Basic Electronics, Reliable Wiring, and Mitigating ESD in FTC Robots FTC Kick-Off

Transcription

1 1 Basic Electronics, Reliable Wiring, and Mitigating ESD in FTC Robots 2014 FTC Kick-Off Andy Driesman FTC4318 Green Machine Reloaded

2 2 Agenda Required and types of Wiring Tips and tricks What are Electro-Static Discharges? Mitigating the effects of ESD.

3 3 Overview of Required FTC Wiring 3000 mah imh BATTERY SAMANTHA MOTOR CONTROLLER SWITCH TWISTED PAIR ANDERSON POWER POLE 20A FUSE USB CABLE w CHOKES

4 4 Wire Gauge (AWG) AWG = American Wire Gauge. Measure of the wire s diameter and current carrying capability. For power applications (motors, motor controllers) use 18 AWG. For signal applications smaller gauges can be used The smaller the gauge, the more fragile the wire and associated connectors. NEC copper wire Copper resistance Copper ampacity at Fusing current, copper Diameter Area (Ω/kft) 60/75/90 C AWG (in) (kcmil) (mω/ft) insulation (A)[7] 1 s 32 ms / 35 / ka 8.9 ka ka 7.1 ka / 25 / ka 5.6 ka A 4.5 ka / 20 / A 3.5 ka A 2.8 ka / / A 2.2 ka A 1.8 ka / / A 1.4 ka

5 5 Neatness Counts Poor Strain Relief Wires next to gears Wires tangled together Exposed Conductors and Bad connections

6 6 Wiring Best Practices Provide adequate strain relief Taught wiring is prone to break, and/or come loose from connectors. Keep wires away from moving parts (e.g.: gears) Can cause wires to break, and/or moving wires to stop. KZS Keep wires straight and untangled Allows for easy dressing of wires and trouble shooting. Avoid exposed conductors Potential for catastrophic shorts. Dress your wires Wires should be tied down in multiple spots to prevent issues. Use twist ties to chassis Always connectors to motors Connectors will fall off during game play.

7 7 Making Connections to the FTC Motor Controllers Allows connection for up to two NXT Motors. Each may or may not have an encoder. Screw connectors are an unavoidable problem. They: Don t hold tight. Don t open completely. Don t easily accommodate larger wire gauges. Allows for exposed conductors. Use 18 AWG wire Do not de-strand wire. It presents the possibility of a short circuit. Assure receptacle is fully open prior to inserting wire. Strip insulation such that there is no exposed insulation. Prevents shorts.

8 8 Making Connections to the FTC Motor and Servo Controllers Servo controllers allow up to six servos to be controlled by a single NXT Servo Connectors ARE NOT polarized. Follow the Yellow-Red- Blue label on the controller. Connections are provided for 12V Power in and one daisy chain. Daisy chains have the potential of catastrophic failures. Connections are provide for input and daisy chain of I2C NXT bus. Connectors are locking type which mitigates potential failures

9 9 Bus Bars and Daisy Chaining (1) X Motors X Motors X Battery Loss a single wire causes loss of all down stream motors or servos.

10 10 Terminal blocks and Daisy Chaining (2) Using a terminal eliminates daisy chaining and allows better strain relief resulting in more reliable wiring + - Battery

11 Tamiya Connectors Provided with the Tetrix Battery are Unreliable Tamiya Connectors Female Tamiya connectors provided with battery are prone to (fail) open after time. Recommended that battery (and charger) connectors are replaced with Anderson Powerpole connectors. Use 30A contacts. Special crimp tool required Tamiya connectors are used on Samantha as well, but they cannot be easily replaced. Use strain relief and minimize un-needed connects and dis-connects. Anderson Powerpole Connectors 11

12 Fusing and Battery Protection 3,000 ma*hr x 12V = 36 W*Hr Thought internal battery resistance limits current there is enough energy in a battery to lift a 25 lbf robot 3,500 feet in the air!! Bottom line Don t mess with the fuse!!! Don t mess with the battery!!!! Be mindful of any sharp edge (e.g.: screw head, channel edge), that can come into contact with the battery. Batteries should be placed on a flat non-abrasive surface. Vibration will cause the edge to cut the battery insulation. Batteries can and will catch on fire if miss-handled. Fusing Battery is fused at 20A. Though it is rare to blow a fuse, always bring extras. Most common reason for blowing a fuse is inadvertent short or stalling multiple drive motors. 12

13 Electro Static Discharge 13

14 14 What is Electro-Static Discharge (ESD)? ESD is the sudden flow of electricity between two objects that have been electrically charged to different voltages. It is caused by contact (or proximity) between the two objects. Lightening is a form of ESD as is getting a shock from a door knob. It is possible to generate voltages in excess of 10,000 volts (10 KV).

15 15 What causes ESD in FTC Robots? The primary source of voltage build up in FTC robots is triboelectric charging. This is caused by rubbing dissimilar materials together. (e.g.: NXT wheels on field matt, manipulators used to gather field elements, conveyors internal to the robot, etc.) Effect is exacerbated by low humidity. Due to the complexity of the physics, it is difficult to predict the polarity of voltage, the rate/amount of charge build-up.

16 16 Why is ESD a serious problem? Observed symptoms of ESD in FTC Robots Hard lock-up of the NXT requiring a battery pull to reset Upset of Samantha Twitching servos

17 ESD and its Effects are not Intuitive Even if a robot doesn t charge, it is still susceptible to another robot that has charged. Charging in and of itself is not typically the problem, rather it is the sudden discharge. Generates large electric and magnetic fields Discharges do not require physical contact to propagate and cause large voltage and/or current spikes in robot electronics; including reversed polarity. In summary, nasty things happen! 17

18 18 Preventing and Mitigating ESD Unless there is a fundamental change to the materials (e.g.: FTC playing surface), ESD cannot be prevented; only mitigated.

19 19 Best Practices Wiring (1) Always use a ferrite choke on the power wires to the Samantha module. Be careful not to crimp the lines when clamping on the choke. Always use a USB cable that has two ferrite chokes, one at each end. Readily available from multiple internet sources (e.g.: Amazon) Keep the USB cable and wires in general as short as possible while still maintaining good wire/cable management techniques.

20 20 Best Practices Wiring (2) Always twist wires (positive and negative) that are carrying power Wires (e.g.: Motor wires) are often delivered this way. Twisted wire can be ordered or easily made. Good solid grounding between all components on a robot is a good-practice that yields a system that is both more reliable and more resistant to ESD.

21 21 Best Practices Other (1) Ensure that no part of the electrical system is able to be contacted by any other external object during game. While it is impossible to avoid the generation of static, consider affinity when selecting materials. This goes for materials that will rub one another within the robot or external to the robot. When possible, fields should be sprayed down with antistatic spray. Tech-Spray Anti Stat 1726QT is used industrially and in other robot competitions Be careful when applying too much or too often as the field can become tacky from spray build-up.

22 22 Best Practices Other (2) If you are consistently getting shocked by your robot, consider discharging your robot with a conductive wand prior to touching the robot. Attaching a 1MΩ to 10MΩ resistor attached to any piece of metal will do. FTC4318 screws a 1MΩ to a 288 mm flat bar. Under current field conditions, trailing bleed wires are to be avoided. They don t work and risk entanglement. Humidity is your friend. Increasing humidity levels to >40% is best.

23 23 Acknowledgment FTC4318 would like to thank engineers at the Johns Hopkins University Applied Physics Laboratory, the Garret Engineering and Robotics Society, Battel Engineering as well as others in the FTC community for their advice, support and mentorship with regards to managing static and ESD.

24 Back-Up 24

25 25 References Designing Robots for an ESD Environment, A. Driesman FTC4318. Source for Anderson connectors:

26 26 The Tribo-Electric Series A rough estimate of charge build up can be made by using the tribo-electric series. Materials further apart will charge up quicker and to higher voltages. Tribo-Electric data for some common robotics material: Material Polyurethane Foam +60 Nylon +30 Polycarbonate (Lexan ) -5 ABS Plastic -5 Gum Rubber -60 Flexible Vinyl (field surface) -75 PVC (rigid vinyl) -100 Affinity (nc/j)