Revision Sept 2014. ELX305. Topics. Reliability Scada Pneumatics Boolean Algebra Timers and timing diagrams PLC programming. RELIABILITY. Definitions.
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(Probability of survival)
(Probability of failure)
Region where l is ~ constantBathtub Curve
R(t) = 1 + U(t) ……………………….(1)
R(t) = e- lt……………………………(2)
MTBF = ………………….(3)
andtherefore l = 1/M ………….……(4)
General formula to be used when l is not constant
(See Lesson 9 student guide for proofs)
i.e. It is assumed that the failure of any unit occurs independently of the failure of the others
From equation (2)
Reliability of the series system
Using Equations (3) M = 1/l therefore (4) l = 1/M failure rate of the system ls and Mean time between Failure Ms can be calculated
EXAMPLE 1 (Tutorial sheet)
In the following example, assuming a series
reliability, find the system MTBF and Reliability in
Unit Type 1 2 3
Number of fails (N) 10 4 5
MTBF (hours) 4x105 1062x106
A system consists of 4 elements, each having a probability of survival to 1000 hours R(1000) = 0.95 . What is the combined 1000 hour reliability of the system.
Rs = R1xR2xR3
= .95 x .95 x .95
Therefore for a simple active parallel system the probability Us that the system is in the failed state is: Us = U1 x U2
Since U(t) = 1 – R(t)
It follows that:
Reliability of the parallel network
Using this information
Two identical PLC units are connected in parallel. The reliability requirement is such that both units have to fail before the complete system fails. Each unit has a Mean Time Between Failure (MTBF) =105 hours. Calculate the reliability of the system RS in 104hours
e.g. Wonderware, Microscan, Labview
Typical applications include:
The transmission type determines the communication speed and noise immunity.
Pros and Cons
Output Port ( Top )
Inlet Port ( Bottom )
2/2 Way Directional Control Valve (Flow Switch)
4/2 Way Directional Pushbutton Control Valve with Spring Return
Spring Return3/2 Pneumatic Control Valve
Open3/2 Pneumatic Control Valve driving a Linear Actuator
What Happens if the Pressure Supply is lost ?
Spring Return5/2 Pneumatic Control Valve
Open5/2 Pneumatic Control Valve driving a Linear Actuator
What Happens if the Pressure Supply is lost ?
2Cascade Valves for higher flows
How does this improve cylinder flow rates ?
Used to vary air flow in one direction only, typical application would be to vary the extension or contraction of an air cylinder. Imagine this element placed into the flow line between the directional control valve and the input port of the actuator. The response of the actuator would be rapid in one direction and slow in the reverse direction, an action that could not easily be achieved using direct PLC control.
What is restricted by each flow valve ?
A+ Actuator direct.
From the previous Circuit note :
B-Two Actuator Operation
For the following sequence
then we require :
(b)TON - Timer on - examples
The following diagram shows a conveyor and sorting system which is used to feed boxes into three separate chutes. The system loads 6 boxes into each chute. There are two pneumatically operated gates and a fixed gate. These are used to divert the boxes from the conveyor down the chutes. There are three through beam photo electric sensors S1, S2 and S3 which are used to detect the boxes as they pass along the conveyor.
The sequence of operation is as follows:
1. When start button PB1 (I:1/4) on the operator control panel is pressed the conveyor motor CM (O:1/3) starts. (This is a 3 phase 415volt delta connected motor).
2. Gates 1 and 2 are in the open position, boxes are conveyed past sensor S1 (I:1/1) which counts 6 boxes into chute 1 via the fixed gate
3. Then Gate 1 (O:1/1) is closed and Sensor S2 (I:1/2) counts 6 boxes into chute 2.
4. Gate 2 (O:1/2) closes and Sensor S3 (I:1/3) counts a further 6 boxes in to chute3
5. The conveyor is then stopped automatically
At any time the loading of the conveyor can be stopped via a manual push button PB2 (I:1/5) which is situated on the operator control panel. A reset button PB3 (I:1/6) is also provided to reset counters that are used in the system.
CREATE AN I/O LIST WITH ALLOCATIONS PROVIDED IN TEXT
DRAW THE INPUT AND OUTPUT MODULES SHOWING THE INPUTS AND OUTPUTS CORRECTLY CONNECTEDAS PER I/O MAP
REMEMBER THE 3 PHASE MOTOR CANNOT BE DIERECTLY CONNECTED TO THE PLC SO NEEDS TO BE INTERFACED VIA AN OUTPUT RELAY AND CONTACTOR
SHOW A BASIC 3 PHASE DOL DIAGRAM
FROM THE SEQUENCE DESCRIPTION PROVIDED IT SHOULD BE CLEAR THAT THE FOLLOWING ELEMENTS ARE NEEDED
A STOP /START CIRCUIT FOR THE MOTOR
3 X ‘UP’ COUNTERS SET AT A COUNT OF ‘6’
SOME CONTROL LOGIC TO LIFT AND LOWER CHUTE 2 AND 3 GATES (CHUTE 1 IS FIXED)
SCHEMATIC WIRING DIAGRAM