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Old January 21st, 2003, 04:00 PM   #1
ready961
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Question encoders and plcs

Hello
I just wanted a quick and simple overveiw on how encoders work and what they are used for. And how you connect one to a plc
Thanks
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Old January 21st, 2003, 05:06 PM   #2
hesham
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Encoders are simply a device which depend on its operation on cutting a ligh beam with a rotray disk (Fixed between the emitter & receiver)
This rotary disk is designed in mainly to ways to produce incremental or absolute encoder.Also the design of this disk determine the most important parameter for the encoder whis is how many pulese will be produced per revolution .
With an electronic Circuit this pules are usually outputed in two Channels Shifted 90 dgree From each other and one index channel.
Enoders are used in many applications Like speed measuring ,Positionning & Legnth Measurment
Almost all PLC manufactures include in their product a set of special devices called High Speed Counters to deal with encoder with a certin inputs assigned for this HSC.
Wiring Encoder to PLC depened on the HSC used in your program(Note the assigned inputs for the HSC you use in your program) and also depend on the output type of the Encoder (Push-Pull,Open collector,Line driver....etc
I hope that will be usefull
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Old January 21st, 2003, 06:10 PM   #3
rsdoran
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More than you probably wanted to know

Question: What is the basic definition of an encoder?
Answer: An encoder comes in two architectures. The first architecture is linear. The second architecture is rotary. Both types sense mechanical motion and translates the information (velocity, position, acceleration) into useful electrical data.

Question: What is the difference between an absolute and an incremental encoder?
Answer: There are a few subtle differences between absolute and incremental rotary encoders. Incremental encoders have output signals that repeat over the full range of motion. It is important to understand that each mechanical position is not uniquely defined. When the incremental encoder is turned on, the position of an incremental encoder is not known since the output signals are not unique to any singular position. Absolute encoders have a unique value (voltage, binary count, etc) for each mechanical position. When an absolute encoder is turned on, the position of an absolute encoder is known (this function resembles a resolver, although the principles of operation have no similarity.) The similarities of both absolute and incremental encoders are form factor and the issues of count and directional information. They can be obtained from both absolute and incremental encoders equally.

Question: What is a channel?
Answer: A channel is an electrical output signal from an encoder. Typical incremental encoders have either two or three channels. An example would be: A, A not, B, B not, and Z, Z not (also referred to I or I not.).

Question: What is quadrature and 4X (4 times) quadrature encoding?
Answer: quadrature is a description of the 90 degrees out of phase between channel A and B. The rising edge to rising edge (referred as the Cycle) on channel A or B indicates that one set of bars (on the internal encoder disk) have passed by the optical sensor. The quadrature state of channels A and B create four unique logic states. When these four unique logic states are decoded, the resolution obtained is 4 times (4X) the resolution of the encoder disk. So with this in mind 250 sets of bars would yield 1000 quadrature states.

Question: What is the Index pulse (strobe, Z, and I.)?
Answer: It is the absolute reference added to an incremental encoder. It is also known as home position. It signifies a full rotation of the encoder disk

Question: How can I reduce the effect of noise when using encoders?
Answer: There are several ways to improve noise immunity. Be careful to separate motor and all power wires away from encoder wires. Always separate the channels from each other. All encoder wire should be shielded and carefully terminated. The pigtail that connects to the control unit using the encoder should be as short as possible. The leads will act as antennas for any stray RF in the vicinity. The addition of other noise suppression devices may be required in high noise environments.

Question: What are "Push Pull" and "Totem Pole" outputs? Why don't they have a differential output?
Answer: Looking at the A and A not channel, the optical sensor internal to the encoder is comparing the two. If A’s light amount is greater than A not, then A is high. When A is less than A not, then A is low. The same reasoning defines channel B. This process is called “Push Pull.” A “Totem Pole” output is the same as a “Push Pull” output. The big difference is the ability of the “Totem Pole” to handle sinking or sourcing less current than the “Push Pull.” It is confusing because the other factor is the amount of voltage usually used to define the difference between the two. “Totem Pole” usually uses 5vdc, where as the “Push Pull” follows the input voltage.

Question: Do I need to shut down my machine to change an encoder?
Answer: The best method is to power down the unit. If the supply voltage is accidentally shorted or grounded, it will damage the encoder.

Question: Q: What equipment do I need to troubleshoot an encoder?
Answer: A: An oscilloscope or specially designed encoder tester. A basic multimeter will do for basic diagnostics such as power supply and low speed pulse outputs. Any advanced pulse detection and diagnostics needs to be performed by a trained technician and an oscilloscope.

Question: What is gray code?
Answer: Gray code is a form of binary. The difference is how it is incremented to the next number. Only one number can increment at a time. Gray code: 0, 1, 3, 2, 6, and 7 Binary: 0, 1, 2, 3, 4, and 5

Basically an encoder is a device that emits X amount of pulsed signals per resolution of a motor or shaft. This can be used as was stated to determine speed, location (or distance traveled), and acceleration/deceleration.

An example of use: We have a flexographic printing press that moves the printing plates toward a drum, at the same time it moves an anilox roller (inking roller) towards the printing plate. This is an automated system so the travel distance from "home" to the "print" postion is preset/determined, therefore when you send it to "print" it knows the precise position to go to via the count given from the encoder. In this application the encoders are incremental which means they are step by step device and have no memory. Absolute encoders are designed to remember position even if powered off.

Naturally the above was a simplification but hopefully it was enough to assist in your assignment.
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Last edited by rsdoran; January 21st, 2003 at 06:22 PM.
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Old January 21st, 2003, 09:32 PM   #4
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rsdoran wrote:
Quote:
Naturally the above was a simplification but hopefully it was enough to assist in your assignment.
... hopefully, he follows up on your descriptions in his book/class materials and gets a real understanding of encoders instead of just copying your answer for his homework.

ready961:
Maybe you can post your teacher's e-mail address so we can forward what you have gotten for free on the web and he/she can judge your answer on what you have learned beyond what you have been given for free.
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Old January 21st, 2003, 10:02 PM   #5
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Norm I knew it was probably a student question but I state some of these things just to "educate" myself or refresh my memory. It was probably overkill but I have a tendency to do that at times.

BTW It probably would be appropriate to read the textbooks but we all know that they and instructors are not always as in depth as they should be. I can hope that maybe I offered something that helped more than hindered.

I live in a dream world, dont I?
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Last edited by rsdoran; January 21st, 2003 at 10:06 PM.
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Old January 21st, 2003, 11:31 PM   #6
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What is a resolver

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When an absolute encoder is turned on, the position of an absolute encoder is known (this function resembles a resolver, although the principles of operation have no similarity.)
rsdoran,

I undertand that a resolver is device which gives sine and cosine waves as feeback ( instead of pulses in case of encoder ) so that both position and speed can be measured , but requires a complex circuitry to analyze and decode position and speed. Am I right ?
Also can a resolver remembers position even after a power recycle?
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Old January 21st, 2003, 11:52 PM   #7
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Quote:
I undertand that a resolver is device which gives sine and cosine waves as feeback ( instead of pulses in case of encoder )
Correct.

Resolver is a magnetic device consisting of 2 static coils 90 degrees apart and one rotating one. The rotating coil is fed with a sinusoidal current, and the induced current in the static coils are measured. The relative intensities of the signals gives you the position. Usually the resolver is part of the motor. The encoder is used for most applications except the following: 1) Hard radiation (fogs the translucent disk) 2) Very high accelerations (damages the coupling) 3) Very high vibrations (damages the coupling, vibrates the disk) 4) Very high temperatures (damages the disk) 5) Very high resolution (optical resolution) The resolver is not used in the following cases: 1) Cost factor 2) High speed applications (the lag through the position decoder means loss of accuracy).

Encoders are electro-mechanical devices that measure the rotation of a glass disk mounted on a rotor within the encoder. Absolute encoders can identify the rotor position precisely, even after a power outage, whereas incremental encoders can only measure the change in the rotors position from any given time. Like the hands on a clock, absolute encoders can indicate the exact time, whereas incremental encoders can only measure how much time has elapsed since the last reading.
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Old January 22nd, 2003, 12:10 AM   #8
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Can a resolver remembers position even after a power recycle?

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can a resolver remembers position even after a power recycle?
I guess not.. right?

Also I broke open some faulty absolute encoders and found only one grating plate inside.. Then how can it remember the 10 bits ( if it is a 10 bit encoder )?
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Old January 22nd, 2003, 03:55 AM   #9
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RS

Thanks for the good post. I asked the same question some time back and also got a good reply. Personally, the problem I had with learning how an encoder worked was finding a good explanation in plain old english for plain old me. I found heaps of stuff on the net but everything I found was pretty complicated.
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Old January 22nd, 2003, 06:07 AM   #10
rsdoran
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Quote:
I guess not.. right?
I am sorry, I wasnt paying attention and didnt realize I had not answered your question

Question: Can a resolver remembers position even after a power recycle?
Answer: YES,the polarities of the voltage sine and cosine outputs are used to determine in which quadrant the shaft is located. As the shaft rotates beyond 90 degrees the polarity of the SINE reverses. Information extracted from the SINE and COSINE is translated into a digital format using analog to digital conversion techniques. The most significant two bits represent which quadrant the shaft is in, and the remaining bits represent the angle the shaft is at from the edge of the quadrant. On power up, the conversion process will track and find the current shaft position for immediate representation to the controller.

Quote:
Also I broke open some faulty absolute encoders and found only one grating plate inside.. Then how can it remember the 10 bits ( if it is a 10 bit encoder )?
An absolute encoder may be designed in different ways and have different interfaces, thats why they usually cost more. The difference is that the absolute measures the shaft position directly similar to a resolver.

I need to add this to my site dont I?
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Old January 22nd, 2003, 08:29 AM   #11
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Some good answers RSDoran. If the question was asked by a student then it should be answered. The Student IS going out to find information and read about it when he posts a question here. What is the difference between reading about encoders on some encoder info site and reading it in a reply here. What is the difference between posting a question here and asking a professor (except that some professors don't know as much as the guys here do).
I agree that it is plain to see that some questions are asked by lazy students (usually simple basic questions like what is a plc and how is it used in industry), but in the case of encoders it can get quite confusing without some sort of nurtuing.
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Old January 22nd, 2003, 08:41 AM   #12
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Thanks rsdoran,

To reiterate what timeflux said, nice post,

I have another question for you, which is better suited to whithstand electrical noise? reslovers or encoders? I have been "told" that resolvers where better suited to withstand noise but I dont have a great deal of knowledge dealing with encoders and the like.

In my systems I have to transfer all electrical signals across "tag lines" that have a combination of 480 VAC and 120 VAC, therefore I have not been convinced to date to use resolvers/encoders for positioning (I presently use lasers, but they are costly).

Thanks,
Chris
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Old January 22nd, 2003, 08:47 AM   #13
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Thanks for all the response. By the way guys Im not a student. Im just messing around with some old plc from work.
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Old January 22nd, 2003, 09:11 AM   #14
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Something else to keep in mind...both rotary resolvers and rotary absolute encoders are available as single turn units and as multi turn units. Multi-turn units consist of two seperate sensing units mechanically geared to each other to provide coarse and fine resolution. The 'fine' unit is typically a single turn unit with direct drive or 1:1 gearing. The coarse unit is typically geared to make one revolution for X number of fine revolutions i.e. 10:1, 64:1, 100:1 etc, where the actual number of revolutions used is dictated by the application. The interface electronics can derive an absolute value in one of two methods. In the first method, the interface electronics will only look at the coarse unit on power up to calculate the number of fine revolutions and load this as a preset. After that occurs, only the fine unit is actively monitored to add or subtract counts. In the second method, both the coarse and fine units are actively monitored and their count values are dynamically summed to generate the final absolute value.

A single turn resolver can be used in multi-turn applications, but the interfacing electronics will have to be 'homed' or referenced in the same manner as an incremental encoder to determine the absolute position on power up. A multi-turn unit does not have to be 'homed' on power up, but you must be sure to not exceed the maximum number of revolutions or else the unit will roll-over i.e. count up to its maximum count value and then roll over to zero and start counting again. For this reason, absolute multi-turn transducers must be carefully installed when the machine is first commissioned to make sure that rollover does not occur during machine travel. Typically this is done by moving the machine to the lowest mechanical limit, and then manually rotating the shaft of the transducer so that is approximately 1 revolution or so from rollover. The unit is then installed and the coupling secured.

I like to use the following analogy for describing incremental vs absolute feedback systems to newbies: Picture a ruler or tape measure permanently attached to the machine being monitored. At any time you can read the absolute position by directly reading the number on the tape or ruler. This is absolute feedback. Now picture the same tape or ruler with all of the numbers erased, but with the lines still marked. At first you glance you don't know where the machine is positioned, but you can incrementally count the number of divisions from a known absolute position i.e. the home position and then continuously count up or down as the machine moves in an increasing or decreasing count direction. This is an example of an incremental feedback system. Incremental systems must always be homed or referenced before the machine can operate.

Last edited by Greg Gauper; January 22nd, 2003 at 09:14 AM.
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Old January 22nd, 2003, 09:47 AM   #15
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SAK-CO TECH stated:
Quote:
I have another question for you, which is better suited to whithstand electrical noise? reslovers or encoders? I have been "told" that resolvers where better suited to withstand noise but I dont have a great deal of knowledge dealing with encoders and the like.
Most controllers in today's market use the quadrature interface scheme. To accommodate these controllers, the resolver based drive will convert its digital format into signal transitions emulating channel A, B and Z of the encoder. Therefore the resolver based drive system, for all intents and purposes, will look as an encoder based drive system. Both transducer types use differential driver and receiver techniques in their electrical interface schemes. These differential techniques are designed to reject electrical noise induced into the cable. The shield wire will drain off the noise into the ground circuit before it can affect the transducer and produce additional error.

The issues on whether to use a resolver, incremental or absolute encoder is more on temperature/environment, speed/velocity, accuracy or cost. If the environment is hot, humid, dirty, oily etc the resolver is a better choice. If precision, smooth torque or stable speed is the issue than an encoder would be a better choice. Its basically application dependent.

I have had minor dealings with using "tag lines" so I can not state what problems you may incur, that you may want to discuss with a manufacturers Engineering Dept.
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