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 <<

Safety NEVER WORK HOT! All Design Work must account for Human

Exposure to Electric Hazards (Idiot-Proof)

Safe by Design

Safe Design Practices include: Enclosing Electrical Components to avoid

incidental human contact Providing a Discharge Path for all 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”) Weatherproofing Hazardous Materials/Containment

Battery Acid Lead Oil

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 Switches Must completely De-Energize Project If your project is mobile, it will require a Local

Kill Switch (Big RED Button, Easily Accessible)

Source: Cartek

Remote Kill A 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)

Grounding

Grounding too often an ‘Afterthought” Soild, Reliable Ground Plane eliminates

many Noise and Reliability Problems Ground 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 Grounds In 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 Protection Electrical Circuits Require Protection

against Fault, Failure or Improper Use Always Know the Failure Modes of

Equipment used in your Design (e.g. A Diode can fail short)

Types of Circuit Protection: Over Current Over/Under Voltage Over Heating Over/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 Protection Circuit Breaker – Rated for a maximum

application voltage, interrupting level and maximum interrupting current (or volt-amps)

Over Current Protection (Cont.)

Fuses Inexpensive Over Current Protection One – Shot Fast or Slow Be Aware of resistance

Over Current Protection (Cont.)

Self-Resetting Fuses Thermistor that is conductive at room temperature If current exceeds rating, heats up and becomes non-

conductive Conductive again after cooling down

Source: commons.wikimedia.org

Over Voltage Protection

It is often desirable, especially in power electronics, to limit transient over-voltages in a circuit

Zener Diodes are an inexpensive means of limiting low-power over-voltages

MOV (Metal-Oxide Varistor) Surge Suppressors provide a heavier duty solution

Circuit Isolation IMPORTANT to electrically isolate delicate

electronics from power circuits (Pulse Width Modulation motor drives, etc)

Transformers Provide Electric Isolation (energy transfer

is through magnetic circuit in core) AC Signals ONLY

Relays Provide Electric Isolation (magnetic

circuit)

Provide “electro-mechanical Amplification” Low Power Signal Controls Large Power Circuit

AC or DC

Not for Repetitive Operations

Opto-Couplers

Provide 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