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Ask The Search Engineer

Ask The Search Engineer

FEA model results . . . Circular crimp rings . . . Step output converter . . . Contact material . . . Inductive charging . . . Sensing cutting tools. . .

Dear Search Engineer: I've designed a vibration fixture. The forcing frequency range was to be 10-2,000 Hz. The mass of the UUT is about 20 kg. The mass of the fixture was to be no heavier than 25 kg due to shaker limitations. After considering all the constraints, I designed a fixture and analyzed its mode shapes and resonant frequencies. In my analysis, I constrained the fixture by assuming the counterbore faces in the bolt holes were fixed. Based on my analysis, I predicted a fundamental resonance at 2,200 Hz, where the fixture was constructed. Upon testing the fixture on a shaker, a fundamental resonance of 1,800 Hz resulted. Why does my FE result differ so much from the actual result? I re-ran the model with no constraints, and my FEA program was able to predict a third fundamental at 1,800 Hz resonance. In fixture design, how should the FE model be constrained to yield the correct results? -A.S., CA

Dear A.S.: Assuming the bolt holes you mentioned are for mounting the fixture to the shaker, the problem is that your FEA model constrains the fixture at points where the shaker is moving it. If you mount a small accelerometer on one of the bolt heads and drive the shaker with a sinusoid of slowly varying frequency, what you will observe should be a minimum response at 2,200 Hz. That is the frequency that your model predicts for the case where the bolt holes do not move.

The proper way to model the vibration fixture is to remove all constraints, divide the moving mass of the shaker armature by the number of mounting bolts, and add the resulting mass to the vibration fixture at each of the bolt holes. Generally speaking, unless you have a very lightweight flexible structure clamped or welded to a large block of steel, a true rigid constraint cannot be achieved easily.

Dear Search Engineer: I'm looking for a circular crimp ring to crimp a round tube to a cylindrical molding. The crimp ring must retain its circularity following crimping. I've looked at tinel lockrings, but they need too much heat. Any ideas? - P.F., U.K.

Dear P.F.: Try the crimping technique used normally for underhood automotive switches. For a visual reference, just look at any heat or oil pressure switch and see how the crimp is formed around the base, usually by a hydraulic press with a form tool.

Dear Search Engineer: I'm looking for a unit that will convert step and direction output from a controller I've been asked to use to run my dc servo systems, which require analog and plus or minus 10V. Alternatively, I could use dc servo amplifiers that take step and direction input, but the only one I can find is too big. -I.M., U.K.

Dear I.M.: Consider amplifiers from Elmo Motion Control that will fit in your shirt pocket. Check them out at

Dear Search Engineer: My electromechanical application has less than 50 mA current at 12.8 and/or 5.0V for 1 million cycles. What kind of plating is required for AGNI contacts or phosphorous bronze, or beryllium copper stamped moving contacts? I have heard multiple theories that fine silver plating is fine, and min 3-micron gold plating is required. What role is there for contact material and moving contact thickness? -K.D., in IN

Dear K.D.: The problem you may run into is oxide buildup because the low current will not burn it off. A Honeywell Microswitch ( may be used for switching a logic signal of 5V through a 10K resistor. If you open the switch up, you can see that they use a wiping action to keep the contacts clean and a grooved pattern to allow oxides to be moved away from the contact surface. Silver may be a good choice; gold is generally a poor choice with wiping action since it is soft enough to be worn away.

Dear Search Engineer: I'm trying to design an inductive charger that will take 12V dc in and give me 10V at 400 mA. The inductor is a ferrite rod 0.25 x 1 inch long; the coil is wound directly on the ferrite rod and is about 0.025 inch thick with about 100t's of #24 wire. The secondary is loosely coupled and consists of 200t's #24 wire and is about 0.05 inch thick. I am using a TC 4427 driver at 100 KHz. Am I dreaming to think I can get 400 mA from such a circuit? Currently, I'm getting about 10V at 10 mA and cannot squeeze any more out of either the circuit or coil. Can this type of technology support inductive charging for anything other than toothbrushes? I know electric cars use such a principle, but they cost $50K. -V.S., Ontario, Canada

Dear V.S.: With only 12V to use, you need to use a LDO regulator instead. The problem with the circuit starts with the TC4427, which may have enough current capability, but the conduction losses are far too high with 7h for the device. In order to regulate the total resistance of switching fet, inductor and rectifier fet/diode would have to be much less. The cost would be high and not very efficient. A flyback would work, but the efficiency would be much lower than using the LDO as a regulator.

Dear Search Engineer: We are currently developing a reciprocating bone-cutting instrument that requires a sensor to determine when the blade has penetrated through the bone and subsequently stop the cutting mechanism (i.e., a sensor to measure the decrease in force on the blade once it has penetrated through the bone). Are there any commercially available products? I remember reading an article some time ago where the Japanese developed sensors to measure the force on the cutting tools of CNC machines to automatically determine tool wear and change the tool. -R.T. in MA

Dear R.T.: Measure the wattage of the motor doing the cutting and monitor it with a time-delay relay to address the fact that power will cycle from zero to maximum each cutting stroke. A typical induction motor will have a nearly constant running current regardless of load, and it is difficult to determine what is happening by reading current alone. If you measure VA (volt-amperes), the problem remains. However, when reading the real power, watts (volts in-phase amperes), the signal will vary with the actual amount of work being done. These types of watt meters are commercially available and were designed for this task.

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