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a pragmatic guide to making your world move with motors!

I like to move it, move it:

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Who am I?

Jonathan Beri ★ @beriberikix ★ +jonathanberi
Maker since we use to call ourselves nerds
Co-author, “Make: JavaScript Robotics”

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Agenda

Why this talk?
The plan
Attributes of motors
Brushed DC
Stepper
Servo
Brushless DC
What next?

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Why this talk?

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The plan

Focus on tl;dr of control
Most common types of motors:

Brushed DC
Stepper
Servo
Brushless

Save Closed-loop for another talk
Develop on hackaday.io/project/8116, github

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Attributes of motors

body dimension

shaft dimension

mounting options

shaft shape

double ended shaft

extended shaft

threaded shaft

cost

gear

pulley

vibrating weight

operating temp

availability

sourceability

starting operating voltage

no load speed

loaded speed, current

no load current

stall current

starting torque

stall torque

operating temp

alternative form factors

operating voltage

encoders

built-in controllers

number of coils

bearing type

gear material

type of encoder

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Brush DC

I'm sure we've all seen Brushed DC (BDC) motors at some point - those little silver stumps with a blunt needle on one and and two wires on the other. These are by far the most common type of motor you'll come across because they're cheap, easy to control and readily-available. They're often called toy or hobby motors because both their cost and ease of use make them great for toys.

Protip: always use a flyback diode. It will prevent your controller from going boom when stored energy in the motor needs a place to go. Also, many App Notes add filter caps as optional but they're a good thing to use if you're in an embedded context where RF noise can mess you up (as opposed to say, an industrial HVAC.)

src: http://www.aliexpress.com/item/DC12V-DC-Motor-and-N20-Motor-Speed-Good-Stall-Characteristics-of-Stepping-Motor-Metal-Gear-Model/32337902961.html?spm=2114.01020208.3.1.AaanFM&ws_ab_test=searchweb201556_3_21_79_78_77_92_91_22_80,searchweb201644_5,searchweb201560_9

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Brush DC: overview

PROS

$ - cheapest discussed today
Basic speed control is easy
Wide-range of form factors
Low to high torque/voltage options
Plenty of add-ons (gear boxes/encoders)

CONS

Brushes can go bad
Lacks precision motion (needs closed-loop)
Magnets == heavy for some applications

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Brush DC: voltage-sensitive devices

Apply enough voltage across a BDC, it moves
Apply more voltage, the motor moves faster
Reverse the voltage, the motor moves in the reverse direction
Remove the power and motor moves freely, aka. "freewheeling"
Stop the motor quickly like a car "brake" with a resistive load to dissipate stored energy

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Brush DC: discrete

Variable resistor

Rotary switch + resistors

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Brush DC: h-bridge

Need more power? Use a Power Mosfet - just don’t forget the heatsink!

Parts like transistors and 555s will allow us to control the speed & direction of BDC motors with decent accuracy + reliability.

The key concept here is the humble h-bridge: an arrangement of transistor pairs that can toggle the flow of power in both directions of a motor. Oh, and it looks like an H in diagrams.

What type of transistor you use depends on the current draw of your motor (look at stall current.) Small hobby motors with light loads can use common BJT PNP/NPN-type transistor (NPNs are cheaper and active-high.) But for anything remotely power-hungry, you'll want to use Power Mosfets. P-channels are a go-to for high-side switching and something like a FQP27P06 can handle 27A with proper heatsinks. That's a hefty motor.

src: https://cdn.sparkfun.com/assets/learn_tutorials/1/9/3/h-bridge-circuit-600w.gif

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Brush DC: h-bridge drivers

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Brush DC: 555 for PWM

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Brush DC: drivers & MCUs

Hardware PWM

Fancy Driver

Protip: use a digital potentiometer when you’re modding

src: http://bildr.org/blog/wp-content/uploads/2011/03/tip120-motor.png

src: http://www.ti.com/ds_dgm/images/fbd_slvsab2f.gif

Utilizing a MCU for motor control makes a lot of sense since it allows for advanced control logic, integration with peripheral components and even more precise control over the output with techniques likeBinary Code Modulation.

Fundamentally we use the hardware PWM generator found on many common MCUs. That means we could easily use the H-bridge we mentioned before. H-bridges will work great for many MCU integrations. If we are limited on I/O pins or processing overhead, we need to switch to specialized motor drivers.

Before you object, you should know that H-bridge ICs are considered drivers too. However, we need to differentiate parts that provide additional logic and/or communicate over I2C/serial.

These drivers generate their own PWM based off serial instructions, saving hardware interrupts, a few pins and general overhead. Plus, if they're the I2C flavor, we can chain multiple controllers together. Examples of both serial and I2C-controlled include the DRV8823 & DRV8830, respectfully

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Brush DC: lots of motors

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Stepper

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Stepper: overview

PROS

$$ - positional accuracy costs but printers are bringing this down
Rotate to very accurate positions (microsteps)
Many controllers for use with MCU
Comes in standard sizes defined by NEMA

CONS

Can be $$$ - high-precision, high-torque
Usually too heavy for light drones
Many coils == more power

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Stepper: 2 motors, basically

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Stepper: discrete

Any H-bridge will work - BJT, Mosfet or Driver

src: http://makezine.com/projects/projects-in-motion-control-three-types-of-motors-with-555-timers/

In a basic Bipolar motor, you cycle through the coils by energizing 1 of the say even coil sets, then the odd and then reversing the polarity on the even, etc. Practically, that's no different than controlling 2 BDC motors, albeit with different logic. Luckily that means any of the H-Bridge solution we've previously discussed will work just fine. If you need to move a stepper continuously and don't care about positioning, a 555, XOR Gate & flip-flop will turn that stepper. You have to make sure you have enough power from your controller, so selecting the right H-bridge/Mosfet for your demand is just as important as before. The L293D can be found in maker kits everywhere because it Just Works™ and therefore breakout boards are super cheap for prototyping.

And braking? We actually kinda get that for free. If the cycling stops but one coil is energized, the motor will stop quickly and even hold in place - something unnatural for the BDC counterpart. To freewheel you actually have to turn off all coils. If you have a 3D printer you might have experienced the need execute a "disable motors" command in order to move around the printer bed or extruder.

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Stepper: drivers & MCUs

Dual hardware PWM

Fancy Drivers

src: https://learn.adafruit.com/system/assets/assets/000/002/495/medium800/learn_arduino_fritzing.jpg?1396783754

src: http://bildr.org/blog/wp-content/uploads/2011/05/EasyDriver-Stepper-Motor-Driver2.png

Any modern MCU can generate the PWM we need to drive a stepper. As before, we just need the right H-bridge for our power demands and at least 4 GPIO. I'd always prefer to use a hardware PWM and a little bit of counter/toggling code but in a pinch you could bit-bang with a software clock signal.

A better option are specialized stepper drivers. They usually require 2 pins (PWM & direction) and are awfully cheap thanks to 3D printers. The A4988 is at the heart of every RepRap and a single unit module with heatsink is a buck and change. In addition to price and pin count, these driver boards also usually have an output for current draw, something key to closed-loop system (a topic for another time.) Other popular chips include the DRV8824, DRV8834 (low-voltage) & A3967 - used in the OSH EasyDriver.

As a third option there exists drivers that can be controlled over the various communication protocols. Pick a vendor and you'll probably find a part with SPI or I2C. As a sample, the DRV8830 & PCA9629 can be controlled by I2C while something like the L6470 & A4980. Note that there are less options for these type of parts and are usually pricier than the step+dir driver.

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Servo

Servo motors are unique in that they can move to the angle your want and stay there - even when something pushes on it. On the inside there is a closed-loop control system but we can treat the thing as a black box (literally, though they sometimes come in blue.)

There's 2* very distinct flavors of Servo - one for the RC plane/animatronic market and one for high-end industrial automation. There are other nuances like plastic vs metal gearing or analog vs digital circuitry, but they are all controlled in generally the same way.

There is a third flavor sometimes called "Smart" servos that have an on-board MCU but they're a different beast and out of scope for the talk.

src: http://www.aliexpress.com/item/MG996R-MG946R-MG995-MG945-Metal-Geared-Tower-Pro-Servo-High-Torque-and-Power-SG172-SG173-SG174/32383111789.html?spm=2114.01020208.6.4.XSMNxb&s=p

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Servo: overview

PROS

$$ (Hobby-grade)
Position shaft easily and stay there
Control protocol is easy, no drivers needed
Easy to mount casing
Lipo-friendly voltages
Industry-defined standard sizes

CONS

$$$ (Industrial-grade)
Continuous rotation exists but kinda hacky
Needs constant signal

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Servo: duty cycle to angle

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Servo: just another PWM device

src: http://i.stack.imgur.com/yrnZY.jpg

A PWM signal is a PWM signal is a PWM signal. So generating the right duty cycle from an MCU is trivial. You can add as many servos as you have PWM pins and even multiplex it of sorts with counters. See Servo.cpp in the Arduino HAL (there, I said it) for an example. Note that if you plan on doing sequenced animatronics, going with an MCU is the only sane solution. If you have a beefy MCU you can use the available pins or rely on a multi-channel driver like the PCA9685 previously mentioned. That leaves you overhead for more advanced processing like inverse kinematics.

There's a nice benefit of Servos I forgot to mention. Because they have a power pin, you don't have to worry about the output current of your driver. Any old logic high will work and the the servo manages it's own current draw.

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Brushless DC

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Brushless DC: overview

PROS

$$ - price dropping b/c drones, e-bikes
High RPMs, torque
Easy to control Electronic Speed Controllers (ESC)
Can be made lightweight

CONS

Not good for fixed-positioning
Sensored type are closed-loop and therefore need more electronics
Despite efficiency, needs lots of current

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Brushless DC: 3 motors, basically

src: http://www.orientalmotor.com/images/technical-articles/No166-fig14.jpg

src: https://upload.wikimedia.org/wikipedia/commons/d/d1/ESC_35A.jpg

BLDC motors are effectively Steppers with three coils instead of two (often called 3-phase.) That means three half bridges and three PWM signals will turn the motor. In fact, if you open up a hobby ESC you'll find six mosfets and either an 8051 or AVR.

BLDC's usually come with built-in Hall effect sensors that help encoding position. If a BLDC doesn't have sensors (sensorless,) or they're being ignored, than you need to account for an initial unknown speed. That usually works when the exact speed is not critical or there is feedback from another source (back EMF or a thing being cooled, for those interested.)

There are of course specialized drivers for BLDCs. There's simpler/cheaper ones just for Fans and full-fledged 3-phase drivers that are driven by PWM inputs (and a few via SPI/I2C.) I personally haven't used these parts but Microchip has a variety of fan drivers like the MTD6502B or a more traditional 3-phase driver like the MCP8024.

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What next?

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Moar online

This presentation is online
More details on hackaday.io/project/8116

plan to continue with A/C, Solenoid & smart servos

Code on Github:

Arduino, soon ChibiOS

Planning a talk on closed-loop

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Appendix of things