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Open House March 26, 10-2, in Bell Hall Suitable Presentation = B Which groups?
No Homework If you feel that you need extra credit, you can do the Washing Machine/PLC homework. Otherwise, I don’t want to impede progress on the project. Schedule your Design Reviews – MWF at 10 or at 2.
Programmable Logic Controllers A Concise Exploration
Industrial Automation Common knowledge: computers are used in factories... Robotic arm, CNC, injection molding
Industrial Automation Not common knowledge: today this is usually accomplished with Programmable Logic Controllers (PLCs) PLCs are the answer to a variety of needs: durability, reliability, flexibility, scalability, reprogrammability, etc...
Industrial Automation Why should you care? Because you will run into PLCs... Median starting salary for entry-level “Electrical Controls Engineer” is $57,452. (EE is $55K, HW Eng is $48K, SW Eng is $53K) [monster.com] As long as there is industry, it will be computer controlled and engineers will earn paychecks.
History Machines become autonomous. Common method: relay logic
Relay Logic Conditional logic can be represented in terms of contacts and coils. Contact: A simple input switch. Coil: An output load, e.g., a relay or motor. Symbolic representation called ladder logic. current
Relay Logic To clarify: “Ladder Logic” is a notation originally used to describe & document relay logic configurations. Later became the basis for PLC programming languages
Ladder Logic Power supply rails drawn as parallel vertical lines on left and right Connection of rails implies current will flow An output is “on” when a connection is completed and current flows through the load’s coil
Ladder Logic Simple “always on” load: [Always_On = 1] note: sometimes illegal Boring... Load controlled by a single contact: [Switch_Con = Switch]
Ladder Logic Boolean logic - C = A and B C = A or B
Ladder Logic C = not A A contact with a slash through it is “normally closed.” This indicates a connection when A is NOT triggered. So when sensor/input A is activated, there is an open circuit
Ladder Logic Each rung of the ladder is a statement that is asynchronous when implemented in relay logic, but evaluated sequentially by the PLC. X = (A or B) and (C or D), Y = ~A and [B or (C and D)]
Ladder Logic Converting between ladder logic and physical electronics is straight forward. So this…
Ladder Logic …becomes this:
PLC The first PLC was invented by Dick Morely in Morely designed a computer with three components: a processor, memory, and a logic solver. “[The logic solver] allowed us to get the speed we needed in this application-specific computer to solve the perceptually simple problem of several cabinets full of relay wiring.” -Morely
Industrial Installation CPU Optical Isolation Wiring to Sensors and Machinary
PLC The first PLC (the 084) was extremely durable and reliable... “We used to test the programmable controllers with a Tesla coil that struck a quarter inch to half-inch arch anywhere on the system, and the programmable controller still had to continue to run.” –Morely FYI, this is a Tesla coil:
PLC Hello World on the PLC. Real hardware: In ladder logic:
PLC Not very interesting… how about a “stay- on” variation? (When the switch is released, the light stays on) PLC benefit: The state of an “output” in one rung may be used as a “contact” in another. In fact, there are “internal utility relays” – virtual outputs that act as intermediate steps toward real outputs.
PLC Latched (“stay-on”) Hello World: When the switch is pressed, “Neon” will be active in the first evaluation. In subsequent evaluations, “Neon” will force itself to stay on. NOTE: OUTPUTS CAN ALSO BE INPUTS
PLC A bit more convoluted: toggling Hello World with a single button. Remember - Switch state: ON OFF ON OFF Figure it out
S1010 Sinv0101 L1100 N0110 Ninv1001 input: Latch = (Switch AND notNeon) OR (Latch AND notSwitch) Neon = (Latch AND notSwitch) OR (Neon AND Switch)
The logic SW1010 SWInv0101 Neon0110 NeonInv1001 Latch1100 A1000 B0100 C0010 A = SW and NeonInv B = SWInv and Latch C = SW and Neon Latch = A or B Neon = B or C
PLC PLCs also support a range of special functions: timers, counters, sequencers, memory instructions, etc… Beyond the basics, they are non-standard and manufacturer-specific.
Timers and Counters Timer: When enabled it counts down, then enables a rung when it reaches zero Counter: counts down when pulsed, then enables a rung when it reaches zero
Exercises Turn 8 LEDs on and off in sequence, repeat. 1 minute Egg Timer. Turn a pump on for 60 seconds, then off for 40 seconds, then repeat. Use a switch to start it off.
LEDs: Using a sequencer Define a Counter output – call it Seq1 Give it a set value 1 greater than your desired sequences: –e.g. if 4, then 0 thru 4 = a set value of 5 Use special bits as inputs Use Seq1:0 to reset the sequencer by defining an output FUNC
Egg Timer IO Table - Inputs: 1/s Clock Outputs: EggDone Counters: Count down from 60 Steps: 1/sec used to trigger counter 60 times When counter at 0, energize EggDone Problem: Use a switch to reset counter Problem: Add a Start Switch
Add a start switch Use a Latch Enable counter with Latch Add ~Reset to Latch Circuit
60 sec ON/ 40 sec OFF Inputs: None Outputs: None Timers: 60 sec, 40 sec. Steps - Count down the 60 sec Timer ONLY WHEN the 40 sec Timer is OFF Count down the 40 sec Timer ONLY WHEN the 60 sec Timer is ON
- A Timer is ON when it is at zero, not when it is counting. - A Timer counts when it is being energized by its circuit. - A Timer is RESET when its circuit is shut off. This is important State1: at startup, Timer40 is OFF so Timer60 starts counting. State2: Timer60 is counting, so Timer60 is OFF, so Timer 40 is not counting State3: Timer60 is zero, therefore Timer40 is counting, therefore -Timer40 keeps Timer60 from counting State4: When Timer40 reaches zero, Timer40 is ON, -Timer40 is OFF, Timer60 gets reset, therefore OFF, then Timer 40 is reset, therefore OFF, and Timer60 starts counting again.
Extend the problem Add a pump, on for 40, off for 60 –The pump should be on when Timer40 is counting –Timer40 counts when Timer60 is at zero –Add the pump output with Timer60 Input Add a Start Switch –Just put a Latch everywhere
Special Bits Use with Counters named “SeqN” Unchangeable SWITCH (always ON)
Special FUNCTION Inputs
Special Bits Normally ON Flag - Norm.ON You can make use of this flag if you need to keep something permanently ON regardless of any input conditions. This is because a coil or a special function is not allowed to connect directly to the power line (the vertical line on the left end of the ladder diagram). If you need to permanently enable a coil, consider using the "Normally-ON" bit from the "Special Bits" menu, as follows: