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Beehive scale utilises inductive sensing

28 May 2014  | Paul Watson

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Editor's note: This entry is one of the runners-up in the TI LDC1000 inductive sensor design contest.

This beehive weight scale employs a Texas Instruments LDC1000EVM inductance processor circuit to measure the weight of a Beehive, by detecting the the change in resonance due to a change in inductance, as the change in weight on the scale occurs. The processor sensed and measured differences; outputs are sent to a collocated laptop computer via a USB port and displayed by the TI software GUI on the screen. For this design concept, it was decided to design for a maximum of 160 lbs. The detailed PDF available below shows a range scale of desired performance, however the LDC1000 was not well understood by me at first, so this concept was a design to fit the unknown performance of the LDC1000, but adjusted to make it work.

The scale consists of the following construction items:

1 — Top board 20" X 20" X 3/4", planed and smooth, leveled

1 — Bottom board 20" X 20" X 3/4", planed and smooth, leveled

16 — compression springs, 1.2"L X 1/2"D, 20lbs/in

32 — metal collars for the mounting the spring ends

4 — top/bottom board stiffeners, 1 1/2" X 20" X 3/4"

16 — Wood screws, 2" — 6

4 — Wood 3/8" — 16 flange nuts for the top board

4 — 9/16" OD Heat Shrink tubing , 4"L

1 — 200 ft spool of 30 gauge transformer wire

1 — Bottle of instant glue

4 — Iron rods, 1/2" X 2 1/4"L, Die threaded for 3/8" — 16

1 — Twisted pair 18 gauge wire 36"L

1 — 1000pf ceramic capacitor

1 — Faraday shield, 1 1/2" ID X 2"

1 — Bottle of Gorilla Glue

1 — USB 36" cable

1 — lot of 4 lb weights, total 40 — I used 20 for this experiment

2 — Gold male pins to fit into the EVM board holes

2 — Gold female pins for tank wire attachment

24 — Small nails 3/4"L

The 20" x 20" top and bottom boards are spaced atop each other for a spacing of 1 1/2". The 3/4" stiffeners decrease the spacing by 3/4" leaving 3/4" air gap. When the 160 lbs is on the scale, the iron rods will penetrate the coil centres to a depth of 1/2", thus there will be 1/4" air gap at max load.

The iron rods were selected and made for the highest induced inductance change. 3/8" machine bolts did not perform as well.

The RF coil sensors were hand wound to a formula given in the ARRL Handbook and then measured by an LCR meter for uniform µH value. The value for design was taken from the EVM board as a goal of 20 to 22µH. Higher values could have been selected, but the LDC performance was too much a mystery at the concept time.

The twisted pair wire was shortened to 3" due to the noise interference, but it was determined that a USB cable was noisy also, so had to use the shortened twisted pair for interface. All tests were performed at 70ºF/21ºC indoors.

Calibration is essential. The design allows for the top board to be removed for iron rod threaded adjustment, for a no load condition.

The GUI screen is used to observe where the calibration line to screen is set. The concept goals are met by the LDC1000, the values obtained by the LDC vs. the LCR meter will differ due to the difference in frequency impedance of the RF coils.

The Top and Bottom boards are stiffened to maintain level and smooth surfaces. Accuracy depends on precision of the scale build, so careful attention must be made to the fitting parts. The iron rods must align with the coil centres and hold adjustment. The LDC is accurate to 1nH, so inaccuracies in scale measurement can occur at the lower load end of the scale. For this design, I did not foresee this effect until long into the project. The change in inductance to change in weight is a linear scale and calculated µH predictions compared well to the test results. Outside temperature and environmental change effects were not considered for this design. The concept was chosen to build something that world work with the LDC1000, not the usual other way around where you choose parts to fit what you want to design.

The 4 lb jars of honey were selected a load weights. The scale was loaded for 8 lb intervals and data recorded using the GUI storage feature to an Excel file. Inductance and proximity values aid in the observance of noise problems encountered during testing and fixes for this problem.

Watch the video on this design here.

About the author
Paul Watson contributed this article.

To download the PDF version of this article, click here.

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