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The easiest tactile (touch) sensor to create is the omnidirectional tactile sensor, or Whisker Sensor, as it will be called here. The whisker sensor is, for all intensive purposes, a momentary contact (SPST) switch. It is either on (closed and allowing current to flow) or off (open and blocking current), making this sensor one of the easiest to not only create, but also use in a circuit.

*** One word of warning for beginners in electronics: when using a contact switch of any kind always include current limiting resistors to protect the rest of your electronic components and avoid wasted energy. ***

Material and Equipment
Let's get started with the tools you will likely need (from left to right):

Recommended Tools

Diagonal Cutter, Small (Hobby) Single Cut Flat and Round Files, Needle Nose Pliers, Small Diameter Tubing (Pipe) Cutter (K & S Engineering makes a pretty good one). In addition, you will also need scissors (or other cutting implement) to cut heat shrink tubing and a lighter (or other heating method) to shrink aforementioned tubing. Ultimately, you will require a soldering iron to attach the completed whiskers to your electrical circuitry (either free-formed or onboard).

You will also need the following components:



Silicone Glue/Sealant (preferred), Super Glue (will work if silicone is not available), 1/16" (1.6mm) Brass or Copper Tube (brass preferred, copper shown), Music Wire or Brass Rod (0.020" shown), and Heat Shrink Tubing (1/16" shown).

Note: Metal Tubing and Rod Materials
There is a variety of metal materials available at most hobby shops and online stores, but keep in mind not all metals are created equal. For instance, while copper is a great conductor of electricity, it does have the propensity to oxidize, especially in outdoor environments. Brass, while not as good of a conductor as copper, has been used aboard seafaring vessels for centuries because of its corrosion resistance qualities. It is best to keep in mind the application for your robot when picking out the type of metal you will use for the tube and wire.

Note: Picking Out Whisker Wire
The music wire and/or brass rod will ultimately become the contact whisker and needs to meet certain characteristics. The wire should be flexible enough for a robot to bend during contact, while elastic enough to return back to shape when the pressure is released. I recommend purchasing from a local hobby shop or hardware store until you become familiar with the different wires, rods and tubes available. Most local hobby shops will carry a variety of metal rods and wires (usually K & S Engineering brand) that you can test to ensure it will meet your robot's requirements.

To test: Hold one end of the metal rod/wire firmly between your thumb and index finger so that 1/2-3/4" is covered and gently flex the wire approximately 15-20 degrees. The pressure you want to apply when flexing the wire is relative to the size and power of the robot that will be using the sensor. Smaller robots with less force will require wires that flex with ease. Larger, more powerful robots will require less flexible (more rigid) wire to avoid bounce-contact being made from quick starts and jolts. If the wire flexes the 15-20 degree distance with relative ease and snaps back to its original form when released, you have a winner.

Safety
Building out the Whisker Sensors requires the use of cutting tools on metal, as well as the application of heat and chemical adhesives. I should not have to say that proper safety precautions should be followed at all times, such as wearing safety glasses when cutting metals or applying chemical adhesives in a well-ventilated area; but the world is full of idiots. So be smart; and if you are not capable of knowing and following a few simple safety precautions, close your browser window and take up underwater basket weaving instead.

Step 1 - Making the Parts
You will need one piece of metal tubing, approximately 3/8" (95mm) to 1/2" (127mm) in length for each whisker you will be assembling. The longer the tube is, the more sensitive the whisker will become as there will be more of the whisker rod that is available to flex for contact. Of course, the longer the tube is, the more room the whisker base will take up on your robot. You will also need to cut down a 6" (15.24cm) piece of whisker wire using your diagonals and two pieces of 1/32" diameter heat shrink tubing: a 1/2" (127mm) piece and a 1/4" (64mm) piece.

The first thing to do when making your omnidirectional tactile whisker sensor is to cut your metal tube to a more appropriate size. I recommend using a small diameter tubing cutter (available at most hobby stores) to accomplish this as your cuts will tend to be straighter and require less cleanup than using a saw. It also reduces the likelihood of crimping the tubing. Regardless of what tool you use to cut the tubing, you will need to clean up the edges using your files.

Place each piece of heat shrink tubing over the whisker wire and apply heat evenly to shrink the tubing down until it is closed tightly around the wire. Once the heat shrink tube has cooled off, you can remove each piece of heat shrink with gentle pressure, taking care not to distort the shape. It is a good idea to test to ensure each piece of heat shrink will fit into the metal tubing; if the heat shrink does not slide smoothly into the metal tubing you will need to use a smaller diameter heat shrink, likewise too loose a fit indicates the need for a slightly larger diameter (or thicker) heat shrink.

Step 2 - Assembly
Begin by lightly sanding and cleaning a 1/4" (64mm) section of each metal wire approximately 1/2" (127mm) up from one end (connector end). Apply a small, even layer of silicone adhesive to this area and slide the 1/4" (64mm) piece of heat shrink tubing into place over the adhesive. It is best to slide the tubing into place from the opposite end of the wire (whisker end) so that any excessive adhesive is pushed toward the connector end. This is piece of heat shrink tubing is your insulator. Allow the adhesive to fully set before continuing.


Slide the 1/2" (127mm) piece of heat shrink tubing onto the wire so that it rests against the insulator on the whisker end side of the wire. This second piece of heat shrink will hold your metal tube evenly. Apply an even layer of adhesive to the insulator and slide the metal tubing in place over it so that the metal tubing end falls just short of the connector end of the insulator. It is vitally important that the metal tubing be slid into place from the whisker end, lest you wind up with adhesive covering the entire inside of the metal tubing. Allow the adhesive to fully set before continuing.

Remove the 1/2" (127mm) piece of heat shrink from the assembly and check the Whisker Sensor to a good fit. The wire should be centered in the metal tubing, not sag when the assembly is held horizontally to the ground, and be capable of flexing easily to make contact with the edge of the metal tubing. If the wire hangs to one side or sags when held horizontally you can add small pieces of heat shrink into the metal tubing until it rests correctly. Use the 1/2" (127mm) piece of heat shrink to push each additional piece of heat shrink fully into the metal tube.

Step 3 - Connecting the Whisker Sensor
Mounting the whisker sensor onto a circuit board is the easiest method of connecting the sensor into your circuit. Using your needle nose pliers, bend the connector end to a 90deg angle and solder in place as you would any through-hole connector. The metal tube gets soldered in place to a surface pad to complete the circuit. I recommend attaching the wire connector to your ground and metal tube to supply voltage, just to be on the safe side. When soldering (or applying any heat to) your Whisker Sensor, it is advisable to reinsert the 1/2" (127mm) heat shrink pieces into the metal tube (seen below).

You can also solder wire directly to the whisker sensor to allow it to be mounted further away from your circuit board using standard freeform techniques. It is a little more difficult to solder a lead wire to the connector end of the whisker, but it provides additional freedom in mounting options. Just ensure you insulate the connectors appropriately with heat shrink.

When mounting your Whisker Sensor, either directly on the PCB or elsewhere, it is best to place each sensor at a slight (20-30deg) angle so that the whisker will flex toward the metal tube when contact is made.

Step 4 - Finishing Up
You will likely want a slight outward bend in your Whisker Sensor in order for it to be most effective. To do this, grasp the Whisker Sensor at the base as show in the first image below. Hold the whisker end between your thumb and forefinger in your other hand, then slide your hand out and away (toward your thumb side) from the base while pressing your thumb into the wire (careful not to cut yourself) as seen in the second image below. As with soldering the sensor, it is advisable to place the 1/2" (127mm) heat shrink back into place prior to making your bend, as this will help to keep the wire centered in the metal tube.

 

The last thing you may wish to do (read: optional) is to insulate the length of the whisker. This can be done by dipping the whisker wire into any of a number of compounds, such as polyurethane or PCB air drying varnish. Just be careful not to allow the insulating coating to drip down into the area of the metal tube.

Posted by Andrew Maxim | on