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Sensor Skin Design

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SkinDesign

The Team

Background

Desirable Properties

  • Dense - Human skin contains thousands of nerve fibers per square inch.
  • Multimodal - The ability to sense pressure, touch, temperature, proximity, location.
  • Flexible - Should be able to bend without breaking.
  • Conformable - Should be able to take the shape of the surface it adheres to.
  • Connectible - The flexibility of the skin should not interfere with rigid wiring schemes.
  • Manufacturable - The process of building skin must be repeatable and based on good manufacturing techniques.
  • Readable - The information that comes from the skin needs to be able to be processed in an efficient manner without losing too much information.
  • Protective - The skin should introduce a layer of safety between the robot and environment.-- DanAukes - 30 Jul 2008
  • Lightweight - The skin's weight should not severely impede performance

Potential Tradeoffs

  • Density vs. Connectivity - The higher the density of sensor information, the greater the challenges to wiring and processing.
  • Flexible skin vs. Rigid wires - Wires bend and break due to stress concentrations at the interface between flexible and rigid structures.
  • Safety vs. Performance - Safe materials can lower performance by introducing hysteresis, signal speed issues.
  • Performance vs. Weight - The heavier the sensing skin, the lower the performance of the arm.-- DanAukes - 30 Jul 2008

Design Questions

Here are some of the preliminary issues I've come up with that we should be thinking about and hopefully answering. Obviously we can't make arbitrary decisions on most of these issues since they are coupled wiith issues affecting other groups; mainly sensing capability and physical construction of the underlying 'bone' structure and how we want to respond to the environment.
  • What exactly do we want to sense? and in what order of importance?
    • Contact location (to what degree of resolution on the arm?)
    • Proximity of objects to the arm (at what length scales?)
    • Normal contact pressure/force (including spatial distribution?)
    • Shear forces (at least need to consider how their presence would affect normal pressure/force)
  • How much energy absorption do we want/need?
    • Rough idea of bulk material properties of the skin
    • How do structural requirements of various embedded sensors affect energy absorption?
  • Skin support structure
    • Can we get away without a dedicated support structure (use the 'bone' and McKibbens)?
    • If not, how stiff does it need to be?
    • Does it need to support the whole skin (a solid shell), or just sensor locations (some sort of mesh)?
  • Connectivity
    • Intelligent busing and wire routing from all sensors
    • Distributed, local "patch" processing by separate local microprocessors?
    • Wireless communication (and power?) between sensors in skin surface and processors deep in skin/in the 'bone' structure. -- BarrettHeyneman - 31 Jan 2008

Existing Solutions

Reading

We have compiled a large list of resources for further reading.

Many (imperfect) Solutions

  • Electrical designs
    • Sensor networks - Individual nodes incorporate communications and sensing.
    • Sensor arrays - Controller scans arrays of sensors and extracts information from the array.
  • Sensor designs
    • Resistive elements – pressure sensing foams, rubbers.
    • Capacitive elements – measure the voltage across plates, between skin and objects.
    • Optical elements – measure the reflectivity between skin regions or to objects.
    • Piezoelectric elements – measure the vibrations due to slip and shear.
  • Mounting solutions - Kapton®, textiles, rubber sheet.-- DanAukes - 30 Jul 2008

Existing Technologies

  • Capacitive: Changes in capacitance can be used to sense and localize pressure/applied force on the skin when arrays of capacitors are embedded in a deformable skin. Additionally, if electric field lines between capacitive plates are forced outside the skin, they form good human detectors; introducing part of a person's body changes the dialectric constant between the plates, thus changing the capacitance.
    • There are IC capacitive or electric field sensors. Simply hook up to the sensor capacitor.
      • Electric field sensor from MIT Media Lab paper above, Motorola 33794: Data Sheet
      • Analog Devices Capacitance-to-Digital IC, AD7143: Data Sheet
    • Capaciflector concept can be used to force electric field lines farther outside the skin, increasing human detection range. Three plate configuration could also be used (by sampling capacitance between different plates) as a pressure sensor? Capaciflector Page
  • Quantum Tunneling Composite sensors: Composite material which (through effects of quantum tunneling I don't really understand) go from an insulator to a conductor as pressure is applied. Effectively they can act as variable resistors, based on pressure.
    • QTC explained by Peratech (the inventors). We can buy "pills" or sheet; check out the product link.
    • QTC comes in sheets, pills, or cables. Currently, they are not available in a liquid polymer form, though applications engineer are currently looking at providing this solution. Attempts to melt it proved unsuccessful; they simply would not turn into a liquid state.
    • New update 4/18/2007. Peratech has changed their business model. They no longer sell sheets. Rather, they do special formulations for each build. Contact with Peratech rep (Martin.kingdon@peratech.com). He will attempt to get us sheet scraps, but feels its very doubtful they will approve of giving us liquid form
  • Infrared Proximity sensors: Transducer sends an IR beam out. It's reflection is picked up by a lens which focuses it onto a CCD. Where the beam is focus can code the distance to the object. A good website that explains the technique From what the webpage says, all have minimum distances before signal strength drops off to a level similar to an object far away, which is bad. We have a sample in our lab. We also have 1 mm IR proximity sensors (ask Sanjay) -- SamsonPhan - 10 Mar 2008
  • Conductive Paths - In order to access the array of skin sensors, we can run wires or incorporate electrical paths into the shell. The possible technologies are:
    • silk screen conductive paths, perhaps followed by electroplating
    • pattern,sputter, then electroplate
    • sputter, pattern, then electroplate
    • conductive epoxies

Interesting websites

Current Projects

We have been exploring some novel sensing technologies (or applications of existing technology) in parallel. Current status of these projects can be found here.

Dual Mode Capaciflector

A capaciflector (see Specific Technologies section, Capacitive) uses a shielding plate to project electric field lines farther into the sensing area and thus increase sensitivity/range of a capacitive proximity sensor. The dual mode capaciflector concept involves using the shielding plate as another sensing plate. If a soft material is placed between the shielding plate and ground, applied loads/pressure will cause the plates to move together which will cause a measurable change in capacitance.

In addition to the new physical sensing arrangement, a new switched capacitance based conversion strategy, Tri-Slope Conversion, is being experimented with. The possible benefits of tri-slope conversion include faster conversion times than dual-slope, conversion times which are roughly constant with changes in accuracy, and flexible recalibration for changing external conditions.

For more details see DualModeCapaciflector.-- BarrettHeyneman - 28 Jul 2008

Optical Pressure Sensors

DanAukes is working on some new optical designs using transparent rubber and infrared emitter/detector pairs. -- DanAukes - 30 Jul 2008

Force Sensing Resistor Array

fsrthumb.jpg JohnUlmen is working on an array of FSR's. Follow this link for more details, ForceSensingResistor.
Force sensing resistors (FSR's) are commercially available piezoresistive sensors that are thin and flexible. They are a good, low cost option for qualitative measurements of force. A skin was designed for use with FSR's to test their suitability as force sensors in a robotic skin. -- JohnUlmen - 05 Aug 2008

Capacitor Array

componentsthumb.jpg JohnUlmen is working on an array of electrically isolated force sensing capacitors. Follow this link for more details, CapacitorArray.
Capacitive sensors are extremely robust because they don't rely on physical contact for sensing. Unfortunately, many capacitive force sensors are plagued with noise problems, especially in an array format where long wires can increase noise coupling. A simple, low cost design is presented that has high resolution while maintaining excellent noise performance. The design also requires a minimal number of wires. -- JohnUlmen - 06 Aug 2008

Skin Networking

We have begun investigating the networking requirements for our sensor skin -- DanAukes - 30 Jul 2008

 
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