Terry Woods
Member
- Join Date
- Apr 2002
- Posts
- 3,170
“Why Some Novices are Confused by the ‘Imaginary Relay’ Model”
Even if we might not be able to explicitly express the basic rules of logic, we all eventually “internalize” (at least, some of) the more basic rules through life-experiences.
When any novice first comes to learn PLC’s, whether he realizes it or not, he is not completely in-the-dark, at least, not with respect to the really basic parts of logic.
I really can’t say anything disparaging about the “intent” behind the “Imaginary Relay” model. It was nothing more, or less, than an Engineer’s attempt to simplify, what was then, apparently, (and still is, apparently) a difficult concept to grasp.
Since the target-audience for the simplification was the electricians/technicians of the day, the simplification had to be based on something they could relate to.
Ergo... the “Imaginary Relay”.
Teacher: So, that’s how the Imaginary-Relay model works.
Student: Uhhhh... I can’t quite put my finger on it, but something just doesn’t seem to make sense.
Teacher: Trust me! I’m a Teacher! I KNOW WHAT I’M DOING! (Just go with it... it’ll make more sense as time goes on.)
As a “solution”, the “Imaginary Relay” model includes its own set of problems. The “Imaginary Relay” model is...
It appears to me that the Imaginary Relay model simply trades one set of problems for another. (Kinda like selling your old car and then buying a brand-new used car! You’ve traded a known set of problems for an unknown set of problems. Whether or not you have actually bettered your situation remains to be seen!)
What the Teacher said... “Just go with it... it’ll make more sense as time goes on.”
What the Teacher is saying... “Have FAITH!”
The Imaginary Relay model asks that you “Just go with it... and, Have FAITH”.
The real problem lays in the fact that the Imaginary Relay model is NOT asking you to “Imagine a Relay”... rather, it is asking you to “Imagine an IMAGINARY RELAY”.
Let’s say we have a normally open, single-contact, limit switch, identified as 2B. At this point, the limit switch is simply lying on the floor. It’s not really monitoring the status of anything. We are simply trying to monitor the status of the limit switch. Now, let’s invoke the Imaginary Relay.
Limit Switch 2B drives the Imaginary Relay. We are asked to imagine this relay with a normally open contact and a normally closed contact.
.
The problem begins right here!
We are being asked to imagine a thing that really exists in reality. We can pick one up and say “this is a relay”. When asked to “imagine a relay” inserted into a circuit (as shown above), we can do so, very easily, because we are very familiar with what a relay is and how it works. We also assume the normal operating characteristics. It’s not hard to imagine a “real” thing.
So... let’s pretend that this “imaginary relay” is Really REAL.
So... this limit switch controls a REAL Relay. There are two outputs from that relay: one output is from the Normally Opened contact and one output is from the Normally Closed contact.
Assuming the system is healthy these two outputs indicate the state of the limit switch.
When the limit switch is OFF, the relay is OFF:
When the limit switch is ON, the relay is ON:
Now, let’s connect each relay-output to a PLC-input. Connect the output from the Normally Opened contact to PLC-Input-0 and the output from the Normally Closed contact to PLC-Input-1.
.
Again, assuming the system is healthy, when the limit switch is OFF, the relay is OFF:
Now, in this scheme, if any programmer wanted to determine the state of the limit switch he could write something like this:
.
Now, anywhere that he wants to, a programmer can reference C1 if he wants to know if the limit switch is ON, or C2 if he wants to know if the limit switch is OFF.
So, using the REAL version of the “Imaginary Relay” model gives us a direct indication of the state of the limit switch: LS-2B is ON; LS-2B is OFF. The “REAL-Relay” model is about as neat, clean, logical, and straight forward as it can be. It’s also Wrong! Huh?????
In reality, we use only ONE PLC INPUT to monitor the status of any single-contact field device.
(This is where the model really fails!)
The reality of using only one PLC-input forces us to imagine that both of the outputs from the relay (either the Real-Relay or the Imaginary-Relay) are connected to the same PLC-Input. Now, remembering that the state of the “imaginary relay” is supposed to indicate the state of LS-2B, we get something like this...
.
Bonus Points if you can “read” this rung. (Thanks, Allen!)
The “–-( ??? )” represents the single, real, PLC Input.
I might not be the sharpest knife in the drawer, but, it looks to me like the PLC-input will ALWAYS be ON! No matter what the state of LS-2B is!
How can the PLC Input differentiate between the two possible conditions? How can you derive “UNIQUE” information from an “OR” source? How can a novice, familiar with even a little bit of logic, accept this concept?
I believe that this is the source of the confusion for many moderately experienced users as well as most novices.
To get past this faux pas, the model asks that you ignore the reasonable, logical conclusion that the model, itself, creates.
By asking novices to ignore that logical conundrum, the model is really asking novices to “Imagine an IMAGINARY-Relay”. This is quite different from being asked to “Imagine a Relay”.
When asked to “imagine a relay”, we can do so, very easily, because we are very familiar with what a relay is and how it works. We also assume the normal operating characteristics.
However, when asked to Imagine an “IMAGINARY Relay”, the word “Magic” is buried in there somewhere.
In any logic situation, you can not determine “UNIQUENESS” from an “OR” source. If, however, it is simply declared that WE WILL HAVE “UNIQUE” INFORMATION, despite the logical inconsistency... then, somehow, the normal characteristics of the Imaginary Relay have to be bent just a little to meet that illogical requirement.
So, whether spoken or not, the novice is being asked to imagine a relay that doesn’t quite follow the rules of logic as they are understood... they are being asked to Imagine an “IMAGINARY-Relay”, a relay that has just a bit of “magic” in it! (....it must be an Irish Relay). Somehow, the Imaginary Relay is expected to impart “UNIQUE” information from an “OR” source.
Hmmmm... how ‘bout this...
.
Even if we might not be able to explicitly express the basic rules of logic, we all eventually “internalize” (at least, some of) the more basic rules through life-experiences.
When any novice first comes to learn PLC’s, whether he realizes it or not, he is not completely in-the-dark, at least, not with respect to the really basic parts of logic.
I really can’t say anything disparaging about the “intent” behind the “Imaginary Relay” model. It was nothing more, or less, than an Engineer’s attempt to simplify, what was then, apparently, (and still is, apparently) a difficult concept to grasp.
Since the target-audience for the simplification was the electricians/technicians of the day, the simplification had to be based on something they could relate to.
Ergo... the “Imaginary Relay”.
Teacher: So, that’s how the Imaginary-Relay model works.
Student: Uhhhh... I can’t quite put my finger on it, but something just doesn’t seem to make sense.
Teacher: Trust me! I’m a Teacher! I KNOW WHAT I’M DOING! (Just go with it... it’ll make more sense as time goes on.)
As a “solution”, the “Imaginary Relay” model includes its own set of problems. The “Imaginary Relay” model is...
- logically incomplete,
- logically contradictory, and eventually,
- counter-intuitive!
It appears to me that the Imaginary Relay model simply trades one set of problems for another. (Kinda like selling your old car and then buying a brand-new used car! You’ve traded a known set of problems for an unknown set of problems. Whether or not you have actually bettered your situation remains to be seen!)
What the Teacher said... “Just go with it... it’ll make more sense as time goes on.”
What the Teacher is saying... “Have FAITH!”
The Imaginary Relay model asks that you “Just go with it... and, Have FAITH”.
The real problem lays in the fact that the Imaginary Relay model is NOT asking you to “Imagine a Relay”... rather, it is asking you to “Imagine an IMAGINARY RELAY”.
Let’s say we have a normally open, single-contact, limit switch, identified as 2B. At this point, the limit switch is simply lying on the floor. It’s not really monitoring the status of anything. We are simply trying to monitor the status of the limit switch. Now, let’s invoke the Imaginary Relay.
Limit Switch 2B drives the Imaginary Relay. We are asked to imagine this relay with a normally open contact and a normally closed contact.
FIGURE-1
REAL IMAGINED
LS-2B
/ relay
------o o------------( coil )
normally open
o----------------- >
-----------------------o
\
o----------------- >
normally closed
.
The problem begins right here!
We are being asked to imagine a thing that really exists in reality. We can pick one up and say “this is a relay”. When asked to “imagine a relay” inserted into a circuit (as shown above), we can do so, very easily, because we are very familiar with what a relay is and how it works. We also assume the normal operating characteristics. It’s not hard to imagine a “real” thing.
So... let’s pretend that this “imaginary relay” is Really REAL.
So... this limit switch controls a REAL Relay. There are two outputs from that relay: one output is from the Normally Opened contact and one output is from the Normally Closed contact.
Assuming the system is healthy these two outputs indicate the state of the limit switch.
When the limit switch is OFF, the relay is OFF:
- the Normally Open Contact is OPENED, and
- the Normally Closed Contact is CLOSED.
When the limit switch is ON, the relay is ON:
- the Normally Open Contact is CLOSED, and
- the Normally Closed Contact is OPENED.
Now, let’s connect each relay-output to a PLC-input. Connect the output from the Normally Opened contact to PLC-Input-0 and the output from the Normally Closed contact to PLC-Input-1.
FIGURE-2
REAL REAL REAL
LS-2B
/ relay
------o o------------( coil )
normally open
o--------------------o (LED) PLC Input-0
-----------------------o
\
o--------------------o (LED) PLC Input-1
normally closed
.
Again, assuming the system is healthy, when the limit switch is OFF, the relay is OFF:
- The Normally Opened Contact is OPENED and the LED, at PLC Input-0, is OFF.
- The Normally Closed Contact is CLOSED and the LED, at PLC Input-1, is ON.
- The Normally Opened Contact is OPENED and the LED, at PLC Input-0, is OFF.
- The Normally Closed Contact is CLOSED and the LED, at PLC Input-1, is ON.
Now, in this scheme, if any programmer wanted to determine the state of the limit switch he could write something like this:
FIGURE-3
The signal That
from --| |-- means…
of relay 2B LS 2B
is ON! IS ON!
-----| |-----------------------------( C1 )
PLC Input
Input-0
The signal That
from --|/|-- means…
of relay 2B LS 2B
is ON! IS OFF!
-----| |-----------------------------( C2 )
PLC Input
Input-1
.
Now, anywhere that he wants to, a programmer can reference C1 if he wants to know if the limit switch is ON, or C2 if he wants to know if the limit switch is OFF.
So, using the REAL version of the “Imaginary Relay” model gives us a direct indication of the state of the limit switch: LS-2B is ON; LS-2B is OFF. The “REAL-Relay” model is about as neat, clean, logical, and straight forward as it can be. It’s also Wrong! Huh?????
In reality, we use only ONE PLC INPUT to monitor the status of any single-contact field device.
(This is where the model really fails!)
The reality of using only one PLC-input forces us to imagine that both of the outputs from the relay (either the Real-Relay or the Imaginary-Relay) are connected to the same PLC-Input. Now, remembering that the state of the “imaginary relay” is supposed to indicate the state of LS-2B, we get something like this...
FIGURE-4
2B
--| |--+--( ??? )
|
2B |
--|/|--+
.
Bonus Points if you can “read” this rung. (Thanks, Allen!)
The “–-( ??? )” represents the single, real, PLC Input.
I might not be the sharpest knife in the drawer, but, it looks to me like the PLC-input will ALWAYS be ON! No matter what the state of LS-2B is!
How can the PLC Input differentiate between the two possible conditions? How can you derive “UNIQUE” information from an “OR” source? How can a novice, familiar with even a little bit of logic, accept this concept?
I believe that this is the source of the confusion for many moderately experienced users as well as most novices.
To get past this faux pas, the model asks that you ignore the reasonable, logical conclusion that the model, itself, creates.
By asking novices to ignore that logical conundrum, the model is really asking novices to “Imagine an IMAGINARY-Relay”. This is quite different from being asked to “Imagine a Relay”.
When asked to “imagine a relay”, we can do so, very easily, because we are very familiar with what a relay is and how it works. We also assume the normal operating characteristics.
However, when asked to Imagine an “IMAGINARY Relay”, the word “Magic” is buried in there somewhere.
In any logic situation, you can not determine “UNIQUENESS” from an “OR” source. If, however, it is simply declared that WE WILL HAVE “UNIQUE” INFORMATION, despite the logical inconsistency... then, somehow, the normal characteristics of the Imaginary Relay have to be bent just a little to meet that illogical requirement.
So, whether spoken or not, the novice is being asked to imagine a relay that doesn’t quite follow the rules of logic as they are understood... they are being asked to Imagine an “IMAGINARY-Relay”, a relay that has just a bit of “magic” in it! (....it must be an Irish Relay). Somehow, the Imaginary Relay is expected to impart “UNIQUE” information from an “OR” source.
Hmmmm... how ‘bout this...
FIGURE-5
Imaginary
LS-2B or REAL
/ relay
------o o------------( coil )
normally open +-------------+
o----------------| |
| M A G I C |
-----------------------o | B O X |-----o (LED) PLC Input-0
\ | |
o----------------| |
normally closed +-------------+
.