DC Motors and H-Bridges Presented By: Sean Huang, Daniel Simon, and Scott Smith [1] [2]

Presentation Outline Electric Motors A (Brief) Review of E&M Physics Brushed DC Motors Controlling DC Motors H-Bridges Brushless Motors and Control References and Further Reading

Electric Motors

What are Electric Motors? Turn electrical energy into mechanical energy. Useful in wide range of embedded applications from automotive to medical. Vary from extremely complex to incredibly simple. [3]

Many Different Types of Motors Electric Motors DC Motors AC Motors Brushless Motors Brushed Motors

A (Brief) Review of E&M Physics

A Brief Review of Ohm’s Law V = I * R [4] I = V / R = 9 / 3000 = 0.003 A = 3 mA

A Brief Review of Magnets

A Brief Review of Magnets

A Brief Review of Magnets

A Brief Review of Lorentz Forces In which direction is the force? N I S Lorentz Force: F = IL x B [5]

A Brief Review of Lorentz Forces In which direction is the force? N I S Lorentz Force: F = IL x B [5]

Brushed DC Motors

How do Brushed DC Motors Work? [6]

How do Brushed DC Motors Work? [6]

How do Brushed DC Motors Work? Commutator Ring [6]

How do Brushed DC Motors Work? Commutator Ring Brushes [6]

How do Brushed DC Motors Work? [6]

How do Brushed DC Motors Work? [7]

How do Brushed DC Motors work? [8] Adding additional coils increases motor stability.

Controlling DC Motors

Goals: Be reliable (Doesn’t blow up) Control Speed/Torque Control Direction

One (Naive) Approach Could work if you had a small motor with a beefy enough microcontroller to supply the power, AND you don’t get control of the direction. Ex. if the motor is 1 ohm, and the microcontroller goes 0 to 3.3v, then the motor try to draw 3.3AMPS. Most microcontrollers can’t supply that much current on a single pin.

One (Naive) Approach ?? How would you change the direction?

One (Naive) Approach Not enough power (usually) No directional control Could work if you had a small motor with a beefy enough microcontroller to supply the power, AND you don’t get control of the direction. Ex. if the motor is 1 ohm, and the microcontroller goes 0 to 3.3v, then the motor try to draw 3.3AMPS. Most microcontrollers can’t supply that much current on a single pin.

Better Approach We add an external voltage supply. This fixes the problem of supplied power. Now we have the GPIO pin controlling a MOSFET or transistor when to supply the motor with power and as long as we have a big enough supply there is no issue with the size of the motor. One problem here: Back EMF.

Better (Dangerous) Approach I

Better (Dangerous) Approach I One problem here: Back EMF. steady state - when the switch is closed for a long time, and then turned off. If we open the switch there is nowhere for the current to go. This translates to a spark or arc at the switch. No good. [13]

Better (Dangerous) Approach Henry I [9] Mr. Henry will not have a good time.

Better Approach Fixes the power supply problem, but Back EMF Sparks Still no directional control We add an external voltage supply. This fixes the problem of supplied power. Now we have the GPIO pin controlling a MOSFET or transistor when to supply the motor with power and as long as we have a big enough supply there is no issue with the size of the motor. One problem here: Back EMF.

Better Approach [10] I Flyback diode creates a path for the current to go after the switch is opened. This will slowly decay the current down.

Better Approach [10] I I Flyback diode acts as it’s not even there.

Better Approach [10] I Path for current to go now.

Better Approach Doesn’t blow up I Control Speed (PWM) [10] Doesn’t blow up Control Speed (PWM) Still doesn’t control direction I

H-Bridges

H-Bridge Components 4 switches centered around a load 4 flyback diodes Power supply Ground [11]

Powering the Motor Create a path for current to flow through motor Switches turned on/off independently using I/O If switch is off (open), no current will flow through the switch If switch is on (closed), current will flow through the switch Current will flow through the load if the diagonally-paired switches are on simultaneously [11] [11]

H-Bridge Hazards (Shoot-through) If both switches on one side are on (closed), we’ll get a short circuit Can happen when trying to reverse the direction of the motor [11]

Hazard Handling Delay is present when toggling the switches Delay can cause both switches on one side to be on/off simultaneously Perform “turn off” before “turn on” 1 1 1 1 So all practical bridge designs are biased in the other way, making sure that the two switches are never on at the same time, but as a consequence they will be both off for a short while during switching. energy is dissipated through losses in the wire by the inductor to draw current from it in a continuous loop, the diode and the resistor. The voltage across the inductor will be a function of the forward voltage drop of the Flyback diode. Total time for dissipation may vary, but it will last for a few millisecond [11] [11] [11]

H-Bridge Hazards (Back EMF) Sudden change in current creates large voltage on motor I -> 0 V -> inf [11]

Flyback Diodes Diodes create a loop that reduces current change Smaller current change means smaller voltage across the motor (no back EMF) 1 1 V = v So all practical bridge designs are biased in the other way, making sure that the two switches are never on at the same time, but as a consequence they will be both off for a short while during switching. energy is dissipated through losses in the wire by the inductor to draw current from it in a continuous loop, the diode and the resistor. The voltage across the inductor will be a function of the forward voltage drop of the Flyback diode. Total time for dissipation may vary, but it will last for a few millisecond [11] I = i [11]

Controlling Motor Speed with PWM [11] 1 1 [11] [11]

Brushless Motors and Control

How do Brushless DC Motors Work? [8] Switch the position of the magnets and coils.

Brushless Control [12]

Brushless Motors Brushed Motors Long lifespan Very high efficiency Low noise generation High power to size ratio Cheap Easy to use Requires few wires Simple control circuitry

Recommended Further Reading Brushed DC Motors: http://pcbheaven.com/wikipages/How_DC_Motors_Work/ Brushless DC Motors: https://www.renesas.com/us/en/support/technical- resources/engineer-school/brushless-dc-motor-01-overview.html H-Bridges: http://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridges- the-basics/ Brushless Control: https://www.digikey.com/en/articles/techzone/2016/dec/how-to- power-and-control-brushless-dc-motors

References [1] DC Motor Photo: https://www.surpluscenter.com/Electrical/DC-Motors/DC-Motors-Face-Mount/1-2-HP-90-Volt-DC-Motor-10-3114.axd [2] H-Bridge Hardware Photo: https://www.robotshop.com/en/l298-dual-h-bridge-dc-motor-controller.html [3] Toy Photo: https://www.walmart.com/ip/Vroom-Rider-Dirt-Bike-Motorcycle-Battery-Powered-Riding-Toy-Orange/33154659 [4] Simple Circuit: https://www.allaboutcircuits.com/textbook/direct-current/chpt-5/simple-series-circuits/ [5] Right Hand Rule Photo: https://en.wikipedia.org/wiki/Lorentz_force#/media/File:Regla_mano_derecha_Laplace.svg [6] DC Motor Still Image and Slow GIF: https://www.explainthatstuff.com/electricmotors.html [7] Alternate Fast DC Motor GIF: https://www.electricaleasy.com/2014/01/basic-working-of-dc-motor.html [8] Better Brushed Motor Photo and Brushless Motor Photo: https://www.renesas.com/us/en/support/technical-resources/engineer-school/brushless-dc-motor-01-overview.html [9] Shock photo: https://dawsonpower.com/blog/2014/09/how-to-help-an-electric-shock-victim/ [10] Flyback Diode Photo:https://resources.altium.com/bill-of-materials-blog/why-you-should-use-a-flyback-diode-in-a-relay-to-prevent-electrical-noise-in-your-circuits [11] H-Bridge Photos and Tables: http://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridges-the-basics/ [12] Brushless Motor Control:: https://www.digikey.com/en/articles/techzone/2016/dec/how-to-power-and-control-brushless-dc-motors [13] Motor Spec sheet: http://hades.mech.northwestern.edu/index.php/Brushed_DC_Motor_Theory