1.3 Multistretch control description
The multistretch drive is a solid state DC motor control especially designed for use in
constant tension mode on stretchwrapping machines. The unit functions as a pay-off
drive, unwinding the pre-stretched film as the turntable and pallet rotate, and
continuously regulating tension as the diameter of the wrapped load changes. Since the
typical pallet load is a square cross section, the effective diameter changes abruptly with
rotation; the film tension is monitored by a tension-arm, held in place by spring pressure.
As the film tension changes, the arm moves, rotating a cam, and the cam orientation is
sense by a hall effect transducer. The transducer is powered at 24VDC by a regulated
supply on the MCIA. The transducer signal varies with the separation between its
sensing head and the cam surface – the output range is from approximately 0.5 to 3.5
volts DC. The amplified transducer signal controls motor torque, increasing or
decreasing it so as to restore the tension arm to its previous position, and to maintain
tension at its preset level. The system response is tailored so that these small
corrections are smooth, continuous and largely imperceptible. The tension defaults to
minimum when the external tension adjustment circuit is opened.
ADJUSTMENTS:
The operator, using the digital interface can set the film tension. In addition to the
above-mentioned digital tension adjustment, there are three trimming potentiometers.
The trimmers are accessible through the faceplate of the plug-in control, (See Fig. 3),
functions as follows:
ZERO (RV1)
This control injects an offset voltage, which adds or subtracts from the tension voltage
reference; this will allow the extremes of adjustment from the digital interface to be set to
levels consistent with appropriate operation. Typically, the “Zero” will be used to center
the operating range in the linear portion of its characteristics. This adjustment is normally
made at the factory and should not require field adjustment. The machine should be
stopped and the film removed for this adjustment.
SPAN (RV2)
This controls the system loop gain, and may be adjusted if the motor continues to turn
when the dancer arm is unloaded. The potentiometer should be adjusted to ensure that
the motor is de-energized when arm is unloaded.
CURRENT LIMIT
To protect the unit against damage, if the motor should stall, jam, or current demands
exceed the rating, a current-limiting circuit is included which keeps motor current at a
safe level regardless of motor load, or input from the tension arm. The current limit is set
at the factory and should not require field adjustment.
CONNECTION
The motor armature attaches to terminals (+) (-) are accessible through the faceplate of
the plug-in module. The unit is suitable for permanent magnet shunt style DC motors
with 90 V armature rating, sized at up to 3/4HP.
Page 3
To Next Page
Loading...
