Consider a normal process change for, say, a mixer in a plastics plant. The mixer stirs a molten mass that undergoes chemical change. A viscosity change usually accompanies such a process change. It causes a mixer-motor load change. With a known motor-load characteristic programmed into the load sensor, you could program load-based trip points that trigger various relay outputs. The outputs might signal the main process controller to move on to the next step, or simply indicate statuses to the operator.

Now consider an unwanted change, such as an automatic drilling machine might feel. If a drill bit becomes dull or its cutting edges chip off, drilled holes suffer size and other quality problems. And without correction, the bit may eventually break off in the workpiece. With acceptable work loads programmed into a load sensor with underload and overload detection, you can sense a broken or overly dull bit. Suitable trip points can annunciate variations or initiate actions.

Boosting belt-sander productivity & quality

In a plant that makes long steel tubing for the nuclear industry, belt-sanding the outsides of tubes to remove burrs and other defects is part of the operation. This requires three belt sanders, each progressively removing a specific amount of material. The sanders use 50-hp, 460-Vac motors. Before using power-measuring load sensors, the plant relied on a currentsensing method to control pressure between the tubes and the sander. By using three load sensors, the manufacturer increased belt life by more than 100%. Finished-product quality also improved.

The photo shows a load sensor mounted on a belt-sander motor at the Babcock & Wilcox tubular products plant in Koppel, Pa.

Similar successes have been reported in grinding applications on other products.

Reducing product defects

A lighting-equipment plant successfully applied a powermeasuring load sensor to detect a process change in a highspeed, sub-fhp, cut-off saw, thus reducing product defects. In normal operation, the saw cuts a cardboard-like material. In the event of a malfunction, however, it might attempt a cut at a metallic substance. By monitoring motor power, the sensors help identify the material being cut, and signal the control to continue or stop the saw accordingly.

The photo shows a power-measuring load sensor installed in a control cabinet for the cut-off saw operation at General Electric’s Bucyrus, Ohio plant.

Watching the drill bits

In a valve manufacturing plant in Ohio, automated drill presses drill and tap holes in steel blocks to make valves. The automatic toolchanger in each machine places six drill bits of 3/16 to ½- in. diameter into the drill head. A power-measuring load sensor that can interface directly with a PLC detects dull and broken bits in this set-up.

A PLC outputs a 4-bit binary code indicating which drill bit is in use. The sensor applies corresponding overload- and-underload limits for that particular tool, to monitor its condition. When the drill is in Idle state, the PLC puts the sensor “to sleep” using the sensor’s remote sleep feature, so it won’t trip on underload.

The illustrations show data taken from a drill press via the sensor’s RS232 interface. The first graph is a load profile with a new ¼-in. drill bit. The second shows a load profile on the same machine after 200 drilling operations. Overload detection sensed the dull bit. Had the bit broken, underload detection would have sensed the absence of load during the cutting cycle.

No-jam screw conveyors

In one application, several power-measuring load sensors have been installed to detect jams in fiber-carrying screw conveyors. The plant had tried thermal overloads, current-sensitive relays, and zerospeed switches. However, they did not have enough sensitivity or they produced nuisance trips due to background drift. So far, power-measuring load sensors programmed in Manual mode have detected eight jam-ups. If undetected, they would have caused mechanical breakdown. Moreover, underload detection helped identify situations where the feeder failed to supply fiber to the conveyor.

Stopping rake damage in settling ponds

In water pollution control, it is important to drive the settling-pond rake without damaging the blades or driveshaft as debris piles up. Mechanical torque limiters on existing equipment would stop the rake in a jam-up, but not fast enough to avoid blade bending. Many facilities now use a power-measuring load sensor to monitor rake-drive motor load. If it senses a jam, that is, excessive power, it shuts off the drive motor and energizes a rake lift motor. To account for a freewheeling drive, underload detection is also available.

Guarding cam-operated pick-&-place robots & indexers

In cam-operated equipment such as indexers and pick-and-place units, the load variation is cyclic. When programmed, a power-measuring load sensor learns the normal load profile of these machines and protects against jams and overloads.

Historically, industry has relied on mechanical overload devices to protect equipment. However, load sensors have no problems related to spring tension, wear, rust, or fretting corrosion. And they are not subject to differing optimum spring requirements at various times in operation, such as starting torque vs. jam torque.

The illustrations show data taken from a pick-and-place robot via a power-measuring load sensor’s RS232 interface. The pick-andplace system has one cam for up-and-down motion; another, for rotary motion. After the inrush peak, alternate bumps look the same, reflecting the dual-cam action. The second graph shows the effect of a light touch on the robot arm. The sensor detected a sharp increase in electric power draw within 16 msec and signaled the drive to apply dynamic braking, bringing the pick-and-place unit to a nearly immediate stop.

How a power-measuring load sensor works

The power-measuring load sensor described in this story measures both voltage and current across motor phases, where power factor changes with load. In ac induction machines the difference between measuring power and measuring current can be substantial. For example, in an application involving a 50-hp induction motor driving a belt-sander (see “Boosting Belt Sander Productivity & Quality”), the power-measuring sensor showed a 600% power increase from idle to full load on the machine. An ammeter measuring the same application showed only a 25% increase!

The reason: The motor’s electric power draw is almost directly proportional to the mechanical load imposed on the motor. Current, however, is much less sensitive to load change. Motor electric power keeps up with the mechanical load imposed on the motor mostly because the power factor changes significantly with load. For a typical induction motor, the power factor may go from a poor 15% at idle to an optimum of around 90% at full load — a 6:1 variation.

The graph shows typical variations of power and current vs. load. A percentage change in load shows up as the same percentage change in power, but the percentage change in current is only a fraction of the percentage change in load. A load sensor’s ability to monitor power instead of merely current allows load-based process monitoring.

The load sensor discussed here works equally well with SCR or chopper drives, ac machines driven with inverter drives, and servomotors driven by ac or dc servo drives. Here, the sensor monitors input ac current to the drive. Contrary to popular misconception, drives with capacitorfiltered input and drives with dc rectification do respond to load changes within tens of milliseconds.

The sensor can also work with Halleffect sensors. Where several servomotors connect to one drive, the sensor can monitor output current to each motor using a Hall-effect current sensor.

You can program trip parameters into a sensor in two ways: Manual and Auto program modes. In Manual you specify:
• Inrush delay time, during which starting inrush power is ignored.
• Trip delay time, during which either an overload or underload must persist before the sensor gives a relay output.
• Lower (prewarn) overload limit.
• Higher (maximum) overload limit.
• Underload limit — if power or current falls and stays below it longer than the trip-delay time, the sensor trips and stays tripped until reset.

In Auto, the sensor learns important load characteristics automatically in one learning session, by following and “memorizing" a routine motor duty cycle. You need not adjust anything to arrive at ideal trip points; the sensor does it automatically. The only programming you must do after the sensor learns the load profile is to input the “threshold” level, which specifies the percent tolerance above learned load parameters.

The sensor described herein includes slope detection: It learns the maximum power slope during the learning session and trips if the power slope during normal running exceeds the learned value.

Besides the basic programmable load sensor already described, there are many application-specific types. For example, one type can interface directly to digital output ports of a PLC to monitor and control up to 16 processes. The PLC gives a 4- bit binary number indicating which process is active. The sensor decodes the number and applies overload and underload limits for that specific process.

Another sensor type lets you monitor, with just one sensor, several motors connected to one machine.

Still another application-specific type, this one primarily for pumps, compressors, and blowers, uses an algorithm to detect surges which might lead to bearing or other mechanical failure. You program the maximum allowable number of surges in a time interval you select.

The programmable load sensor described in this article is, in most cases, the MLS Machine Load Sensor by ASPAC Inc., Ann Arbor, Mich..

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