An encoder is a device that is used in many industries to provide feedback.
In the most basic terms, an encoder,
regardless of the type, which we will cover later,
senses position, direction, speed, or counts.
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Encoders will use motion, under a variety of technologies,
and translate it into an electrical signal.
That signal is then sent back to a controlling device,
such as a PLC, and is interpreted, meaning scaled,
to represent a value that will then be used within the program.
Some of the technologies involved in encoders are “magnetic”,
“mechanical”, “resistive”, and “optical”.
“Optical” is the most widely used encoder motion translating technology.
There are different types of encoders
such as “absolute” and “incremental”.
We will describe those in greater detail in a future lesson.
But for now, an example of an incremental, optical type encoder
uses a beam of light that passes through a disk
that has opaque lines in a specific pattern,
somewhat like the spokes of a wheel.
On the other side of the disk is a photo sensing device
that will interpret the light, based on the pattern on the disk,
picture a shutter, blocking and unblocking the light.
The pulses of light are then converted to an electrical signal
to be sent back to the processor,
through the encoder's output.
Encoders have a wide range of uses
which include “Closed-Loop” applications
such as “servo or VFD control”, “measuring”, and “counts”.
Here are some examples of processes that may use an encoder.
For VFD control, you may be running a pump,
on a VFD, to fill a tank full of a liquid.
You are requesting a certain speed
and want to verify that the pumps VFD is at the requested speed.
An “encoder” on the VFD may be used for feedback of speed.
Next, let’s give an example for a “measuring” process.
In this application, you will need to cut some aluminum product to a particular size.
You are passing a long roll, meaning hundreds of feet,
of the aluminum sheet through a cutting mechanism.
You need to determine the amount of aluminum fed,
so that you can cut the sheets to the proper size
that will be used in a separate manufacturing process.
An encoder, attached to the conveyor
and reading the material that is feeding through your cutting assembly,
will indicate the length of material that has been fed since the last cut.
That feedback can then be used to adjust the cutting blade
to sever the length required.
For an example of “counts”, consider this process.
You have a conveyor line that has bottles running on a conveyor.
They are counted by a photo eye sensor when entering the assembly.
They have a cap with an aluminum tamper proof foil on top
that needs to adhere to the bottle.
Once the foil is sealed, the bottle will then move down the conveyor line
and verified that it exited the cap sealing assembly
via an exit photo eye sensor.
Some of the requirements for this station are:
The same number of bottles that enter the assembly
must exit in a “predetermined time frame”.
The bottle must not remain in front of an “entrance” or “exit” sensor.
The bottle must not be exposed to the inductive sealer
longer than a predetermined amount of time.
You must make this assembly flexible enough
to handle many types of bottles
and entrance and exit sensor placement.
Consider a prescription bottle, easy right?
No handles, just a bottle that is a standard size.
Now consider an antifreeze bottle
with a somewhat small cap and a very large handle.
How in the world are you going to meet the requirements for the machine
and remain flexible?
If you put a pill bottle in, yep, easy peasy right?
bottle in, seal, bottle out, no sensors blocked.
If any of the requirements fail,
bottle in, seal, tips over and isn’t counted out,
machine halts and alerts an operator of a malfunction.
How about the “antifreeze” bottle?
Bottle in, cap passes in front of an entrance sensor,
bottle counted, then the handle;
the system faults because it sees a blockage at the entrance.
How do you tell the machine that this is expected behavior?
“An encoder” of course, and a “selector switch” for a setup mode.
To set up, insert your product, in this case, the antifreeze bottle.
You make sure to feed cap first
and it must contain a foil.
Your program reads an entrance on the sensor
and records “an encoder reading”
as well as determines that this is a “cap” and records that fact.
Your program then reads another entrance
and determines that this is not “cap”
and must be some other acceptable part of the bottle, possibly a handle.
The single bottle continues down the line
where the exit sensor is triggered.
At that point, you again record the encoder value.
You now have an encoder count
that registered at the entrance and exit.
In the code, you subtract the entrance from the exit
and you have a bottle “count”.
You now know precisely how long it will take that bottle
to move through the system,
meeting the “must enter and exit” in a predetermined timeframe,
in this case, encoder counts.
You have also made the flexibility aspect of different types of bottles
as well as being able to place the entrance and exit sensors
any reasonable distance apart.
When back in run mode,
the code knows that there’s a handle to ignore
when it comes into the process and doesn’t fault the machine.
You register the encoder at each bottle entrance
and predetermine when the bottle must exit.
If the bottle doesn’t exit the machine in a timely manner,
which is based on your projection
and the current encoder count, the machine faults.
This is a great use of an encoder that is fully configurable.
Count in, count out, subtract, done.
Don’t miss the next lesson
where we are going to discuss incremental and absolute encoders.
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