o
), the “normally closed” contacts of
the “up” limit switch (in the automatic contour module) “open,” and operation
of the knee section is stopped—the head up function will continue to operate
independently (if activated). When the head down function is activated, current
is also provided to the “normally open” contacts of the “down” limit switch in
the automatic contour module—this “down” limit switch is “closed” and
completes the circuit to allow current to the relay coil corresponding to the
knee down function (lowering the knee section while the head section is being
lowered). When the automatic contour isolation switch on the control lockout
box is placed in the locked position, automatic contour is disabled, allowing
the head and knee sections to operate independently.