Transcript Completing the circuit

Basic electricity
Some basics for FIRST Robotics
ROBOTICS ACADEMY: FRC Basic electricity
Safety First
Q: What voltages / currents are safe and which are dangerous?
A: It depends.
What body parts is it running through?
Are contacts with the power damp?
Is the path to complete the circuit clear?
Are you lucky?
Completing the circuit – battery
Path to complete circuit for a battery requires
contact with both + and – terminals. Circuit is not
completed if only one terminal is connected.
ROBOTICS ACADEMY: FRC Basic electricity
Completing the circuit – Household 110V/220V
Ground
Phase f
Common
Phase f+180
220V across f and f+180
110V from common to f or f+180
The common is connected to the ground wire in the
box and to a stake driven in the ground outside.
The circuit is completed by connecting to anything
conducting to the ground outside, i.e. everything!
Only have to touch one wire since whatever you
stand on or sit on completes the circuit. Dampness
improves the connection. What about birds?
ROBOTICS ACADEMY: FRC Basic electricity
Physiological response
Data from NASA “Man-Systems
Integration Standards”
20 M
10 M
NASA-STD-300 Handbook Vol. III,
August 1994, Figure 6.4.3-1
Amperes Current Milliamperes
Total Circuit Resistance, Ohms
1M
100 K
10 K
5
Data are based on current flow
from arm-to-arm or arm-to leg
of 60 Hz AC on 150 lb human
DC limits ~ 50% higher
NASA-STD-300 Handbook Vol. I,
Rev B, July, 1995, Table 5.4.2-2
2
1000
5
2
100
10
2
5 100 2
5 1000
Voltage, 60Hz AC
10 K
100 K
Current is major factor, but voltage also
important since resistance is not known
ROBOTICS ACADEMY: FRC Basic electricity
So what is safe?
12V DC generally considered safe if touching with hands or dry skin.
(Licking terminals could give shock, lead poisoning, acid burns.)
Our cordless power tools are 18V DC, so manufactures apparently
see little risk with that voltage when used properly.
Many people get shocked with 110V AC with just a strong jolt.
Others get shocked with 110V AC and die. Investigations generally
reveal a good path to ground, often blamed on moisture, but that is
not the only factor.
Treat 110V as if it could kill you – it can.
Lightning is nearly always fatal. There are a few survivors of branch
strikes. Can exceed a billion volts and 300 kA of current.
ROBOTICS ACADEMY: FRC Basic electricity
Complete circuits
Circuits must be complete
Should be able to trace current flow from
battery, through switch, controller, and
motor back to other pole on the battery
The wire and motors in the
circuits have resistance to
current flow and also create
magnetic fields. These are
resisters and inductors in
circuits.
ROBOTICS ACADEMY: FRC Basic electricity
Definitions
Voltage – The force pushing the electricity through the circuit.
Measure in volts, denoted V. “V” in equations.
Also called potential and electromotive force
Current – The quantity of electrons passed in a given time.
Measured in Amperes (Amps), denoted A. “I” in equations.
Resistance – Obstacles impeding flow of electrons, generates heat.
Measured in Ohms, denoted W. “R” in equations
Capacitance – Storage of electrical charge
Measured in Farads, denoted F. “C” in equations
Inductance – Electrical inertia to changing charge and magnetic fields
Measured in Henrys, denoted H. “L” in equations
ROBOTICS ACADEMY: FRC Basic electricity
Power and thermal management
Voltage drop (Volts) across a resistor is current (Amps) times resistance (Ohms)
V IR
Power (Watts) dissipated in a resistor is voltage (Volts) times current (Amps)
V2
P V  I  I  R 
R
2

1 HP = 745.7 Watts
All wires have resistance equal to resistivity times length divided by area
 increases with temperature
R    L/A
 Dissipated electrical power turns to heat, and temperature must be controlled or
wires will melt and components will smoke. Smoke is from burning components.
Heating rate increases as wire area decreases, and ability to transfer the heat
 to the environment decreases as wire size decreases.
Light bulb filaments have a stable glow because resistance increases with
temperature, dropping the power, while heat transfer increases with temperature.
Flash bulbs flare a bright white because they melt and vaporize rapidly.
ROBOTICS ACADEMY: FRC Basic electricity
Selecting fuse and wire sizes
Check motor or controller limits and use fuse or circuit breaker at or slightly above
the maximum current draw for normal use.
Main breaker:
120 Amps
Note that 4 motors drawing 30+ Amps
Motor Controller up to 40 Amps
each will trip the main breaker!
Relay module
up to 20 Amps
Digital sidecar
20 Amps
Fuses and breakers must protect the entire circuit, including the wire
Select wire size based on fuse or circuit breaker amperage. Using wires thicker
than required is okay. The requirements are for minimum diameter.
Application
Main power
30-40 Amp circuit
20-30 Amp circuit
5-10 Amp circuit
Pneumatic valves
Minimum wire size
6 AWG
12 AWG (2.052 mm)
14 AWG (1.628 mm)
18 AWG (1.024 mm)
24 AWG (0.5106 mm)
ROBOTICS ACADEMY: FRC Basic electricity
Simple circuit with potentiometer
This simple circuit contains a battery, resistance from the wire and a variable
resistor in the form of a potentiometer.
Voltage drop across resistor is
Other circuit resistance
V IR
f=1
Analyze voltage through entire
Voltmeter
R
c
Battery
circuit. Voltage rise and drop
should sum to zero
Vbat
 Vbat  I  Rc  I  Rpot  0
Rpot
f=0
Current, I
Meter voltage
Vmet  I  Rmet 
f  Rpot  Vbat
Rc  Rpot
Voltage and resistances are
known, solve for current
Vmet

Resistance measured
across the voltmeter is
Rmet  f  Rpot

I
Vbat
Rc  Rpot
ROBOTICS ACADEMY: FRC Basic electricity
Current and magnetic fields
Current in a wire induces a magnetic
field around the wire.
Current in a loop of wire creates toroidal
shaped magnetic fields
Field around wire with illustration
of the right hand rule
Magnetic field in coils of wire is additive
to create regions of strong, consistent
magnetic fields in the coil interior.
Electromagnet
Toroidal field around a wire loop
Field in a coil with different materials in core
ROBOTICS ACADEMY: FRC Basic electricity
Relays and solenoids
Relays use a small current to move
a switch handling large current
Large-current
switch
Material within a magnetic coil will move
to increase the coil inductance.
A metal rod partially filling the coil will
move into the coil. This strong and fast
linear actuator is called a solenoid.
Field around wire with illustration
of the right hand rule
Small current
The air compressor on our test
pneumatic system uses a relay
Solenoids are used to engage car starters
ROBOTICS ACADEMY: FRC Basic electricity
Permanent-magnet motor basics
Magnetic field induced by current in the armature interacts with outer magnets
Permanent
magnets
Repulsive
Repulsive-attractive
Attractive
Armature
Commutator
Brushes
The magnetic field in the armature reverses when the split in the commutator
reverses the current path. The torque on the armature is strongest in the
repulsive-attractive configuration.
ROBOTICS ACADEMY: FRC Basic electricity
Increased efficiency with segmented armature
Segmented armatures are used to create
multiple independent circuits and magnets
which are active only when near the orientation
for maximum torque.
Pairs of brushes contact opposite sides of
the commutator to excite different coils
Commutator segments are
connected to independent
armature coils
Brushes
ROBOTICS ACADEMY: FRC Basic electricity
Inductance effects on motor circuit response
Armature
resistance
Vcomsin(wt)
Oscillating
voltage from
commutator.
Volts
Voltage-time
curve looks like
Time
but will model
as a sine wave
Armature
inductance
Rarm

Larm
Current, I 
Voltage balance on circuit with inductor
dI
Vcom sin( wt)  L   R  I  0
dt
The current satisfying the 1st order ODE is
Vcom sin( wt)  wRL cos(wt)
I
;
R
1 (wRL )2
IRMS 
Vcom
1
R 2 1 (wRL ) 2
RMS voltage drop across resistor and inductor
wL
Vcom
1
Vcom
R
VR 
;
V

L
R 2 1 ( wRL ) 2
R 2 1 (wRL ) 2
Observations: As motor speed increases, current decreases
and the voltage drop is primarily across the inductor.

Most of the voltage goes into flipping the magnetic field.
Magnetic field strength drops, motor torque drops.
ROBOTICS ACADEMY: FRC Basic electricity
Torque, speed and power characteristics
Power analysis
Notional torque-speed
curve for CIM motor
P t w
Torque (t)
Most torque at stall
No torque at max
speed
Speed (w)
Notional torque equation

w 
t  t stall1 

 w max 
Should obtain real torquespeed curve for accurate
analysis
Substituting notional torque relation

w 
1

 P  t stall  w  
 w max 
Find speed at maximum power by
setting derivative to zero
 2w 
dP
  0  t stall  1 
 ; w  w max /2
dw
 w max 
Characteristics: Max torque at stall
Max power at ½ max speed

Can’t have both; what is important for application?
ROBOTICS ACADEMY: FRC Basic electricity
Limitations on power and delivered torque
Power:
•Power cannot be increased from that provided by the source.
•Power on an FRC robot is limited by the current of the battery ~1400 W
•Power surge can be obtained from sudden release of energy stored
electrically, mechanically and pneumatically. (chemical not permitted!)
Torque:
•Torque be increased arbitrarily mechanically through gear reduction,
but rotational speed drops commensurately.
•Motor torque is limited by FRC motor specifications.
•Wheel friction and other factors will eventually limit performance as
torque is increased.
ROBOTICS ACADEMY: FRC Basic electricity
Wrap up

Do not be concerned about shock from 12V circuits under normal use

Size motors based on application and motor characteristics
Don’t forget solenoids and electromagnets in design
Use circuit breakers and fuses consistent with intended load
Select minimum wire size appropriate for circuit






Plan wire routing to ease assembly and troubleshooting
Create wiring schematic for notebook showing motor and port numbers
Ensure connections are secure and bundle wires to keep neat.
ROBOTICS ACADEMY: FRC Basic electricity
Questions?