ece 4591 – design workshop
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ECE 4591 – Design Workshop. Lecture 1: Safety and Protection. The Three Laws of Robotics. A robot may not injure a human being or, through inaction, allow a human being to come to harm. - PowerPoint PPT PresentationTRANSCRIPT
ECE 4591 – Design Workshop
Lecture 1:Safety and Protection
The Three Laws of Robotics
1. A robot may not injure a human being or, through inaction, allow a human being to come to harm.
2. A robot must obey the orders given to it by human beings, except where such orders would conflict with the First Law.
3. A robot must protect its own existence as long as such protection does not conflict with the First or Second Laws.
>> Isaac Asimov <<
SafetyNEVER WORK HOT!All Design Work must account for Human
Exposure to Electric Hazards (Idiot-Proof)
Safe by DesignSafe Design Practices include:
Enclosing Electrical Components to avoid incidental human contact
Providing a Discharge Path for all Energy Storage Devices (like Capacitors)Design for discharge to under 5V in 2 seconds
Safe Design Procedures (cont.)
Design for Environment Internal Cooling/Heating (entire exterior must be
“touchable”)WeatherproofingHazardous Materials/Containment
Battery AcidLeadOil
Peltier Devices – Exploit the Thermoelectric Effect to convert a temperature differential into an electric voltage (Reversible!)
Source: commons.wikimedia.org
Safe Design Procedures (cont.)
Kill SwitchesMust completely De-Energize Project If your project is mobile, it will require a Local Kill
Switch (Big RED Button, Easily Accessible)
Source: Cartek
Remote KillA remote Kill Switch could act via one of
many “wireless” technologies: Infra-Red, AM Radio, FM Radio, IEEE 802.11g, Bluetooth, etc.
Range and Noise will be issues in the Contest Environment
Example: http://www.kitsrus.com/pdf/k180.pdf UHF Radio 2-Channel (Control 2 things) Relay Backend (Heavy Duty)
GroundingGrounding too often an ‘Afterthought”Soild, Reliable Ground Plane eliminates many
Noise and Reliability ProblemsGround Plane on Mobile Platform cannot reliably
be joined to Earth Ground. (Floating with Respect to Earth) Induced Voltages on Vehicle must be considered when designing and maintaining Vehicle
Proper Grounding (Bonding)Good Example of Terminating a Ground Wire (or
any wire) – Be aware of Vibration!
Proper Grounding (Cont.)Proper Termination of Shielded Cable
Need for Separate GroundsIn the case where a low power analog signal
(such as from a radio antenna) requires a ground plane, AVOID direct connection to a ground plane used for digital equipment (Noisy).
In the absence of Earth Ground, establish an Analog Ground Plane and a Digital Ground Plane and connect the two with a resistive path to attenuate noise.
Circuit ProtectionElectrical Circuits Require Protection against Fault,
Failure or Improper UseAlways Know the Failure Modes of Equipment
used in your Design (e.g. A Diode can fail short)Types of Circuit Protection:
Over CurrentOver/Under VoltageOver HeatingOver/Under Frequency (AC Systems)
Over Current Since most circuit designs assume established
voltage levels within the circuit, regulating current will regulate the power in the circuit.
Conductors must be sized to handle the maximum load current and any transient short-circuit current level available. Example: NEC indicates 14 Gauge Solid Copper will
safely carry 15 Amps when properly protected by a circuit breaker
Know When to Use Solid Wire and When to Use Braided Wire
Over Current ProtectionCircuit Breaker – Rated for a maximum
application voltage, interrupting level and maximum interrupting current (or volt-amps)
Over Current Protection (Cont.)
Fuses Inexpensive Over Current ProtectionOne – ShotFast or Slow Be Aware of resistance
Over Current Protection (Cont.)
Self-Resetting FusesThermistor that is conductive at room temperature If current exceeds rating, heats up and becomes non-
conductiveConductive again after cooling down
Source: commons.wikimedia.org
Simple Fuse Status Indicator
Size R to limit current through LED
Over Voltage Protection It is often desirable, especially in power electronics, to
limit transient over-voltages in a circuitZener Diodes are an
inexpensive means of limiting low-power over-voltages
MOV (Metal-Oxide Varistor) Surge Suppressors provide a heavier duty solution
Bring Critical Voltage Test Points Out to A Measuring Block
Circuit IsolationIMPORTANT to electrically isolate delicate
electronics from power circuits (Pulse Width Modulation motor drives, etc)
TransformersProvide Electric Isolation (energy transfer is
through magnetic circuit in core)AC Signals ONLY – V2 proportional to change in
flux
RelaysProvide Electric Isolation (magnetic circuit)
Provide “electro-mechanical Amplification”Low Power Signal Controls Large Power Circuit
AC or DC
Not for Repetitive Operations
Opto-CouplersProvide Electric Isolation (Energy Transfer via
Photons)Many Types of Output: BJT, Darlington Pair, SCR,
etc
Tri-State Drivers (Buffers)Enable Pin = 0 puts driver in High Impedance
State (Open Circuit A to B)High Input Z, Low Output Z (10 GE output)Non-Inverting or Inverting
BatteriesIt is IMPORTANT to understand that Batteries have
significant internal resistance: Fast Charge or Discharge leads to Internal Heating and Loss of Life.OSHA Std: 1926.441 (www.osha.gov)
Energy Storage in batteries is rated in Amp-HoursA general rule of thumb is that storage batteries
should never be discharged below 50% of capacity to avoid undue aging
Lead-Acid Storage Batteries
Oldest technology, but still the most cost-effective in terms of energy density (most amp-hours per kilogram per dollar)
Checking Charge level with a voltmeter:
Open Circuit (Resting) Voltage VS. State of Charge 12V Lead Acid: 12.66V 100%
12.45V 75% 12.24V 50% 12.06V 25% 11.89V 0%
Power SuppliesConventional Power Supplies: AC
to DC using Transformer and Diode Bridge
Switch-Mode Power Supplies: AC to DC using Solid State Electronics
DC to DC Conversion is called Chopping
Solid State Voltage Regulators
Conventional PS
DC-DC Chopper
Chopper Output Waveforms
Power MOSFET “Chops” the DC supply voltage
Inductor smoothes the Current, Capacitors smooth the Voltage
Voltage RegulatorsSolid State DC/DC Buck ChopperCan provide a fixed output voltage: +5, -5, +12,
etcSome can provide a variable output: +4 to +12
Volts, etc
Assignment:Using a breadboard, LM7805 Voltage Regulator,
a 5.1 V Zener Diode and any other parts:Build a simple 12 Vdc to 5 Vdc step-down circuit
and protect the output against voltage transients
Submit to me: An electronic copy of your working design schematic