04 Programmable Logic Controllers (PLC) and Motion Control (MC)

Question 1

Karnaugh-Veitch-Map

Answer 1

• Graphical procedure to determine the disjunctive normal form. -> It is always possible to identify the minimal disjunctive normal form, when applied correctly.
  -> Disjunctive normal form means that the equation cannot be simplified any more.
• Number of fields in a KV-Map must be the same as the number of rows of the truth table
• Each variable is denoted along the side of the KV-Map
• Index in the right bottom corner of each field resemble the appropriate row number of the truth table

Question 2

Flip-Flop Memory

Answer 2

• The flip-flop (bistable multi-vibrator) is a basic block which is used to store a binary signal.
• Two inputs and two outputs
• If a signal is present at S, then according to the truth table there will be a direct output at Q (first line) and this will be held until the flip-flop is re-set by the re-setting input R (second line). If no signal is present at S, then the current value of output Q will be held (third line)
• The output always has the complementary condition (i.e. opposite signal to that of output Q.) -> This circuit is also called R-S flip-flop

Question 3

1:2 Scaler

Answer 3

• A 1:2 scaler is realized by repatriating (zurückführen) the inverted output Q to the D-Input. -> Output is changed from true to false with each positive edge of the clock signal. The clock frequency is therefore divided by 2.
• When separate 1:2 scalers are connected to a series, e.g. a counter can be realized.
  o Each scaler divides the input frequency by two, the number of input pulses can be read as a binary number.

Question 4

Programmable Logic controllers (PLCs):
Definition technical process

Answer 4

A technical process is a process, whose physical values can be captured and affected by using technical resources.

Question 5

Programmable Logic controllers (PLCs):
Tasks of a Control

Answer 5

o Capture all required analogue and digital process signals
o Process those signals and calculate (usually according to a program) all appropriate control signals
o Control the actuators accordingly to the calculated control signals

Question 6

Programmable Logic controllers (PLCs):
Input Level

Answer 6

o All process signals are read in
o Signal sources can vary from inputs of the user, sensor signals or data/values from data storages
o Sensors can be used to capture machine or process states
o Control elements are direct human-machine-interfaces

Question 7

Programmable Logic controllers (PLCs):
Signal Processing

Answer 7

o Output signals are calculated according to logic connections from the input signals
o Logic connections can be realized as a program (software), electromagnetic (relais) or by using semiconductor elements.

Question 8

Programmable Logic controllers (PLCs):
Process Computers

Answer 8

Digital, Programmable Devices^

Question 9

Set-Up of Process Computer Systems

Answer 9

Digital processor with hard- and software extensions

 Process interfacing
 Operating System including real-time behavior
 Response time of a processor must be smaller than the process-related machining time
 Use of a real-time clock to synchronize processor task and technical process
 Signal sources (Sensors etc.) and actuators are directly connected to input- and output devices of the PLC
* Task of input devices: Electrically decouple of all input signals and adapt their voltage level

 Output devices inherit signal amplifiers which modulate the internal 5V DC to the appropriate level to control actuators.  Power contractors can be used if the max. amplitude of the currents is not sufficient to control field devices (e.g. large motors)
 Data Storage of a PLC can be subdivided in system, program and data storage.
 Operating System: Responsible for controlling the cycle time as well as the correct processing of signal and the application program  Stored in a EPROM and cannot be manipulated by the programmer

Question 10

Hard-Wired Control –> Coding, Technology, Application, Mode of operation

Answer 10

• Coding: Hardware (wiring)
• Technology: Electro mechanic (relay), cordless (semiconductor)
• Suitable for simple applications
• Mode or operation: faster than PLC because of parallel program processing
• HWC in safety critical applications

Question 11

Programmable Logic Controllers (PLC) –> Coding, Technology, Mode of operation, advantages

Answer 11

• Coding: Software (single instruction)
• Technology: Contac less (semiconductor, microcontroller)
• Mode of operation: serial program processing
• More flexible and higher performance
• Cheaper and clearer than HWC for highly significant applications

Question 12

PLC configurations and types

Answer 12

• Hardware-PLC
• Slot-PLC
• Soft-PLC

Question 13

Hardware PLC

Answer 13

 Most common type

 Power supply, CPU, digital/analogue in- and outputs

 Fieldbus or proprietary bus system –> Communication, programming interface

Question 14

Slot-PLC

Answer 14

 Independent of the operating system
 Hard real time conditions possible
 Low failure ratio
 Direct access to the host-PC
 High arithmetical capacity

Question 15

Soft-PLC

Answer 15

 PLC based on Windows-NT
 Soft real time conditions
 Higher error risk
 Short life span

Rarely integrated to control real production processes. The application focus is more on the simulation of PLC functionality in field tests.

Question 16

PLC - Different forms of Program processing

Answer 16

• Cyclic processing of the program
• Event-controlled processing (interrupt)
• Time-controlled processing

Question 17

PLC - Cyclic processing

Answer 17

Normal operation mode: Cyclic processing of instruction sequence which are stored in the program memory
 Whole program is processed continuously
 Length of a cycle depends on the clock speed of the CPU and the length of the number of operations
 IPO Principle (Input -> Processing -> Output)

 Output values are calculated and stored in the process output image (POI) -> After completion of cycle the values are transferred to the (hardware) output

 PII: Process Input Image -> All inputs are loaded into this picture at the beginning

Question 18

PLC: Event-controlled processing (interrupt)

Answer 18

 Program cycle is interrupted by a processing signal (e.g. emergency stop) to run a special routine
 After processing this routine, the regular program cycle is continued (unless main program has been generally interrupted)

Question 19

PLC: Time-controlled processing

Answer 19

 Program cycle is interrupted by an internal time signal to run a special routine
 After processing this routine, the regular program cycle is continued

Question 20

Conventional PLC Programming methods

Answer 20

• Ladder
• Functional Block Diagram (FDB)
• Sequential Function Chart (SFC)
• Instruction List
• Structured Text

Question 21

PLC Programming: Ladder

Answer 21

Based on the representation of electrical schematics, the signal flow is always from left to right.
 Other elements: Closer (>E1), Opener (>E2) or Assignments (>A1)

Question 22

PLC Programming: Function Block Diagram (FBD)

Answer 22

Signal flow is usually from top to bottom or from left to right. Signals and inputs are connected to logic gates (AND gates, NOR gates etc.) which calculate values from the input signals  Values can either be used in other gate circuits or can be loaded as results in the PIO

Question 23

PLC Programming: Sequential Function Chart (SFC)

Answer 23

In SFC the program is not calculated completely once per cycle. -> Execution of states happens sequentially

 Process always starts in the “INIT”-State
 Transition connects the state to one (or more) subsequent state(s)  If transition is linked with a condition, the change to the next state only takes place when the transition condition is valid/true. If condition is false, the program may persist for several cycles in one state. While a state is active, all listed state instructions to the right are being executed

Question 24

PLC Programming: Instruction List

Answer 24

A set of instructions is available, which can be used to process variables and signals logically and arithmetically

Question 25

PLC Programming: Structured Text

Answer 25

Language very similar to the C programming language. There are several functions, loops and other control structures.

Question 26

PLC Solutions for machine tools (5)

Answer 26

o Control of functional units
o Monitoring and diagnose
o Graphical user interface shared with NC
o Specialized tasks
o Data exchange with other controls

Question 27

Errors and costs during PLC software programming

Answer 27

o Most software failures occur in early stages of development phase and when creating the program
o Most important requirement: Avoid expensive troubleshooting processes through systematic specification/planning of the control task
o Close cooperation between the mechanical and electrical design as well as the early coordination of solution concepts are therefore fundamental prerequisites for creating error-free PLC programs

Question 28

Easy Test procedure for PLC Programs

Answer 28

 Connect the programmed control hardware to the hardware (machine or test dashboard) and test by manually setting inputs
 By observing the outputs the test engineer can verify the correctness of the system
 For each observed error, the code can be corrected -> a re-test can be carried out

Disadvantage:
User must recreate the process behavior of the machine manually  Requires a high degree of abstraction (Replaces the time behavior of machine only approximate, dynamic processes are hardly verifiable)

Question 29

Complex Test Procedure for PLC Programs

Answer 29

Hardware-In-The-Loop (HIL) Simulation

 Controller is integrated into a simulation environment
 No remote inputs and outputs needed -> They are simulated by the Profibus installation card in the PC
 Mechatronic behavior can be configured and verified on the control computer

Characteristics of HIL simulation
* To simulate original NC and PLC hardware is used
* Simulation is run on the computer to simulate the response of the connected components
* Real-time communication between the components involved

Question 30

Fieldbus

Answer 30

o Connects sensors and actuators to the controller -> Controller itself is directly integrated into the machine tool
o Commonly used at the lowest level of the automation pyramid

Question 31

Direct Connection (conventional, no fieldbus)

Answer 31

• Single control unit
• Low space requirements
• Intensive wiring effort

Question 32

Decentral Connection (Fieldbus)

Answer 32

• Reduced wiring effort for actors and sesors
• Small, single units
• Signal processing “slower” compared to bus architecture
• Less planning complexity
• Easier troubleshooting

Question 33

Industrial Ethernet

Answer 33

o Commonly used in higher levels of the automation pyramid
o Real Time capability through fast ethernet and full-duplex
o Reduced wiring effort of actors and sensor
o Consistent usage of ethernet within the company (ERP -> MES -> Shopfloor)
o Transfer of higher data volume possible -> Enables image processing applications
o Easy wiring of field devices
o Usage of various transfer media (Wifi, flass fibre, copper cable)
o Real-time capable star architecture demands specialized switches
o High availability of cost-effective components (due to standardized connectors)