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Absolute encoder on conventional Digital inputs

Created by: Nikzitronik at: 7/19/2012 10:55 PM (6 Replies)

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7/19/2012 10:55 PM	Rate (2)
Nikzitronik Posts: 150 Rating: (17)	Dear all, Usually, absolute encoders come with a high cost as it requires expensive hardware. Absolute encoders usually connect to the PLC using SSI cards or a network like Profibus-DP. However in less demanding, low speed applications where accuracy is not super-critical, an absolute encoder can be connected to the normal inputs of a PLC. First of all, the encoder has to be capable of outputing 24Vdc at its outputs. Some encoders output 5Vdc which is no good for conventional inputs. Next care needs to be taken in the output type of the encoder. Most encoders output grey code which is usefull for many reasons, but it needs conversion to decimal so that it is meaningful. Moreover, care needs to be taken in the connection of the encoder to the PLC (source or sink inputs, MSB, LSB). Lastly the number of inputs required depends no the resolution of the encoder. For instance an 8bit encoder uses 8 inputs and splits the one revolution of the encoder in 256 steps. In degrees this gives a resolution of 360/256=1.406 degrees. Not great, but maybe ok for non-critical positioning applications. One can use a higher resolution encoder, that requires more inputs. For instance a 10bit encoder (not uncommon) uses 10 inputs and splits the one revolution of the encoder in 1024 steps. In degrees this gives a resolution of 360/1024=0.351 degrees. Not bad at all. For my test I used an OMRON E6CP-AG5C encoder. It is an 8 bit encoder with resolution of 256. It has sourcing outputs so the 1M terminal of the PLC needs to be connected to +24Vdc. LSB is connected to I0.0 and MSB is connected to I0.7. The program basically does the conversion of grey code to decimal, and then it scales the decimal output to 360 degrees, so that it is more usefull. The conversion of the grey code is not a big deal. Take the MSB form the inputs and link it directly to the MSB of the decimal word. Then XOR the next input with the previous bit of the decimal word, and so on. I0.7 = Decimal_word.bit7 (I0.6 XOR Decimal_word.bit7) = Decimal_word.bit6 (I0.5 XOR Decimal_word.bit6) = Decimal_word.bit5 (I0.4 XOR Decimal_word.bit5) = Decimal_word.bit4 (I0.3 XOR Decimal_word.bit4) = Decimal_word.bit3 (I0.2 XOR Decimal_word.bit3) = Decimal_word.bit2 (I0.1 XOR Decimal_word.bit2) = Decimal_word.bit1 (I0.0 XOR Decimal_word.bit1) = Decimal_word.bit0 See the attached program that runs in a 1212 DC/DC/DC CPU. I hope this will help someone. Regards Nik Attachment Absolute_encoder.zip (1054 Downloads)
Nikzitronik Posts: 150 Rating: (17)	Last edited by: O_Moderator at: 7/20/2012 7:42 AM attachment converted to "real" zip-file
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11/16/2017 10:05 PM	Rate (1)
Milosh Silver Member Joined: 8/16/2010 Last visit: 6/10/2023 Posts: 628 Rating: (11)	Hi all. I see that post is old But how to detect if wire is broken? If encoder have 10 bits, or 10 digital inputs, How to detect if one input is down durin encoder turn?

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11/16/2017 11:43 PM	Rate (0)
0_Einstein Diamond Member Joined: 7/7/2010 Last visit: 6/9/2023 Posts: 13374 Rating: (2185)	If at any time during encoder shaft rotation, more than 1 bit changes from the last values presented to the PLC digital inputs, one of two things has happened: 1. Encoder is moving too fast for the plc to handle (you can use hardware interrupts for s7-1200 plc inputs for those inputs on-board the plc module to help with this). 2. A line is broken. If you have a proper gray code decoder in your logic, you should be able to detect an impossible move if one wire is broken, provided that wire is currently in scope of the gray code and encoder position. Milosh Hi all. I see that post is old But how to detect if wire is broken? If encoder have 10 bits, or 10 digital inputs, How to detect if one input is down durin encoder turn?
	science guy
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11/17/2017 9:14 AM	Rate (0)
Milosh Silver Member Joined: 8/16/2010 Last visit: 6/10/2023 Posts: 628 Rating: (11)	huggy_d1 If at any time during encoder shaft rotation, more than 1 bit changes from the last values presented to the PLC digital inputs, one of two things has happened: 1. Encoder is moving too fast for the plc to handle (you can use hardware interrupts for s7-1200 plc inputs for those inputs on-board the plc module to help with this). 2. A line is broken. If you have a proper gray code decoder in your logic, you should be able to detect an impossible move if one wire is broken, provided that wire is currently in scope of the gray code and encoder position. Thanks, huggy, But how to implement that in code? How to detect impossible move?

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11/17/2017 2:49 PM	Rate (1)
jacek d Platinum Expert Joined: 9/23/2005 Last visit: 6/9/2023 Posts: 4181 Rating: (645)	Solution 1 Take actualGray XOR previousGray and count 1's if the count >1 -> error Set previousGray:=actualGray Solution 2 Decode actualGray to natural binary ->actualBinary If ABS(actualBinary-previousBinary)>1 -> error Set previousBinary:=actualBinary Of course both solutions require that you can capture all state changes at inputs.
	Regards, Jacek What is confusing you is the fact that you never see a number but always its digit representation. So you end up thinking that the number is the representation. - Andrej Bauer Any claims against me - based on whatever legal reason - resulting from the use of any code and/or ideas presented on this forum shall be excluded. #StayWithUkraine
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11/20/2017 12:01 PM	Rate (0)
Milosh Silver Member Joined: 8/16/2010 Last visit: 6/10/2023 Posts: 628 Rating: (11)	Thanks a lot all of you. The second solution is better if inputs are fast enough. The first solution have some lack. If wire broken, for bit 0 of encoder. It is always 0. If we done XOR actual and previous, this bit is always 0. So everything looks good, but it is not. jacek d Solution 1 Take actualGray XOR previousGray and count 1's if the count >1 -> error Set previousGray:=actualGray Solution 2 Decode actualGray to natural binary ->actualBinary If ABS(actualBinary-previousBinary)>1 -> error Set previousBinary:=actualBinary Of course both solutions require that you can capture all state changes at inputs.

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11/20/2017 3:41 PM	Rate (0)
0_Einstein Diamond Member Joined: 7/7/2010 Last visit: 6/9/2023 Posts: 13374 Rating: (2185)	If your inputs are not fast enough, you cannot use the digital inputs to capture the encoder position AND detect a broken wire. At 0.2ms update rate, that gives you 500 Hz pulse changes per input, but with encoders, you should only see one bit change per pulse, so in reality, you could have twice that in real-world encoder input update rate, or 1000 Hz. That is nearly 60 RPM for 1024 PPR encoder. There might be a way to use HSC's (with a good algorithm) and achieve up to 100 kHz per input, or 200 kHz overall max per input. The problem is you would only detect when 0's become 1's, and values with 0's would get lost. You could infer the position, but that inference would lose broken wire detection options. I say connect it to a purpose built device that can do what you want, and output the value you need (whether via profinet, or analog value). Milosh Thanks a lot all of you. The second solution is better if inputs are fast enough. The first solution have some lack. If wire broken, for bit 0 of encoder. It is always 0. If we done XOR actual and previous, this bit is always 0. So everything looks good, but it is not. jacek d Solution 1 Take actualGray XOR previousGray and count 1's if the count >1 -> error Set previousGray:=actualGray Solution 2 Decode actualGray to natural binary ->actualBinary If ABS(actualBinary-previousBinary)>1 -> error Set previousBinary:=actualBinary Of course both solutions require that you can capture all state changes at inputs.
	science guy
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