Rockwell Automation 1398-PDM-xxx IQ Master Version 3.2.4 for IA-2000 and IQ-5000 Positioning Drive Modules, IQ-55 - Manual (Page 383)

Application Examples • SmartBelt Application 361
Publication 1398-PM601A-EN-P — October 2000
APPENDIXES
• Microprocessor control provides maximum accuracy and speed
Design Considerations
Sensor Placement & Wiring
The sensor to detect products should be placed so products are detected only after the majority of prod-
uct weight is on the correction belt so that products will not slip. The sensor to detect flights should be
placed so the flight that is detected is the same flight that the current product will be positioned to. This
minimizes effects of mechanical differences in flight spacing. The sensors are wired to the interrupt
inputs on the ULTRA Plus or IQ for maximum accuracy and response. The example program which fol-
lows uses the flight detect sensor wired to Interrupt 1; the product sensor is wired to Interrupt 2. The
flight sensor may be mechanically adjustable to eliminate software offset adjustment. The interrupt
inputs are edge triggered. Sensors with sinking outputs (NPN) should be used. The interrupt occurs
when the input transitions from off to on. Response times of the interrupt inputs are 35 microseconds
±
15 microseconds, which determines maximum position accuracy.
Mechanics
The choice of the servo motor and its connection to the belt must be chosen carefully to minimize
reflected inertia to the motor. While gear reduction may be used to reduce reflected inertia, make sure
motor top speed allows maximum correction belt speed. The system should be tuned to provide maxi-
mum response and stability for all ranges of product weights.
Correction Belt Length
The length of the correction belt must be chosen so that worst case correction moves are completed
before the product leaves the correction belt. This depends on the acceleration rate used for correction
moves and the maximum speed attainable at top machine speed. Make sure that a second product does
not enter the correction belt before the first product leaves (based on nominal product spacing). This
would cause an inaccurate correction. These two criteria determine the minimum and maximum correc-
tion belt lengths. If a belt length cannot be chosen to satisfy both criteria, an alternate approach is to use
two correction belts sequentially. The first belt would be limited to correcting only one half of the worst
case correction distance, and the second belt would complete the correction. This approach requires a
second smart belt servo system and a product sensor at the entrance of the second belt. Both systems
may monitor the same flight sensor.
Product Slippage
The acceleration rate used for correction moves must be chosen to be small enough so product does not
slip on the surface of the belt but large enough so worst case corrections can be completed over the belt
length. If friction between product and belt is so low that worst case corrections cannot be completed
with out slippage, an alternate approach is to “sandwich” the products between two belts. The two belts
are mechanically coupled together either above and below the product or side to side. This minimizes
product slippage and allows higher acceleration rates.
Performance/Accuracy
Typically, the maximum throughput is limited by product spacing, which limits the correction belt
length. If product spacing is small, the corrections need to be made in the positive direction only to pre-
vent product collisions caused by correcting one product negative and the next product positive, target-
ing the same position. One method of increasing effective correction belt length is to use two correction
belts working in conjunction with each other rather than independently. This approach requires both
belts to look at product position at the front of the first belt. Both axes calculate a gear ratio which over
the length of both belts would properly correct product position. The first belt immediately changes to
this gear ratio, while the second belt waits until the product reaches the second belt before changing
gear ratios. This allows one product to be on each belt at any time, yet doubles the effective correction
belt length.
The positions captured with the flight and product sensors can be motor position or master encoder
position. Whichever position has the higher resolution should determine which one to use. A software
offset variable can be used to fine tune the target position of products.