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EENG421 Ch. 2
Intro to PLCs, Input Devices, Actuators, Timers
| Question | Answer |
|---|---|
| Three types of manually operated industrial switches discussed in this class? | Toggle, push button, selector |
| What does a toggle switch do? What are common pole-throw configurations? | Toggles between two or more options using a lever actuator. SPST, DPST, SPDT, DPDT |
| What does double break mean? | Both ends of the pole move when the switch is activated |
| What are the two contact types for a push button switch? | Momentary and maintained |
| What are the styles of a pushbutton switch? | no guard, full guard, extended guard, mushroom button |
| How many positions can you have for a selector switch? | two, three, or four |
| What are the knob styles of a selector switch? | Winged or keyed |
| What is hysteresis? What type of switch does it relate to? | Hysteresis is a separation between the activation point and the deactivation point of a switch. It allows for the level switch to remain turned on until the level moves past turn on to deactivation |
| What are the two types of triggers for a pressure switch? | Absolute - triggers at a specific pressure value Differential - triggers on the difference between two pressures |
| What are the two styles of an industrial sensor? | Contact (includes switches) and non-contact |
| What is the sensing range of an inductive proximity sensor? | up to 50mm for steel, up to 15mm for other metal |
| What are ideal applications for an inductive proximity sensor? | Counting metal parts from underneath a conveyor belt, sensing a foil seasoning packet in a cardboard container, sensing a bottle cap or metal cans from above a conveyor belt, |
| What are the performance characteristics of an inductive proximity sensor? | Targets are electrical conductors, targets are not limited to magnetic materials, targets are stationary and moving, target is a flat, smooth object, energy levels are low so they do not create RF interference or generate heat in the target. |
| What is the sensing range of a capacitive proximity sensor? | up to 30mm |
| What are the performance characteristics of a capacitive sensor? | Conductors, insulators, plastics, glass, ceramics, oils and greases, water, and all materials with a high moisture content or dielectric constant greater than 1.2, stationary and moving objects, flat smooth objects, low energy levels |
| Why are inductive capacitors commonly chosen over capacitive? | Lower cost and high reliability |
| What are the performance characteristics of an ultrasonic sensor? | Ideal target has a flat, smooth surface, object must be +/- 4 degrees or less from the center axis, doesn't work with soft materials, reflective surfaces must be aligned to reflect the ultrasonic waves back to the receiver, temp must be less than 100C. |
| What are the two basic components of a photoelectric sensor? | Light source (LED) and light detector (photodiode or phototransistor) |
| Is the radiation pattern of the LED large or small with or without a lens? What about the field of view of the detector? | Large radiation pattern and field of view without a lens, small with a lens. |
| What is the housing for a through/transmitted beam? | Source and receiver are in separate housings |
| What is the housing for a retroreflective? | Both the light source and receiver are in the same housing |
| What is the housing for the polarized retroreflective? | Polarizing filters in front of the light source and receiver that are 90 degrees out of phase with each other |
| What is the housing for the diffused? | Light source and receiver are in the same package. |
| How does a through beam sensor work? | The target must break the beam between the light source and the receiver (requires both sides) |
| How does a retroreflective sensor work? | The emitted light beam is reflected back to the receiver, the object is detected when it breaks the light beam. |
| How does a polarized retroreflective sensor work? | Light from the source has a horizontal polarization, only light that can reach the receiver can have a vertical polarization. |
| How does the diffused mode sensor work? | Object to be sensed becomes the target that reflects the light back to the receiver. |
| Does diffuse mode depend on the color of an object? | Yes |
| How does background suppressed diffuse mode work? | If the object falls between the focal plane and the receiver, the beam falls on the first receiver. If the object moves out of the focal plane, it falls on the second receiver. The signal from the second signal is then electronically suppressed. |
| How does fixed focus diffuse mode work? | The beam from the light source and the detection area are focused to a very narrow point at a fixed distance in front of the sensor |
| Why is the fixed focus diffuse mode have reliable detection of small objects? | Because the sensor is very sensitive at the focal point |
| How does wide angle diffuse mode work? | The sensors project the light source and detection area of the receiver over a wide area |
| What are typical applications of a wide angle diffuse sensor? | Thread detection, ignoring holes or imperfections in targets |
| How many sides of the object does through beam need to access? | Both |
| How many sides of the object does diffuse need to access? | One |
| How many sides of the object does reflective need to access? | One |
| What properties of a target does diffuse mode take advantage of? | Reflective |
| What type of targets does retroreflective detect? | Opaque |
| What type of targets does polarized retroreflective detect? | Shiny |
| What are advantages of through-beam sensor | - High margin for contaminated environments - Longest sensing distances - Not affected by second surface reflections - Most reliable when you have highly reflective objects |
| What are disadvantages of through-beam sensor? | - More expensive bc of separate light source and receiver. Costly wiring - Alignment is important - Avoids detecting clear objects |
| What are advantages of retroreflective sensor? | - Moderate sensing distances - Less expensive than through beam bc of simpler wiring - Ease of alignment |
| What are disadvantages of retroreflective sensor? | - Shorter sensing distance than through beam - Less margin than through beam - May detect reflections from shiny objects (use polarized instead) |
| What are advantages of polarized retroreflective sensor? | - Ignores first surface reflections - Uses visible red beam for ease of alignment |
| What are disadvantages of polarized retroreflective sesnors? | - Shorter sensing distance than retroreflective - May see second surface reflections |
| What are advantages of standard diffuse mode? | - Access to both sides not required - No reflector needed - Ease of alignment |
| What are disadvantages of standard diffuse mode? | - Can be difficult to apply if the background is highly reflective and close to the object |
| What are advantages of sharp cutoff diffuse mode? | - Access to both sides not required - Provides protection against sensing close backgrounds - Detects objects regardless of color |
| What are disadvantages of sharp cutoff diffuse mode? | - Only useful for short distance sensing - Not used with backgrounds close to object |
| What are advantages to background suppression diffuse mode? | - Access to both sides not required - Ignores backgrounds beyond rated sensing distance regardless of reflectivity - Detects objects regardless of color at specified distance |
| What are disadvantages to background suppression diffuse mode? | - More expensive than other diffuse modes - Limited maximum sensing distance |
| What are advantages to fixed focus diffuse mode? | - Accurate detection of small objects in a specific location |
| What are disadvantages to fixed focus diffuse mode? | - Very short distance sensing - Not suitable for general purpose sensing - Object must be accurately positioned |
| What are advantages to wide angle diffuse mode? | - Good at ignoring background reflections - Detects objects that are not accurately positioned - No reflector needed |
| What are disadvantages to wide angle diffuse mode? | - Short distance sensing |
| What are advantages to fiber optics? | - Glass fiber optic cables for high ambient temp applications - Shock and vibration resistant - Plastic cables for areas where continuous movement is required - Insert in limited spaces - Noise immunity - Corrosive areas placement |
| What are disadvantages to fiber optics? | - More expensive than lensed sensors - Short distance sensing |
| What is the sensing distance of ultrasonic sensors? | 50mm - 3.5m |
| What is the sensing distance of retro-reflective sensors? | 600mm - 5m |
| What is the sensing distance of diffused sensors? | 30mm-1m |
| What is the sensing distance of transmitted beam? | 0m - 70m |
| What are the voltages used to power 2- & 3- wire devices? | 10-30 VDC, 20-130 VAC, 90-250 VAC, 20-250VAC/DC |
| How do you configure current sinking sensors? | By matching them to current sourcing PLC inputs |
| How do you configure current sourcing sensors? | By matching them to current sinking inputs |
| Define electromagnetic output actuator | Any device that contains a magnetic winding or coil that converts electrical energy into mechanical movement |
| What are the types of electromagnetic output actuators that are driven by PLCs? | solenoids, relays, contactors, motor starters |
| What does rated voltage mean? | The suggested operation voltage for the coil |
| What does rated current mean? | The maximum current before contact damage |
| What do holding contacts / sealing contacts do? | Provide a method of maintaining current flow to the coil after a momentary switch has been pressed and released. They carry the full power of the circuit output |
| What are the components of a latching relay? | A latch coil and an unlatch coil |
| How does a latching relay work? | Its contacts remain open and/or closed even after the power has been removed from the coil |
| What are contactors? | Relays that switch large currents from large voltage sources |
| Why are field devices interfaced to PLC relay outputs? | To manage high current requirements |
| Why are field devices interfaced to PLC solid-state outputs? | To control low-power circuitry |
| What are important notes for powering output field devices? | - Power for output field devices is not supplied by PLC - User must ensure current requirements of the devices are compatible with the PLC output capability |
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