Fur in Motionv1.1

Nordic Fuzzcon – 28/02/2019

Floere T. Pillowcase, Devourer of Automobiles (floere@robocow.be)

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Disclaimer

This presentation is intended for educational purposes only and does not replace independent professional judgement.

The presenter, nor the convention, nor RoboCow Industries assume any responsibility for the content, accuracy or completeness of the information presented.

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What is this Talk About?

● A basic introduction on:

– How to drive DC motors and RC servos– Power solutions– Safety

● The focus is on the WHAT and WHY, rather than on the HOW

● These slides can be downloaded after the talk:

– https://www.robocow.be/events/

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● Celebrate the craftmanship in our community!

– I’ll be there with my first animatronics WIP– There’s beer brewing, waffle nomming and a

variety of untimely-yet-hilarious demises● Bring safety goggles and a hard hat *

● Don’t forget to pay-up your insurance!

● Re-load the website. Catch them all…https://fluufff.org/

* Survival with all body parts intact is not guaranteed. Hugs at your own risk.

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Content

● MotionTypes of motors and how to drive them

● PowerPower sources and stability

● No FireHow to stay alive to tell the story

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Content

● MotionTypes of motors and how to drive them

● PowerPower sources and stability

● No FireHow to stay alive to tell the story

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Let’s Watch Some Videos...

● WMW66 Costumes – Animatronic Fursuit Headhttps://www.youtube.com/watch?v=eEEZLMv56xQ

● Hemms Fox – Coolest Fursuit Everhttps://www.youtube.com/watch?v=_stG5w_ExAs

● ElminsCosplay – My Giant Motorized Aether Wing Kayle Cosplayhttps://www.youtube.com/watch?v=BT5mDQA6gnY

● mostudio – Animatronic Lion Full-Head Maskhttps://www.youtube.com/watch?v=evcbnY3Cl90

● Control is clearly the issue, but that’s next year’s subject!

● The good news: we don’t need Hollywood budgets

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Motors for Fursuits

● General Input/Output Taxonomy:

– Input power: AC or DC● All motors are AC on the inside● Alternating fields are generated using an AC supply or

made from a DC supply (using brushes and a commutator or electronically)

– Output shaft: geared or direct– Output motion: rotation or linear

● Control: open-loop or closed-loop (e.g.: servos)● Drive circuit: highly motor dependent

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Motors for Fursuits

● Costume applications will generally use:

– DC motors with a gear box– RC servo motors (= small DC motor + gear

box + motor driver + feed-back control)● Easy to get, easy to drive and cost-effective● Low operating voltages handy with batteries● Integrated gearboxes yield high torque (N/m)● Rotational and linear motion available

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Types of (Geared) DC Motors

Plain motor● High speed● Low torque

Geared motor● Low speed● High torque

Linear actuator● Low speed● High force● Linear motion

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Anatomy of a DC Motor

https://learn.sparkfun.com/tutorials/motors-and-selecting-the-right-one/dc-brush-motors---the-classic

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Anatomy of a DC Motor

● Advantages:

– Simple and generally “inexpensive”– Good starting torque– Geared versions readily available– Easy to drive

● Disadvantages:

– Brushes wear out– Sparking causes interference– High stall current can cause issues

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DC Motor Stall Current● When the motor can’t

turn, the current becomes very high

– Only limited by the resistance of the winding and brushes

● When the motor runs, the current is lower

– Given by the resistance of the winding + brushes and the back-EMF

https://www.pololu.com/product/1117/faqs

Cur

rent

Spe

ed

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DC Motor Stall Current

● Rule of thumb: limit the continuous operation of small motors to 20-30% of the stall current

– Unless the manufacturer data says otherwise● Gear boxes can be damaged at stall torque

● Motor windings and brushes can overheat

● Mind your electronics!

– Fuses will be too slow to protect the power drivers, they may need active current limiting

● Fuse and wire gauge selection is important with high-current capable batteries

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Types of RC Servo Motors

Regular servo● Rotation● < 10 ms for

60° exists!

Linear servo● Push-pull● ~20 mm/s

max speed

Control boards● For generic

actuators with feed-back pots

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Anatomy of a RC Servo

http://www.robotpark.com/academy/servo-motors-51057/

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Driving RC Servo Motors● Pulse-Width Modulation

– The ratio between on and off is changed (modulated)

– Expressed in % duty cycle

● With RC servos: timing defines the set-point (= desired position)

– Eg: 1.5 ms = 90° centre

– 1 ms – 2 ms → 0° - 180°

– Usually updated every 20 ms (= 50 Hz)

● Control sources:

– MCU (eg: Arduino)

– Dedicated controller

– RC remote control

Word of Warning

● RC servos are open-loop with respect to the driving controller

● There is no feed-back if the set-point is not actually reached!

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Protecting RC Servo Motors

● The gear train has some fragility

– Forcing it can strip gear teeth

● Protect gears from excessive force

– Servo saver– Spring linkage

● Cheap servos can over-heat due the motor stall current

Traxxas

Kimbrough

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Driving DC Motors: Switches● NC limit switches + diodes

+ DPDT switch

– Control direction and travel

– Use relays for:● Bigger motors● MCU control

● Switch variations

– on – on

– on – off – on

– (on) – off – (on)● DPDT = double-pole

double-throw

● NC = normally-closed

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Driving DC Motors: PWM● H-Bridge using PWM

– Control speed and direction

– Replaces DPDT switch● Position / travel control

– Use electronic position feed-back (eg: pot)

– Keep the limit switches!● Protection is important

– Back-EMF (FWD) diodes

– Over-current

– Overheating● Best use COTS modules!

Word of Warning: avoid the old L298

● No real over-current protection

● High voltage drop (~4V @ 2A)

– Lots of heat

– Limited use with 1S, 2S LiPo

– Acts as a current limiter...

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Protecting DC Motors● Gear boxes can break

– Limit torque using a slip clutch, spring, shear pin, etc...

● Consider a PolyFuse to protect against prolonged over-loads

● Motor electronics:

– Able to accommodate the stall current

– Protected againstback-EMF, U, I, T

● Observe the section on fire safety!

MCD Pro-Bite

DAGU RS022

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Content

● MotionTypes of motors and how to drive them

● PowerPower sources and stability

● No FireHow to stay alive to tell the story

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Sources of Energy

● Batteries for now…...which are a mess

● Motors require:

– Significant energy → Watt-hours

– High peak current → Amps or xxC

● Fuses require high short-circuit currents

● Most costumes need small power packs

● Usable battery types

– NiMH

– LiFePO4

– LiPo– Alcaline + supercap– Pb gell cell / AGM

● Usable ≠ safe!● Air travel restrictions

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Sources of Energy● In practice:

– NiMH

– LiFePO4

– LiPo● First two are much

safer than LiPo● Last has ~double

the energy density● All exist in versions

that can source high (peak) currents

● Battery internal resistance is key

– Cell voltage drops when loaded

– Udrop = Rcell x Iload

● Maximum discharge current is important

– In Amps or xxC● Read the battery

data sheet!

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27 / 62Panasonic

Charge Time (min.)

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Enix Energies

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Enix Energies

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Power System Stability● Battery and wiring

have resistance● High motor start

current will cause bus voltage to drop

– Deeper, partial dips are “brown-outs”

– This has an adverse impact on the other system components

● Impact of voltage fluctuations:

– MCUs can crash– Analog circuits

can have their operating points disturbed

– General undefined and unexpected behaviours and interactions

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Basic Suit Power Bus Design

● Star Ground● Isolate sections● Buffer capacitors● Sufficient wire gauge● DC/DC converters

– Eg: buck-boost● Multiple power

sources (avoid, method of last resort)

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Stupidity Diode

● If your batteries are:

– Loose cells– Can be connected

in reverse● Put a Schottky diode

in series! (After the battery fuse.)

● Not always practical

– Voltage drop(0.2 V – 2 V)

● Consider P-MOS protection circuit

– Use FET with low enough Vth!

Infineon: “Automotive MOSFETs – Reverse Battery Protection” June 2009

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Content

● MotionTypes of motors and how to drive them

● PowerPower sources and stability

● No FireHow to stay alive to tell the story

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Danger Will Robinson!

● There are many things that can go wrong when adding electricity, electronics and mechanisms to a costume.

● A more extensive, but certainly non-exhaustive, discussion can be found in: – “Silly Electronics for Fun and Absolutely no Profit”

Both the V1 and V2 series of talks

– “Paws-On Basic Fursuit Electronics Workshop WIP”

● Here, only some highlights regarding fire prevention when using high-current loads and high-capacity batteries are discussed.

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Common Sources of Fire

● Batteries● Wiring● Electronics● Electro-mechanical

components

● Root Causes:

– Joule heatingP = R x I²

– Chemical● Thermal

runaway● Ignition of H2

● Ignition of Li– Arcing

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LiPo Battery Fire

● LiPo batteries are VERY sensitive

● Do not abuse

● DO NOT ABUSE!

● FOR THE LOVE OF ALL THAT IS DEAR, DO NOT ABUSE A LiPo!!!

● Use protected cells if at all possible

– Usually limited to 2C and much less…

● But, really, just DON’T use LiPo in a costume!

https://www.youtube.com/watch?v=gisdMQbtJqk

Also, have a good look at the link below. It is a series of test ofvarious, common LiPO storage options:https://www.youtube.com/watch?v=CnNId0mDnBo

https://www.youtube.com/watch?v=gz3hCqjk4yc

Adafruit Industries

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Battery Selection● NiMH

– Restrained failure modes

– Resistant to much abuse

– Energy density ½ LiPO

– Weight at least 2x LiPO

– Lower discharge current vs LiPO

– Long life (1000 cycles)

– Caution: may still vent, explode, etc…

● LiPo

– Vigorous failuremodes

– Highly sensitive toabuse

– High energy density

– Light-weight

– 50C and up discharge current

– Short life (few 100 cycles)

https://rogershobbycenter.com/lipoguide/

Consider LiFePO4as a safer middle-ground

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Batteries - SSDD

● Consider them all as dangerous and follow LiPo handling safety recommendations

● Isolate them from your body and by-standers

– Mechanicallygoing in → impacts, deformationgoing out → shrapnel, flames, smoke, fumes

– Thermally● Implement a safety “pack eject” procedure

● Store and charge them in a suitable, fire-proof container

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LiPo (and others) Safety● Insure mechanical +

thermal protection

● Always use a LiPo bag

● Use the smallest possible battery

● Use a battery fuse

● Use an under-voltage detector (+ cut-out)

● Keep the pack external to your suit

● Have a quick-eject for the pack (and use a separable connector)

HobbyKing

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Battery Connectors● The RC people have this

figured-out

– Several options

– High-current types must be soldered

● Solder them properly

– Safety-critical!

– Tinning thick wires takes special care

– Follow a tutorial!● Or buy pre-assembled

● Don’t cheap-out!

https://www.rchelicopterfun.com/rc-lipo-battery-connector.html

Traxxas Deans EC3

Amass XT-30

https://rogershobbycenter.com/lipoguide/

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Wire Selection● Main battery wiring → high-flex silicone

● Use sufficient gauge

– Allows fuse to blow

– Prevents wire from over-heating and the isolation from catching fire

● Max. current limit set by fuse→ for the portion AFTER the fuse

● Max. current limit set by battery→ for the portion BEFORE the fuse

● Conservative rule-of-thumb:700 circular mil / A = 0.35 mm² / A

● Temperature coefficients

– Cu: ~0.00386 per °C

– Al : ~0.00429 per °C

● (Ohms @ T) = (Ohms @ 20 °C) x [1 + (T – 20 °C) x coeff]

AWG Ohms per km0000 0.46 11.684 107 0.049 0.16072000 0.4096 10.40384 84.9 0.0618 0.20270400 0.3648 9.26592 67.4 0.0779 0.2555120 0.3249 8.25246 53.5 0.0983 0.3224241 0.2893 7.34822 42.4 0.1239 0.4063922 0.2576 6.54304 33.6 0.1563 0.5126643 0.2294 5.82676 26.7 0.197 0.646164 0.2043 5.18922 21.1 0.2485 0.815085 0.1819 4.62026 16.8 0.3133 1.0276246 0.162 4.1148 13.3 0.3951 1.2959287 0.1443 3.66522 10.6 0.4982 1.6340968 0.1285 3.2639 8.37 0.6282 2.0604969 0.1144 2.90576 6.63 0.7921 2.59808810 0.1019 2.58826 5.26 0.9989 3.27639211 0.0907 2.30378 4.17 1.26 4.132812 0.0808 2.05232 3.31 1.588 5.2086413 0.072 1.8288 2.63 2.003 6.5698414 0.0641 1.62814 2.08 2.525 8.28215 0.0571 1.45034 1.65 3.184 10.4435216 0.0508 1.29032 1.31 4.016 13.1724817 0.0453 1.15062 1.04 5.064 16.6099218 0.0403 1.02362 0.823 6.385 20.942819 0.0359 0.91186 0.653 8.051 26.4072820 0.032 0.8128 0.519 10.15 33.29221 0.0285 0.7239 0.412 12.8 41.98422 0.0253 0.64516 0.327 16.14 52.939223 0.0226 0.57404 0.259 20.36 66.780824 0.0201 0.51054 0.205 25.67 84.197625 0.0179 0.45466 0.162 32.37 106.173626 0.0159 0.40386 0.128 40.81 133.856827 0.0142 0.36068 0.102 51.47 168.821628 0.0126 0.32004 0.08 64.9 212.87229 0.0113 0.28702 0.0647 81.83 268.402430 0.01 0.254 0.0507 103.2 338.49631 0.0089 0.22606 0.0401 130.1 426.72832 0.008 0.2032 0.0324 164.1 538.24833 0.0071 0.18034 0.0255 206.9 678.63234 0.0063 0.16002 0.0201 260.9 855.75235 0.0056 0.14224 0.0159 329 1079.1236 0.005 0.127 0.0127 414.8 136037 0.0045 0.1143 0.0103 523.1 171538 0.004 0.1016 0.00811 659.6 216339 0.0035 0.0889 0.00621 831.8 272840 0.0031 0.07874 0.00487 1049 3440

Conductor diameter inch

Conductor diameter mm

Conductor cross-section mm²

Ohms per 1000 ft.

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Switch / Relay Selection● Usually rated for AC + DC

– DC rating is relevant, at the system voltage

– Inductive loads (motors) reduce the rating

– (Tungsten) lamp loads are worst → near-short when cold (like a capacitor)

● Do not cheap-out!

– Contacts spot-weld on

– Burned-in contacts overheat

– DC switching is much harder than AC as there are no zero-crossings to extinguish the arc www.nkkswitches.com/pdf/electricalratings.pdf

Apem

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Switch / Relay Selection

● Select the right contact plating material!

● Silver for high currents

– Ag contacts oxidise

– Arcing cleans the contact plating

– No arc = problems● Gold for low currents

– Au does not oxidise

– Arcing destroys the contact plating

www.nkkswitches.com/pdf/electricalratings.pdf

NKK - Small Toggle Switches

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DC Arc Demo Video

● Partial, slow opening of a knife switch

– Contact distance ~representative of a typical, compact AC switch

● 150 V DC, 10-20 A● Resistive + inductive load (15 mH + 8 Ohm)

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Fuse Selection● Automotive ATO blade

fuses are convenient

– Up to 30 A (40 A)– Fairly slow-blow

● Some voltage drop is inevitable

● Understand I²t curves!

LittleFuse

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Fuse Holder Selection

● Make sure it is rated for the nominal DC current

● Buy in-line fuse holder with factory-crimped wires if possible

– You need a special tool to do this well!

● The holder has some contact resistance

– Do they mean in total or per contact?

– Probably excludes the wire

Multicomp

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The PolyFuse● Not your main battery fuse

● Good to protect motors, etc...

● Self-resetting

● Gets hot during fault→ enclose in a box!

● Mind the Ron!

Littlefuse

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Short-Circuit Current

● Must be able to blow the fuse when a short occurs

– Need sufficient current to do this!

● Ishort = Ubat / Rsum, where:

– Ubat = battery voltage

– Rsum= sum of all resistances

● Calculate at lowest Ubat and highest Twire

● Caution: partial short may not blow fuse!

● Say: 1S LiPo = 2.0 V min.

– Rbat = 20 mΩ

– Rfuse = 10 mΩ

– Rholder = 10 mΩ

– Rswitch = 10 mΩ

– Rwire = 2X 20 mΩ

● Ishort = 2.0 V / 90 mΩ = 22 A

● Fuse of 10 A blows in 0.5s → OK

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Dynamic Impedance

● Semiconductors are very non-linear!

● Diode voltage drop depends on current

● MOSFET regions: triode - saturation

– Can desaturate!– Limits current, but

lots of heat!● Huge impact on fuse

calculation!

ON

-Sem

i 1N

400x

Vis

hay

IRF

640

SaturationSaturation

Triode

Triode

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Power Distribution Fusing

● Battery fuse protects the battery and main bus wire

– It does usually not protect the various loads, (if more than one)

● You need a fused power distribution box!

– Same process as main fuse selection, but computed for each load group individually

– Offers selectivity!→ a section can fail without taking-down the rest of the system

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Build Quality

Work Cleanly!Safety and Reliability utterly depend on it!

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Wiring Harnesses● Bundle wires and use

connectors

– More reliable– Less restriction of

motion– Easier for suiting-up

and down– Easier for suit

cleaning– Easier for testing

● Choose properly-rated connectors, though!

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Wire Termination● Use insulated crimp

terminals for fast-on and screw connectors

● Do not tin stranded wire for use in screw connectors

– Solder is not elastic and wires will work loose

– Use wire-end ferrules!

● Do tin wires for soldering

● Use lots of heat shrink!

● Use a good crimp tool

– Use the ratchet type!

– The “pliers” type is unreliable

To solderTo solder To screwTo screw

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Strain Relief

● Soldered joints are stiff due to the solder wicked into the wire strands

– Stress fatigue will happen fast

● Thin wires have little pull strength at the joint (thin conductor)

● Take the strain by clamping the wire some distance from this point

● Need to observe the wire minimal bending radius as well!

● Relieve various pulling forces through smart cable routing

– Use ‘S’ or ‘U’ wire routing to add ‘stretch’ to the wires!

● Follow body plan (e.g.: spine) to reduce motion

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Electronics Enclosures● Things may get hot

● Parts can explode

– Shrapnel, nasty fumes

– Electrolytic capacitors are especially nasty

● You will sweat, bump into things, spill drinks

– Partial shorts, corrosion● Prevent fire, burns, injuries

but also improve reliability → put it in a box!

● Good base to attach strain relief, connectors, controls

Kaiborg Studios

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Dealing with Sweat● It gets everywhere!

– Both conductive and corrosive

– Leaves salt deposits● Remediation

– Conformal coatings on circuit boards

– Adhesive-lined heat shrink (stronger splices!)

– Suitable, IP-rated switches, enclosures, connectors, etc...

White = salt depositWhite = salt deposit

Tarnished copper in spliceTarnished copper in splice== higher contact resistance! higher contact resistance!

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Brief - Chemical Risks

● Batteries

– Violent exothermic reactions (esp. LiPo)

– Prolific quantities of fire and smoke

– Electrolytes vent when overheating

– Caustic / acidic spray + vapours

– Shrapnel may eject

– Combustion products?

● Electrolytic capacitors

– Electrolyte vents when overheating

– Caustic / acidic spray + vapours

– Shrapnel may eject

– Combustion products?

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Brief – Mechanical Risks

● Mechanism

– Guard against the intrusion of body parts (yours + others)

– Beware of long hair

– Parts can break and shatter → shrapnel

– Limit maximum force and speed

– If, say, a wing hinge pivot breaks, do you end-up with a rapier going into your body?

● System Build: what if you fall or others fall on you?

– Sharp edges

– Long, narrow structures can impale you + others

● Screws, horns, etc...● Trim, cap, or mount

so they collapse or easily detach

– Padding compresses when you fall on it!

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Always

● Keep your batteries in a fire-proof bag

● Use properly-rated fuses, the main one right at the battery

● Use properly-rated connectors and wires

● Enclose your electronics

● Work cleanly

● And for all that is good in this world:

– Respect batteries – Have a safety

STOP switch readily at hand

– Have a battery safety cut-out + ejection solution

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So, We’re All Gonna Die?

- NO -

The risks can be largely mitigated.All it takes is to be aware of them,

and to ask for / offer help where needed!

We’re Furries! Let’s do what we do best:help each other and have fun!

And do the Safety Prance https://www.youtube.com/watch?v=5dIu8gHkzEs

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Questions? Ideas? Let’s Talk!