Opens, shorts, and a condition in between the two are the faults most obvious at the system level. Motors and feedback encoders are usually located tens to hundreds of feet from the servo-system controller/amplifier. Connectors terminate these long cable runs at both ends, and it's possible for wires to fall out of the connectors, for connectors to break, and for cables to be inadvertently opened. When end connectors break open due to machine vibration, the open fault often exhibits several open/reconnect cycles similar to the contact bounce in a switch, before opening up completely.
Similarly, shorts from severed wires may exhibit several short/open cycles, also similar to switch contact bounce, before shorting completely. Because feedback-encoder signals are usually transmitted down a twisted pair, the differential signals are likely to short together during this type of fault.
Intermediate faults can develop when the resistance or capacitance of a feedback wire increases, as is possible when a poor installation pinches the cable. Such problems can also manifest during later operation. When moisture enters a damaged cable jacket, for example, the cable capacitance can increase over time, causing signal strength to decrease. That condition is common in heavy industrial environments, where automated equipment may require a daily wash-down. The cable remains operational even though its performance degrades over time. As a precaution, you should include a circuit for detecting moisture-contamination faults.
Noise faults can be the most difficult to eliminate, because noise can originate from electromagnetic interference (EMI), radio-frequency interference (RFI), and ground or system-level ground loops. System-level noise sources (radiators) include:
- Arcing of dc-motor brushes during commutation
- High-speed dv/dt switching noise from pulse-width-modulation (PWM) motor amplifiers
- High-power relays, switches, and actuators such as solenoids
- Random turn-on of SCRs and TRIACs during the 60-Hz ac cycle
- Switching power supplies
- Electrostatic discharge
Noise receivers (antennas) include long cables, ground connections, long high-impedance traces on pc boards, and transformers. Noise problems require a method of coupling between a receiver and a radiator, such as capacitive, inductive, or conductive coupling. Capacitive coupling typically occurs in high-impedance circuits when wires or other ungrounded pieces of metal pick up or generate electric fields. For a circuit to couple noise capacitively, the circuit's loop impedance must exceed the intrinsic impedance of air (376.7 Ω).
Inductive coupling typically occurs with low-impedance circuits for which the circuit's loop impedance is less than 376.7 Ω. Wires, open-core inductors, and transformers pick up or generate magnetic fields that can cause EMI noise. The current loops for these circuits must be minimized during design and installation.
Conductively coupled noise usually enters the circuit at ground. For dc noise, it takes the path of least dc resistance in the ground plane, and for ac-coupled noise the least impedance. As a result, the circuit reference point (ground) tends to be at a voltage above or below its normal value, or (worst case) a dynamically changing value.
Ground loops formed between the ac-power neutral and system-level ground can generate random noise by causing ground current to flow. Ground current can be driven by voltage differences, induction from other cables or devices, wiring errors, ground faults, or the normal equipment leakage that occurs in an industrial environment.
Common-mode noise, defined as common to two nodes that may be floating or exhibiting high impedance, can be ac or dc. It can be inherent in the system design, but common-mode noise is usually coupled inductively or capacitively from an external source. For instance, a 60-Hz signal from the power line, lying adjacent to a pair of signal wires from an analog sensor, can couple inductively onto the wires and drown out the low-level sensor signal.
Electrostatic discharge (ESD) develops when two dissimilar materials come together, transfer charge, and move apart, producing a voltage between them. IC pins that connect to external connectors are susceptible to ESD when a technician connects or disconnects those cables during maintenance.
ESD injected into the pins of an IC can cause the IC to latch up or fail completely. Very high currents can flow during latchup, causing the main power supply to limit current or the system to enter an uncontrolled shutdown. IC pins exposed to external signals or connectors without internal ESD protection must incorporate ESD protection such as metal-oxide varistors, or silicon avalanche suppressors like Trans-Zorbs. ICs with built-in ESD protection save pc space and thereby support the push for smaller form factors and smaller industrial enclosures.