RoboTag^TM Thoughts on Implementation

Photo sensors in the Target Sensors

• The photo sensors in the target sensors need to be capable of working over a very wide dynamic range.  All the way from a 0.5 mW 635 nm Red Laser at 90 degrees off perpindicular at 20 m to a 5.0 mW 670 nm Red laser at point blank range without saturation of the photo sensor or the amplification / filtering circutry.
• Many helpfull hints on handling such a wide input range can be found on discussions of implementing IRDA links.
• Of inportance to detection at angle very far away from perpendicular are the lens losses, or diffusion losses incured in the structure used to gather high incidence angle laser beams into the photo sensor.
• It may seam strange, but placing a glob of slightly opaque silicon caulk on top of the photo sensor may infact help improve the ability of the electronic circuit to handle such a wide input range.  It will help increase high incidence light gathering, and attenuate low incidence light.  This is the method I'm going to try first.
• Filtering of the light hitting the photo sensor.
• It may be desireable to use a IR cutoff filter to remove the light pulses generated by IR wiskers.
• It may also be desireable to use a Red Pass filter (Most red pass filters don't cut off IR light.) to get rid of all visible light outside of the range of the red laser diodes allowed spectrum.

Finding that pesky Target in the sea of light out there

• I chose the target spectrum deliberatly so it dosen't interfere with IR wishers and the Laser Beams.
• The 15 Hz stobing frequency of the Target LEDS is chose so it can be detected by
• The spectrum chosen is easily selected out by using the proper Green Pass filter.
• With a cheep photo transistor, lense, and a green pass filter it would be easy to look for the 15 Hz output of the Target LEDs.
• With a BW video camera more accurate positioning can easily be attained.

Aiming the Laser

• A simple base of two micro RC servos can be used.
• For lower cost implementations one micro servo for verticle and turning the robot body for lateral aiming.
• Fixed laser mounting isn't recomended as the sensors can be at different heights on different robots.
• Homing in on the exact location of the Target can be done by turning on the laser in an aming mode (no modulation), then firing it when you acheive target lock.
• Note there isn't anything in the rules to prevent someone from having an array of sensors all over ther robot to tell them when it's being aimed at, then scooting real fast out of the way.
• If your wish you can do extensive calibration of your input sensor to laser aiming hardware to allow a quick direct hit with one shot.
• A simple laser aimer I though of would combine a set of photo diodes in an array with a lense in front of them.  This array would be swepted across the area where it is thought that a robot is.  When one of the sensors detects the Target LED lights, the control circuit would then home in on the target and fire the laser.

Sensor interference

• As there will be many different robots all running in the same area, interference between sensors is a destinct possibility to the point of certainty.
• Spread spectrum type slewing of signals can be used, but it could be a real bear to get working, but still possible.
• With a MCU, IR LED, photodiode, and OP-AMP filter/amplifyer with compairator it should be possible to make a IR wisker that can operate at many different frequencies.
• If designed right it can even be used to give a reasonable distance measure.
• I personally think passive type sensors will provide a much better means of sensing other robots and the environment.  Much less chance of interference.  Also much less likely to give away your location.

• First see the documents in the Safety section.  They contain inportant information on lasers and possible health issues involved with them.  Specifically, but not limited to possible eye dammage.
• CDRH (FDA) Regulations require warning lables, and emission indicators that are plainly visible.

• Laser diodes, and most laser modules are highly Electro Static Discharge (ESD), and power surge sensitive.
• For protection against power surges, I recommend a multi tiered approach.
• First, don't even think of powering it with the same supply that you power your motors or RC servos with.  Spikes that don't even show up on an ossiliscope can destroy a laser module or laser diode.
• I seriously think that the same supply you use for the digital logic on your robot may be to noisey.
• I reccomend using some sort of solid state device to turn on and off power to the laser module.  This is to avoide spikes caused by switch and relay contact bounces. (That's the only thing I can attribute the death of one of my laser modules to.)
• I'm looking at using a quality linear low dropout voltage regulator with a shutdown control for switching laser module power on and off.
• To help filter the ground line some, while maintaining electrical connection, I'll be using a small ferite bead between the batery pack's ground (Also the digital logic's ground) and the laser module ground.
• The shutdown control will be connected to a pin on the MCU on my robot.  It will be held low till I'm ready to use the laser module.
• To keep me from energising the laser module signal line with the module turned off, I AND the signal line and power shutdown lines with an AND gate before going to the signal line on the connector.
• If you are doing surface mount you can get single AND gates in super small packages.  Digi-Key sells them.
• For protection from ESD dammage, I'm applying some tecniques used to protect CMOS circuits, as well as using a plug that keeps phisical contact away from any metal surfaces connected to the circutry.
• WARNING, many laser modules have the outter case electrically connected to the circutry inside.  If it dosen't say its isolated, assume the case is electrically connected to the internal circutry.
• To give a measure of protection to a CW laser module that I have, I used a small piece of heat shrink tubing slipped over the metal case, but NOT heat shrunk.  WARNING Actually heat shrinking the tubing will likely cause the tempeture sensitive mirrors on the laser diode to break down.  If they break down the laser module becomes an expensive LED.
• I personally think it's worth it to buy a module that is electrically isolated.
• The CMOS ESD protection structure I plan on using, uses a series inline resistor with two voltage clamping diodes, one to GND, and the other to Vcc.
• Note: The inline series resistor will effect the signal slew rate and thus cap the maximum efective requency of transmition.
• If needed I will add a schmidt trigger to the signal line to cleanup the rise slope for the module.
• The plug I've selected for connecting my Laser Diode to power is a 3 circuit Molex Waldom 0.120" Pocket Connector Plug.  (Note: include photograph or drawing to show parts and final assembly.)
• It has a clip on one side which will hold it in place when connected, which also helps me keep the polarization correct.
• It covers over the electrical contacts with it's housing.
• The lines will be aranged GND, Signal, Vcc.
• I can build a small PCB that has the resistor, and diodes on it, along with a power on LED, and signal LED.
• I will crimp/solider the contacts for the Plug Housing so they have different heights for GND/Vcc versus signal.  This will allow me to get power and ground connected before I connect up the signal line.  (Which is more sensitive to ESD than the power/ground lines.)
• The top half of the Plug Housing, PCB and a short length of the wire will be covered with clear heat shrink tubing for electrical isolation and viewing the power and emission LEDs.

• A document from OSHA http://www.osha-slc.gov/OshDoc/Directive_data/PUB_8-1_7.html
• A nice page with many Laser related Formulas http://www.wilsonind.com/safetydata.htm
• Center for Devices and Radiological Health (FDA http://www.fda.gov/)
• x http://www.fda.gov/cdrh/radhlth/index.html
• Check out the sub heading "Laser Products, Including Laser Light Shows and Displays"
•     Performance Standard - Lasers and Products Incorporating Lasers
•     Performance Standard - Specific Laser Products (Includes Display, Survey, and
• Medical Laser Products)
• Laser Pointers
• Tabulated Values of Emissions (for hazard classification)
• Compliance Guide
• Notices to Industry
• Reporting Guide - Radiation Safety Product Report for Laser Products
• Variances from Standards
• Variance Application for Laser Products used for Light Shows and Displays
• Reporting Guide - Radiation Safety Product Report for Laser Light
• Shows/Displays
• Laser Light Show Safety: Who's Responsible?
• Reporting Guide - Annual Reports
• Laser Quality Control Guide
• General Information on Importation and Exportation of Electronic Products which
• Emit Radiation
• General Information on Reporting and Recordkeeping
• x
• x
• x
• x
• x
• x