Control Systems Application Guide
We’ve encountered a number of applications in which customers want to transport or handle a material web while making minimal contact with it. These webs still must pass through various processes and operations that require zone tension control. Where these processes might traditionally be separated by a load cell or dancer controlled loop of material, we have to use alternate methods for non-contact sensing. One popular non-contact sensing method utilizes ultrasonic distance measurement.
1.) Ultrasonic Loop Control
There are a number of “conditions of use” that affect the success of ultrasonic sensing.
A.) TARGET DENSITY:
Since ultrasonics work much like a bat in transmitting an ultrasonic sound and waiting for a reflected echo, the density of the target material affects how much sound signal is reflected. Traditional sound dampening or suppressing materials work by dispersing the echo instead of reflecting it back directly to the source. Open cell foam is a good example of this. Some very sheer fabrics and materials like cotton batting lack the density to disperse or reflect much echo – the sound passes through them.
B.) TARGET DISTANCE:
The amplitude of a sound echo attenuates greatly with distance. A target at a near distance is more likely to be detected than the same target further away – especially with low density materials such as mentioned above. When you have a choice, mount the sensor as close to the target as practical while maintaining any required minimum distance.
C.) TARGET SIZE, SHAPE & ORIENTATION
Ultrasonic distance measurement depends on processing the round trip travel time of a transmitted sound and its echo return from the target. Since the transmitted sound is emitted in a direction perpendicular from the face of the transducer, some portion of the target must be perpendicular to the emitted sound (or parallel to the transducer face) for the echo to return to the transducer.
In actuality, the transmitted signal is emitted in a cone shape with about 12 degrees beam spread. Greater distance from the transducer then means a larger diameter “detection cone”. At 5 feet, the cone diameter is between 12 and 18 inches diameter. At 10 feet the diameter is between 24 and 36 inches. This means a target doesn’t have to be directly in front of the transducer to be detected but, this also means that undesirable targets at the cone perimeter may be detected. An ideal target shape would be a parabolic or dish shape which would focus the echo back at the transducer.
When sensing a loop of web material, several things should be considered:
1. The sensor should be positioned directly above the “valley’ or lowest and flattest part of the loop to assure the reflection of an echo back to the sensor.
2. When viewing across the width of a loop or the length of a roll, the surface should lie flat – again to create a perpendicular surface to assure the reflection of an echo back to the sensor.
3. Use care in positioning the sensor if one side of the loop is supported by a “center driven” roll. As the diameter of this roll changes, this side and the valley position of the loop will shift. The sensor can usually be pointed toward the center of the movement range to provide satisfactory operation.
D.) OTHER SOUND SOURCES
Other sources of ultrasonic sound can interfere with ultrasonic sensors. Some other possible sources of ultrasonic sound are compressed air leaks, high speed spindles, metal halide light fixtures and other ultrasonic measuring units. Avoid pointing the sensor directly at any suspected ultrasonic sound sources. Carotron’s ultrasonic sensors can be interconnected so that they will take turns in making measurements to alleviate interaction.
In one application, we’re unwinding rolls of steel sheeting and feeding into a forming and stamping process. The process is a slow one where a length of material is fed into a press and stopped while the cutting and forming actions take place. Then the next length is pulled in to repeat the process. Refer to Figure J.2.
The low speed of this application allows simple and direct control of a material loop by an ultrasonic sensor. The ultrasonic sensor is set up to provide a + DC reference signal to a regenerative drive whose output direction is controlled by the reference polarity. The regenerative capability is also required to handle the high inertia of the material roll.
The loop sensor outputs zero volts at the desired loop null or parking position. When the nip rolls ahead of the stamping section starts pulling material, the loop is lifted. The sensor will output a positive reference proportional to the loop distance – a higher loop closer to the sensor produces higher output. This causes the unwinder to motor forward – paying out material until the loop drops to the null position. If the loop were to drop below the null position, the sensor will produce a proportionally increasing negative signal that will tell the unwinder to run reverse until loop null position is again reached.
2.) Optical or Proximity Detection Loop Control
Other non-contact sensors are available for materials where ultrasonic detection is not appropriate. The following control scheme utilizes sensors that simply detect the presence or absence of material in a loop control application. Capacitive proximity sensors or photo-electric sensors – whatever is appropriate for the web material can be used. The control is operating a drive on the input of the loop – increased speed will lower the loop and decreased speed will raise it.
A Carotron MOP (electronic Motor Operated Pot) function is controlled by loop “upper” and “lower” position sensors. A contact closure from each sensor is used to operate the “increase” or “decrease” inputs of the MOP. The upper sensor contact closes when not sensing the loop material. This operates the “increase” input which increases motor speed until the loop again falls in front of the sensor. Presence of the loop in front of the lower sensor operates the “decrease” input to slow the drive and raise the loop. When either contact is opened, the output voltage freezes at its present level. The rate of output signal change is controlled by acceleration and deceleration adjustments.
The ability of the electronic MOP circuit to source its output from an external signal, such as the line speed reference, enables the trim function to be speed proportional. The acceleration adjustment in the circuit allows the rate of the correction to be optimized for the loop size and desired response.