In the existing machines the motion
of the loadwheel carriage is controlled by a brush
type D.C. motor via a Dodge 15”1 gear box reduction and a 54T-23T chain-sprocket
drive increase with a resulting reduction of :.39:1 to
a 0.25 inch pitch ball screw (.5” pitch for X100).
The typical DC drive system uses a
4 quadrant SCR drive (Saftronics DF8/DF9 or equivalent)
with a +/- 10 VDC velocity setpoint signal and an enable
relay (LM) from the TUO controller. The DC drive produces an amplified
DC voltage to the brush type DC motor that is proportional
to the velocity setpoint. When the enable relay
is acitivated, the electromechanical brake is released
at the same time the drive is enabled to run the DC motor
at the analog setpoint speed. The full scale DC
motor speed of 1750 RPM moves the carriage at about 1.14
inches per second.
The load routine advances the loadwheel
at full speed until the load is within 100 lbs of the
load setpoint. The
analog velocity signal then is proportional to the difference
between the instantaneous load and the load setpoint. Loading
is completed when the average load value remains within
the load tolerance for 1 revolution. The SCR DC
drive is disabled and a electromechanical brake is engaged
to maintain the final position of the loadwheel.
Retracting is done at full speed in
a variety of methods – retract
to zero load (load < 10 lbs) and then a programmable
time, retract to zero load and then a programmable distance,
or retract to a specified position.
The existing method has several deficiencies:
> The full
speed advance and retract is relatively slow – 1.14
inches per second.
>
When the carriage is stopped a large
velocity command is required in order to overcome
the friction in the carriage.
>
The brake on the motor
is a source of many problems – it
must resist the back drive forces of the loaded tire
when engaged and
must fully release when moving the
carriage.
>
Making a small Jog movement is very
difficult because there is a significant delay to release
the brake and the velocity setpoint
must be large enough
to “start” motion. This is especially
noticeable when trying to make a small load change
with a
Dynamic Calibrator.
>
The potentiometer
settings on the DC drive sometimes need to be changed
for different load setpoints.
>
When the load tolerance
is small, if the load is not achieved on the initial
move an unload/reload cycle is often the only option.
SOLUTIONS
Akron Special Machinery offers a number
of servo solutions for the loading problems. Call today
to find out more
