Inclinometer Head (A2065) Manual

Contents

Description

The Inclinometer Head (A2065) is a Long-Wire Data Acquisition (LWDAQ) Device that reads out a liquid-level tilt sensor. The sensor is on a second circuit board, called the Inclinometer Sensor Head (A2066). The sensor that mounts on the A2066A is a five-pin liquid level, part number 59560 from Applied Geomechanics. We present the accuracy and precision of the A2065 in our Inclinometer Performance report. For links to all Inclinometer documentation, go to the Inclinometer Home Page.

Figure: The Inclinometer Head (A2065B) and Inclinometer Sensor Head (A2066B). Note that the enclosure is resting on its side. During operation, the glass sensor is on the bottom. See the drawing below for the correct orienation. The two are joined by a three-inch six-way one-millimeter pitch flex cable, and secured in an anodized aluminum enclosure. The lid is shown in part on the right.

The following versions of the A2065 exist.

The A2065 drives its liquid-level tilt sensor with a sine wave of frequency 1.1703 kHz, which is 32.768 kHz divided by twenty-eight.

Below is a drawing of the A2065B showing its mounting plate and lid.

Figure: View of A2065B and A2066B in Enclosure. Shown are (1) tilt sensor, (2) Inclinometer Sensor Head (A2066B), (3) Inclinometer Head (A2065B), (4) LWDAQ device socket, (5) base plate with three steel balls for kinematic mounting of inclinometer enclosure, (6) six-way flex socket for six-way flex cable to A2066B, (7) lid, (8) enclosure with holes for screwdriver to tighten A2066B to wall.

The A2065B uses the A206501C printed circuit board. For hole and connector locations on the A206501C, see here.

Specification

The A2065 is complies with the LWDAQ Specification, except in its power consumption. The A2065 consumes 11 mA from the +5V supply when it is asleep, which exceeds the 5 mA limit set by the LWDAQ Specification.

The A2065 is LWDAQ Device type 0 for device-dependent jobs. It has no device elements for device-dependent jobs. Its command bit allocation is as follows.

When the A2065B is asleep, its sin wave drives the sensor so that the sensor will be ready for immediate read-out. If we disconnect the sensor between measurements, some of the AC coupling capacitors discharge, and add an offset voltage to the returned signals. This offset voltage can be sufficient to put the signal outside the dynamic range of the LWDAQ driver's ADC.

Operation

Use the A2065B with the Inclinometer Instrument in our LWDAQ Software.

The Inclinometer instrument uses the A2065 to apply a sine wave across the X+/X− terminals of a tilt sensor, and then the Y+/Y− terminals.

The Inclinometer calculates the amplitude of the n'th harmonic of each recorded waveform, where n is the value of config(analysis_harmonic) in the Inclinometer window.

Figure: Sinusoidal Waveforms Plotted in Inclinometer Instrument Panel.

When it measures X-direction tilt, the Inclinometer drives the X+/X− terminals of the sensor. In the A2065B, X+ is connnected to the sine wave, and X− is connected to VCOM. The sine wave is centered upon VCOM, so the net DC voltage applied to the sensor is zero. Even if the sine wave were not centered upon VCOM, capacitors in series with ever tilt sensor electrode prevent any DC current flow into the sensor. The Inclinometer Instrument records the signal on X+ and the sensor's center electrode, which we call CTR. It calculates the ratio of the amplitude on CTR to the amplitude on X+. We call this ratio X.

Aside: When we first started working with the A2065B, we monitored X− as well, on the grounds that there is a slight voltage drop across the X− coupling capacitor. But our long-term studies showed that we gained nothing from using X− in our measurement. All we did was add noise into the calculations.

When it measures the Y-direction inclination, the Inclinometer Instrument drives the Y+/Y− terminals of the sensor, and determines the ratio of the resistance between CTR and Y− to the resistance between Y+ and Y−. We call this ratio Y.

Both X and Y are nominally 0.5 when the tilt sensor is horizontal. For the slope of X and Y with inclination, see Inclinometer Performance.

The Inclinometer Instrument result is a string containing the amplitudes of four sinusoids, followed by X and Y, as shown in the following table.

We use the LWDAQ Driver's (A2037) delay timer and repeat counter, plus the fact that we can digitize any integer number of periods of the sinusoid, to make sure that our sampled version of the waveform contains a whole number of periods of the sine wave we apply to the tilt sensor. The period of this sine wave is 854.50 μs. The A2065 obtains the 1.170-kHz frequency by dividing a its 32.768-kHz reference by 28. It turns a 1.170-kHz square wave into a sine wave by passing it through a 1-kHz low-pass filter (U7 in the schematic, for more on filters see our Filter Design Guide).

The Inclinometer Instrument records sine waves with the LWDAQ Driver's eight-bit ADC. To obtain an accurate measurement of the amplitude of each sine wave, we must be sure that we record a whole number of periods. In the A2037E driver, the eight-bit ADC sampling job takes 500 ns. We can add to this daq_delay_ticks 125-ns delay ticks. We take daq_num_samples samples.

Example: We set the delay timer to 28, and execute the 500-ns Eight-Bit ADC Job 3418 times to obtain samples spanning a 13.672-ms interval, which contains exactly 16 periods of 854.50 μs.

Example: We set the delay timer to 4, and execute the 500-ns Eight-Bit ADC Job 1709 times to obtain samples spanning a 1.709-ms interval, which contains exactly 2 periods of 854.50 μs.

Example: We set the delay timer to 12, and execute the 500-ns Eight-Bit ADC Job 6836 times to obtain samples spanning a 13.672-ms interval, which contains exactly 16 periods of 854.50 μs.

We use our dummy sensor, made out of resistors, to test the inherent resolution of the A2065, as we describe elsewhere.

Figure: Dummy Sensor. We create our X signal with two 1-kΩ resistors, and our Y signal with two 10-kΩ resistors.

We obtain our best performance with more samples and more complete periods of the sine wave. The default Inclinometer settings are 6836 samples over 16 periods. With the dummy sensor we obtain a resolution of better than 100 μV/V for the X and Y ratios.

Waveform Clipping

There is a danger that the reference sinusoids will be too large, and so end up being clipped by the LWDAQ driver's ADC, as shown below.

Figure: Clipped Waveforms

When the reference waveforms are clipped off at the top, as shown above, the clipping introduces an apparant tilt in the sensor. If you see such clipping, your A2065B needs to be corrected, so as to reduce the amplitude of its sinusoids. In the case of the circuit that gave the waveforms you see above, we replaces the 330-Ω R5 with a 470-Ω resistor (schematic)

Electronics

Note: We distrubute our schematics and gerber files under the terms of the GNU General Public License.