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The present invention relates to systems for the real-time control of hot glue dispensing equipment and in particular to a closed-loop control system employing a thermal-infrared sensor for detecting variation in glue times.
Hot glue dispensed from a hot glue gun may be used for the rapid automated assembly of products, for example, cardboard boxes, the latter which have joints held together with hot glue. Unlike conventional adhesives, hot glue provides fast setting times without the need for dangerous solvents or the mixing of multiple part formulations. The glue, when heated, may be dispensed in a tacky state under pressure through a nozzle. When the glue cools, a strong bond is created.
Precise timing of the dispensing of hot glue is extremely important in a high-speed assembly line. Delays or advances in the dispensing time, as products move by the glue gun, can leave beads or strings of glue extending from the seams or create seams that are improperly or incompletely glued and hence sealed. The dispensing of hot glue at times when the product is not properly aligned with the glue gun can dispense glue on the conveyor system creating costly downtime for high-speed assembly machines.
Repeatable and precise and high-speed operation of the valving mechanism of a hot glue gun is difficult. The speed at which the valve opens and closes is highly dependent on variations in the glue batch and temperature and can shift as the valve operates. The intrinsic variations in the response time of the valve place significant limits on the throughput of assembly machines using hot glue dispensers.
One solution to variations in hot glue gun response times is to observe the dispensed glue beads and from this observation, correct the trigger signal provided to the glue gun valve to compensate for any timing errors. Unfortunately imaging of the glue beads at high speed is difficult because the glue is transparent or light in color and often dispensed on a light surface, for example, light paper stock. One solution is to dye the glue, for example, with fluorescent dye normally invisible to the consumer. This approach is not always practical for reasons of consumer acceptance or expense.
It is also known to image hot glue beads using thermal-infrared sensors that can distinguish hot glue from the substrate on the basis of temperature. Practical thermal-infrared imagers are either relatively sluggish in performance, noisy, or require expensive and unreliable cryogenic cooling, and thus have not been used for real-time, closed-loop control but only for quality assessment purposes where the errors are analyzed off line and used to schedule maintenance for adjustment of the glue gun.
The present invention provides closed-loop control of a glue gun using thermal-infrared sensing. Key to this breakthrough is the development of a practical high-speed thermal-infrared sensor providing improved signal to noise ratio and reduced threshold drift.
Specifically, the present invention provides a system including a hot glue gun receiving a trigger signal at a trigger time to actuate a dispensing of glue on a substrate at a dispensing time. A thermal-infrared sensor views a pattern of dispensed glue on the substrate by detecting a temperature difference between the substrate and the glue to produce a detection signal and a comparison circuit receives the detection signal to detect an error caused by variations between the trigger time and a dispensing time. A modification circuit modifies the trigger signal based on this error to reduce the detected error.
Thus it is one feature of at least one embodiment of the invention to provide for real-time correction of the operation of a hot glue gun at commercially practical assembly line speeds.
The system may include a transport mechanism moving the substrate with respect to the hot glue dispenser and the thermal-infrared sensor, the transport mechanism providing a displacement output, and the comparison circuit may receive the displacement output and detect error by comparing the displacement output at a time of the detection signal to a known displacement output at the trigger time modified by an offset in displacement between the hot glue dispenser and the thermal-infrared sensor.
Alternatively, the comparison circuit may detect error by comparing a time of the detection signal to the trigger time modified by an offset in time between alignment of the substrate with the hot glue gun and the thermal-infrared sensor.
Thus it is an feature of at least one embodiment of the invention to permit displacement of the thermal-infrared sensor from the glue gun for practical manufacturing, using either an encoder on a conveyor belt or the like, or knowledge of the time delay between dispensing and detection.
The thermal-infrared sensor may be a photoconductive thermal-infrared sensor.
Thus it is a feature of at least one embodiment of the invention to provide a detector providing improved response time over photo-resistive detectors
The thermal-infrared sensor may be a PbSe thermal-infrared sensor.
It is thus one feature of at least one embodiment of the invention to provide for a commercially practical thermal-infrared sensor.
The thermal-infrared sensor may have an aspect ratio of greater than three to one and the long dimension of the thermal-infrared sensor may be positioned to image perpendicularly to the motion of the substrate.
It is thus another feature of at least one embodiment of the invention to accommodate conveyor belt lateral shifting while minimizing the acceptance of detector noise, which in a thermal-IR detector increases with detector area.
The thermal-infrared sensor may be operated with a constant voltage bias.
It is thus another feature of at least one embodiment of the invention to manage the voltage drift of available thermal-infrared sensors.
The thermal-infrared sensor may be mounted to a temperature-controlled substrate.
It is another feature of at least one embodiment of the invention to reduce temperature drift of the sensor for practical use in an industrial environment.
The thermal-infrared sensor may include a filter optically blocking light above and below a 3.5-μm wavelength.
It is thus another feature of at least one embodiment of the invention to minimize lighting interference necessarily a part of an industrial environment.
The thermal-infrared sensor may be positioned behind a window of halogenated plastic.
It is thus another feature of at least one embodiment of the invention to provide a practical, rugged, and low absorption protection of the sensor in the industrial environment.
The thermal-infrared sensor may receive an image of the substrate projected by reflective optics on the sensor. The thermal-infrared sensor may be offset from a path of light from the substrate to the reflective optics.
It is thus another feature of at least one embodiment of the invention to provide improved detector sensitivity without the need for expensive optical materials.
The thermal-infrared sensor may further include an illuminated focus target in an image plane of the thermal-infrared sensor projected by the imaging optics onto the substrate. The focus target may indicate an axis of the substrate, as well as proper standoff distance.
It is thus a feature of at least one embodiment of the invention to allow for precise alignment of the detector to maximize the signal to noise ratio of the signal, as well as to simplify the process for the user of determining the precise target position monitored by the sensor.
The comparison circuit ensemble averages signals from the thermal-infrared sensor from multiple substrates to obtain the detection signal.
It is thus a feature of at least one embodiment of the invention of providing for a flexible trade-off between response speed and detection accuracy.
The detection signal may provide a comparison of the output of the thermal-infrared sensor to a threshold dependent on a temperature of the thermal-infrared sensor.
It is thus another feature of at least one embodiment of the invention to accommodate temperature sensitivity of inexpensive thermal-infrared detectors.
The detection signal provides a response time of less than 500 μs or a response time relative to a movement of the substrates such that less than ten substrates have passed before the detection signal with less than 5 mm positional error.
It is thus a feature of at least one embodiment of the invention to provide for real-time correction of glue gun timing errors in a timescale comparable to actual changes in the mechanism of the glue gun to allow high-speed operation without offsetting waste or manufacturing line downtime.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.
Referring now to
A glue gun 24 may be positioned at an upstream end 22 of the conveyor belt 12, the glue gun 24 having a pressurized hot glue reservoir 26 connected to a nozzle 28 by means of electrically actuated valve 30. The valve 30 may receive a trigger signal 32 to open the valve to cause a dispensing of glue through the nozzle 28 in a glue bead 34 on to substrate 14. As is understood in the art, the speed of response of the valve 30 will change, being dependent on the characteristics of the glue, including its viscosity and chemical formulation, as well as wear and heating of the valve 30.
An industrial controller 35 or the like, may provide a timing signal 33, which is received by a timing signal shifter 36 which may advance or retard the timing signal 33 to correct the trigger signal 32 and hence the position of the glue bead 34. Advance and retard of the timing signal 33 may be readily accomplished within the regular periodicity of the timing signal through a phase locked loop or the like, or may be accomplished within the industrial controller 35 itself by varying delays based on signals precedent to the timing signal 33. The industrial controller 35 may also actuate glue gun 24 through an actuation signal 38 for example controlling the glue pump and glue heaters (not shown).
A thermal-infrared detector assembly 40 (e.g. having a detector sensitivity around 3.5 microns) may be positioned downstream from the glue gun 24 to detect substrates 14′ having had a glue bead 34 applied to their top surface. The detector assembly 40 is located at a known displacement from the nozzle 28 or a known time delay (for known speed of conveyor belt 12) from the nozzle 28. The detector assembly may receive infrared radiation from the glue bead 34 while the glue bead 34 is still at an elevated temperature, for example, before adhesion to a second component to be attached to the substrate 14′, so that the glue bead 34 may be readily distinguished from the substrate 14 by temperature alone without the need for dyes or other techniques.
The detector assembly 40 produces an error signal 42 that is received by timing signal shifter 36 and which indicates whether the glue bead 34 has been shifted to the right or to the left with respect to the substrate 14′ caused by advance or delay in the operation of valve 30 of the glue gun 24. This error signal 42 may be deduced by detecting, for example, the leading edge of the glue bead 34 and comparing it to a reference signal 44. The reference signal 44 may in a first embodiment be the signal from the encoder 18 at the time when the substrate 14′ was beneath the glue gun 24 and the trigger signal 32 occurred, summed with the offset between the nozzle 28 and the detector assembly 40. Alternatively reference signal 44 may be a time signal equal to the time when the substrate 14′ was positioned beneath the glue gun 24 and the trigger signal 32 occurred, summed to a time delay between the time substrate 14′ was beneath the glue gun nozzle 28 and the time when the substrate 14 arrived beneath the detector assembly 40.
The error signal (advance or delay) for turning on the glue gun (correlated to the rising edge of the infrared signature), and the error signal for turning off the glue gun (correlated to the falling edge of the infrared signature) may or may not be the same, as changes in glue gun turn on and turn off delays may or may not track each other perfectly. It will be understood that the present invention may also be used for separately correcting the turn off time of the glue gun using a similar procedure.
Critical to the feedback control of the valve 30 of the glue gun 24 is that a spatially accurate detector signal can be produced to effect corrections to the trigger signal 32 as the next substrate 14 is being glued or as a practical matter before five substrates have passed. The present invention provides a detector signal having a response time of greater than 2 kHz with a better than 5 mm positional accuracy.
It should be understood that a detector assembly having slower response speed and/or lesser positional accuracy can still be used for quality control purposes even though it is impractical for closed loop control. For example, if it is desired to determine the length of the glue bead 34 only, then an arbitrary and/or variable delay in the response time of detector assembly is of no concern. Further, if it is intended only to track long-term trends in the shifting of the glue bead 34 then high-speed detection is not required and positional accuracy can be improved by long averaging periods. Thus there is a trade-off between accuracy of detection and speed of detection and both are required for real-time corrective control.
Referring now to
The detector 54 is held on a temperature controlled substrate 56, for example, being a Peltier device, that is held at a constant temperature by a local controller 58 receiving a temperature signal 60 from a temperature sensor and 62 in thermal communication with the detector 54.
The temperature signal 60 is also provided to a comparison circuit 64 whose use of this temperature signal will be described below. The comparison circuit 64 also receives a detector signal 66 from the detector 54.
An optical filter 68 may be positioned on the upper surface of the detector 54 to filter out light having a frequency outside of the desired infrared band being centered at approximately 3.5 μm in wavelength. The filter 68 may be a chip of germanium anti-reflection coated for the 3.5 μm range to reject frequencies in the visible and near infrared range.
The reflective optics 48 may be protected from the environment by an opaque housing (not shown) which admits the infrared energy 50 through a protective window 70 formed of a halogenated plastic so as to prevent absorption of the desired infrared bandwidth. A suitable material for this window 70 is PolyIR5 commercially available from Fresnel Technologies of Fort Worth, Tex. The use of halogenated plastic avoids the hydrogen-carbon bonds that are opaque at the desired thermal-infrared frequency. Non-carbon based plastics such as silicon based plastics may also be employed.
The sensor 54 is offset from the path of the infrared energy 50 to provide for maximum received radiation.
Referring now to
Referring to
Referring now to
The output from the filter 90 is provided to an ensemble averager 92 which may average readings from up to nine successive substrates 14 to obtain improved signal-to-noise discrimination.
Referring now to
By averaging the signals only within each bin 88 over several substrates, random noise is decreased, without blurring the leading edge of the signal used to detect the beginning of the glue bead 34. The number of substrates 14 averaged controls the reduction of noise at the cost of decreasing the effective response speed of the detector assembly 40. Typically as few as five substrates 14 will be sampled.
The histogram 93 is compared against a threshold 94 to identify a start 96 of the glue bead 34 to extremely high precision on the order of one to 2 mm. The threshold 94 may be fixed or may be changed based on an empirical measurement of the change in the sensitivity of the detector 54 with temperature as deduced from the substrate temperature sensor 62. The result of this comparison is a threshold signal 97.
The threshold signal 97 may be compared to the reference signal 44 (as corrected by the inherent delay between the detection and the dispensing of the glue caused by their spatial separation) at error generator 95. When the leading edge of threshold signal 97 is after the leading edge of reference signal 44, a negative error 100 is measured while when the leading edge of threshold signal 97 comes before leading edge of reference signal 44 a positive error 102 is measured.
Referring again to
If desired, the histogram value for the present bin can be fed to a digital to analog converter as each electronic bin is 88 is revised when its corresponding position on the moving substrate 14 falls below the sensor. In so doing, a real-time signal can be provided to an oscilloscope, allowing the user to see a plot of the glue sensor reading signal levels. This may aid in diagnostics by allowing the user to determine if the proper threshold is being set, and by allowing the user to assess the signal to noise ratio for the chosen number of substrates averaged. This information could also be mapped onto a display (such as a liquid crystal dot matrix) on the side of the sensor.
It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.
This application claims the benefit of the U.S. provisional application 60/766,710 entitled: “Hot Glue And Thermal Web Sensor For Inspection And Control Of High-Speed Processes” filed on Feb. 7, 2006 and hereby incorporated by reference.
Number | Date | Country | |
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60766710 | Feb 2006 | US |