EDNE June 2012 - (Page 60)

Ta l e s F r o M T h e c u b e Mike Page • Free-lance Designer Pulp packs a punch few years ago, I was working with servo controls for winding 20­foot­wide rolls of paper from pulp to paper and processing. Servo loops with various feedback and monitoring devices hydraulically controlled the rolls. When you wind paper, the tension on the surface has to decrease as the diameter increases. Otherwise, when you remove the roll from the spindle, it could shoot across the room and possibly kill someone. At the time, operators normally performed tension control manually by watching a pressure gauge and using trial and error. A taper­tension technique automatically reduced the tension on the take­up roll. A contact­type “dancer” roller had previously measured the diameter of each roll, but, in this case, we wanted to go “no­contact” with an ultrasonic­transceiver circuit. This approach involved no microprocessor; it used simple op amps and drivers controlling Moog valves. The system used a National Semi­ conductor LM1812 ultrasonic trans­ ceiver and a 75­kHz transducer. I built a prototype on perforated board that was copper­clad on one side so it provided a ground plane for the RF circuit. Around each through­hole component, I used an oversized drill bit to clear away the copper and then hard­wired everything. As usual, National Semi’s application notes provided a nice starting point, and I only slightly modified one of the exam­ ples. The prototype powered up with no problems—and no smoke. I set up a target fixture and started to fine­tune everything, and it worked—sort of. I had never been proficient with RF circuits, so I was pleased with myself. Even with my old Tektronix 465 oscilloscope, I could see the transmitting and echo pulses, which went to my start­stop counter cir­ cuit and displayed an arbitrary count with A no timebase for distance. This count then went to a DAC for the analog control in the hydraulic­control loop. All of a sudden, everything flatlined. I couldn’t see any reason for it, so I start­ ed to adjust the coil slugs, which got it to work again. This flatlining scenario kept happening every so often, and it was driving me nuts. I began to time it and found that it would quit for about 30 seconds every 10 minutes. I figured that something in the area was jamming my frequency, and I tried all kinds of shield­ ing by placing boxes around the LM1812 and certain other components. I even added a twin­T filter tuned to the spe­ cific frequency. Nevertheless, the prob­ lem recurred about every 10 minutes. I couldn’t track it down with my scope; everything went fuzzy during the 30­sec­ ond period. I figured that it would act differently in the field in which it would operate, so I made plans to take it there. I updated and made copies of my designs. From the copier about 20 yards away, I could see my oscilloscope flatline. Meanwhile, I was leaning on the copier and felt a slight click and heard a faint whir. The damned copier was doing it! A heater and small fan inside the machine for heating and cooling turned on every now and then—30 seconds every 10 minutes! The secretary yelled at me every time she needed to make copies because I always pulled the plug while I was “cali­ brating.” She would then have to wait for the machine to warm up before she could continue copying. I put a line filter in line with my power supply and boxed it all up with good grounding, and everything was fine. I wonder how this copy machine passed Underwriters Laboratories, CE, and whatever other emission tests exist­ ed. It was apparently throwing wideband noise of significant power back into the 120V­ac line. Nevertheless, in the war between our no­contact tension­control system and the copy machine, no lives were lost due to flying rolls of paper or irate office workers—both out to kill.EDN Mike Page is a retired custom industrial system control and instrumentation designer who now works as a free-lance power, ADC circuit designer. 60 EDN EUROPE | JUNE 2012 www.edn-europe.com Daniel Vasconcellos http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE June 2012

Cover
Agilent Technologies
Contents
International Rectifier
RS Components
Masthead
International Rectifier
Comment
Pulse
Analog Devices
Digi-Key
Farnell
NXP
Test & Measurement
Silicon Labs
Digi-Key
Test-driven development for embedded C: why debug?
Digi-Key
Baker’s best
Cover story
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Pico-projector design uses color LEDs
Digital isolation in smart energy metering applications
Mechatronics in design
Teardown
Design Idea
Product Roundup
Tales from the cube

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