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@@ -6,6 +6,7 @@ We can also measure strain of a member by measuring the displacement of a point
Non-contact position measurements have historically not had high enough resolution for load cell applications, but have been used commonly for applications that can tolerate larger displacements. For instance, joystick inputs for machines are often sensed using a non-contact magnetic field measurement (c.f. US5831596A). Similarly, many automotive applications (gas pedals, shifter positions, etc.) that sense large displacements are turning to non-contact measurement for the robustness it offers.
+Advances in high sensitivity measurement has made it possible to use non-contact measurement for the small displacements required by force sensing applications. For instance, capacitive sensing is used in high resolution digital calipers and has found use in load cells (US10582023). We breifly describe a prototype 3 axis capacitive load cell in a section below. One downside to these techniques is that relatively large overlapping areas are required for a low noise measurement. In many compact load cell applications (particularly for measuring many degrees of freedom), realizing these geometries can be challenging.
## Capacitive
@@ -18,10 +19,13 @@ It is based on a discrete electrostatics solver (shown below), and also document
<img src='capacitive/moving-dialectric.mp4' width=400px>
-
## Magnetic
-Another position measurement uses the magnetic field instead of the electric field. Among magnetic field sensing technologies, Hall effect sensors are the most ubiquitous. Integrated circuits with arrays of hall effect sensors are available at extremely low cost in very dense packages. Using differential pairs of these elements, non-contact rotary and linear encoders can be made. A great resource for designing such magnetic devices is the <a href='https://sensing.honeywell.com/hallbook.pdf'>Honeywell Hall Effect Handbook</a>. Austrian Microsystems makes a variety of these devices for sensing position or rotation of a magnet.
+Another position measurement uses the magnetic field instead of the electric field. These devices consist of one or more magnetic field generating devices and one or more magnetic field sensing devices, usually arranged in pairs on opposing sides of a flexure. The magnetic field generating devices are arrangements of permanent magnets or electrically driven coils. The magnetic field sensing devices are usually spatial arrays of sensor elements, the measurements of which can be combined differentially to nullify external fields or unintentional displacements. The sensing devices are usually positioned at the region of greatest rate of change of magnetic field strength with respect to the degrees of freedom of the flexure.
+
+### Magnetic sensing technologies
+
+Among magnetic field sensing technologies, Hall effect sensors are the most ubiquitous. Integrated circuits with arrays of hall effect sensors are available at extremely low cost in very dense packages. Using differential pairs of these elements, non-contact rotary and linear encoders can be made. A great resource for designing such magnetic devices is the <a href='https://sensing.honeywell.com/hallbook.pdf'>Honeywell Hall Effect Handbook</a>. Austrian Microsystems makes a variety of these devices for sensing position or rotation of a magnet.
For more sensitive, low field devices magnetoresistive sensing elements are often used. <a href='http://www.physics.nyu.edu/kentlab/Lectures/Pappas_Tutorial_APSMM2008.pdf'>David Pappas of NIST compares</a> noise floors of these technologies, noting Hall effect sensors register around 300,000 $`pT/\sqrt{Hz}`$, while magnetoresistive show roughly 200 $`pT/\sqrt{Hz}`$. Honeywell sells a variety of very sensitive magnetoresistive magnetometers, and NVE Corporation sells sensors based on giant magnetoresistance for very low field measurement. Despite these favorable properties, sensors with these technologies are usually more expensive and less dense than those using the Hall effect.
@@ -32,6 +36,16 @@ A similar description holds for fluxgate technology, which can achieve the lowes
Finally, <a href='http://aip.scitation.org/doi/pdf/10.1063/1.2836410'>magneto-electric sensors</a> are a nascent technology with exciting prospects (100 $`pT/\sqrt{Hz}`$ according to Pappas). In these sensors, a magnetostrictive material (usually a metglas) is combined with a piezoelectric material (usually lead zirconate titonate) into stacked layers. The magnetic field produces strain in the stack, and the piezoelectric materials produce a corresponding voltage that can be read. There is a <a href='https://www.nature.com/nmat/journal/v7/n2/pdf/nmat2106.pdf
'>great Nature Materials paper</a> demonstrating that a common SMD 1206 ceramic capacitor can be used as a 1 cent magnetic field sensor based on this physical phenomenon. Again, this technology is promising, but not yet commercially available.
+### Advantages
+
+* As described above, the non-contact, flexural nature of these sensing devices means the output can be free from most sources of hysteresis.
+* These devices can be made so all electrically connect parts of the device are on one side of a flexure, and no cables, slip rings, etc. are required to bridge a spatially varying gap.
+* Hall effect elements are easily produced in arrays, allowing differential measurement across many axes, nullifying external fields and unwanted components of displacement. This means we can use large gain values, creating very sensitive devices.
+* These gain values are also adjustable on-the-fly, which broadens working dynamic range.
+* The digital interfaces to these sensors can be efficiently bussed because the measurement time (100-1000 microseconds, typically) is much longer than the transmission time (10 microseconds, typically). Thus, we can use many sensors on the same bus with little timing overhead.
+* Temperature sensor circuits can also be included on these busses, offering fine-grained temperature compensation.
+* Because the magnetic field generating and sensing devices are compact, the flexures can be designed to be easily manufacturable and have nice features like over travel limits.
+
### AS5510 1 DOF Loadcell