More speed and accuracy, less scrap, fewer jams — these are just some of the factors leading design and plant engineers to consider servo drives for indexing applications, such as case packers, press, shear, and welder feeders, flying cutoffs, auger fillers, and high-speed diverters.
Where index distances and times change at almost every cycle — for example, where piece lengths change for every “pick” — a servo is worth its cost and programming effort. But a servo may not be the best choice to upgrade a drive that merely requires repeatable indexing to a setpoint.
Many indexing applications have been served well by open-loop drives using a clutch-brake, perhaps with different mechanical devices (arms or cams) to make periodic setup changes. But many of today’s production and quick-changeover demands exceed the capabilities of these drives.
Usually, the first step up in performance for these open-loop drives is closed-loop control of the clutch-brake with a PLC. But when PLC scan time becomes too slow or too variable for high repeatability at high cycle rates, most engineers think “servo.” However, servos offers features that may be unnecessary, and therefore too costly, for repeatable high-speed indexing.
A relatively new combination of an oil-shear clutch-brake equipped with an encoder and a closed-loop controller, Figure 1, solves the need for economical precision positioning system. Applications typically solved with servos can now be controlled with this new control system.
The combination of controller and oilshear clutch-brake has been applied to several high-speed, high-cycle indexing machines, including those that stack shingles and others that cut and seal plastic bags. Here are two examples.
Shingle catcher. In the roofing shingle industry, devices known as “autocatchers” catch and stack finished shingles into bundles for wrapping. The 36-in.- long shingles virtually fly into the catcher at conveyor speeds of 500 to 900 fpm, with an 18-in. gap between them. After catching a group of seven to eight shingles on two counter-rotating blades, called “star wheels,” the wheels must index 90 deg (±2 deg) in 60 to 90 msec to drop the shingles into a bundle-forming chamber, Figure 2.
PLC-based controls were unable to hold positioning accuracy of the blades at the high cycle rates needed. A new closed-loop, position-controlled oil-shear drive, developed in conjunction with Reichel & Drews Company Inc., the leading shingle industry OEM, has met this need in dozens of installations. The new drive indexes the blades at rates from 30 cpm for three-tab shingles to 180 cpm for laminated shingles. Accuracy must be maintained while the machinery warms up to operating temperature from ambient, which may be subfreezing because the equipment is usually located in sheltered, but unheated, areas.
The typical shingle catcher is timingbelt driven. The clutch output shaft usually runs at 933 rpm and is connected to a common shaft for the 4:1 ratio counter-rotating gearboxes that turn the two star wheels in opposite directions. Accel/decel times range from 0.02 to 0.04 sec. The kinetic energy per engagement is about 50 ft-lb, and the clutchbrake dissipates an average of about 0.50 thermal horsepower.
Feeding plastic bags in cut-and-seal machine. Cutting and sealing plastic bags from a web of material requires high-speed feeding with precision registration. Typical production rates are 120 to 180 bags per minute, with required indexing accuracy of 1/8 to 1/4 in.
Cut-and-seal machines use a variety of adjustable cranks, rack-and-pinion drives, and clutch-brakes to index the pinch rollers that feed material. These complex mechanisms are inaccurate and difficult to change for different bag lengths.
An oil-shear clutch-brake and closedloop position control retrofit package has proven to be a simpler, more reliable approach. With a timing belt connecting the clutch-brake to the pinch rollers, the controller gets its home signal from a photoelectric sensor that reads a registration mark on the web.
Drives that were retrofitted with this system typically use a 3/4-hp motor at 1,750 rpm, with a 1.175:1 timing belt ratio. The pinch rolls, operating at 1,489 rpm, index a 21-in. bag in 0.112 sec. Roll inertia is approximately 0.0306 lb-ft2, requiring 35 lb-in. to accelerate the feed mechanism in 0.050 sec. Bag length can be changed from the front panel of the controller.
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Closed-loop controller provides accurate indexing
A recent control development, known as the Closed Loop Position Control (CLPC), addresses both the PLC scan-time issue and the servo cost issue associated with indexing drives. Simpler than a PLC, this controller enables oil-shear clutch-brakes to index with accuracy approaching that of a servo, at moderate rates of 20 to 30 cycles/min (cpm) up to 600 cpm.
The CLPC is a dedicated, closed-loop, errorcompensating clutch-brake controller for precision indexing. An incremental encoder — usually on the clutch-brake output shaft — sends data to the controller indicating how many encoder counts, if any, the clutch-brake under or overshoots absolute position, Figure 1. The controller’s software uses a running average of any error to advance or retard the brake’s trigger point to hit absolute position. Accuracy of ±1 encoder count from absolute position is easily achievable at cycle rates of 600 cpm. This is equivalent to ±3 deg at 1,800 rpm.
Even in drives that operate under very consistent conditions, there are always small changes in dynamic load because of product variations, such as when rolls of material unwind. Friction in a drive can also change substantially during warm-up, again causing a variation in drive loading. The controller constantly corrects for these changes.
The CLPC is 16-bit microcontroller using proprietary software in EPROMs. Scan time is approximately 20 μsec for every scan — virtually eliminating scan time as a variable. The short scan time increases both accuracy and the cycle rate at which high accuracy can be maintained.
This controller can be used with virtually any incremental encoder. It accepts up to 65,535 index counts, allowing multiple shaft revolutions with high resolution. The home position switch can be incorporated into the encoder, or the signal can come from an external device, such as a proximity sensor, light beam, or limit switch.
Several safeguards are built in. For example, a watchdog timer with 16 selectable values detects jams and shuts the drive down if the timer does not sense a specified number of encoder counts during a set period of time. As an independent, dedicated controller, the CLPC interfaces directly to the machine or to a host control.
A “teaching” circuit in the controller lets the operator rezero the drive to its home position and recalibrate after a stoppage. The controller also allows manual advance or retard of the brake trigger point (up to ±25% of the count) via touch-pad and LED-readout. This simplifies fine-tuning of the final position of the driven component.
In addition, the indexing values can be changed for different product batches. Using the touch control, the operator simply keys in the encoder count for the new index, then pushes a “calibrate” button. The controller automatically establishes the correct brake trigger point.
This combination of controller and clutchbrake offers other advantages including:
• Can be used with a standard, inexpensive ac electric motor.
• Inertia of the motor rotor is less of a factor in stopping or starting, as it is with a servo. Low inertia of oil-shear clutch-brakes (up to 3.18 lb-ft2 for a 24,000 lb-in. unit) lets more of the motor torque go into driving the load.
• Unlike some servo applications, the controller does not power the drive, therefore, it can’t overheat if the drive is pushed to its design limits.
• Allows a wide range of torque control by changing actuation pressure. Oil-shear units offer accel/decel times ranging from 20 msec to 1.5 sec.
• Because the heat of engagement is mostly absorbed by transmission fluid, oilshear clutch-brakes exhibit negligible torque changes throughout their lives, or during cold-to-hot phase shift.
Reg Kelley is vice president of Force Control Industries Inc., Fairfield, Ohio.