Skip to Content

Register Login

Industry Online Support

Technical Forum

Navigation

Measuring PWM Signals

Created by: fimbulwinter at: 10/13/2016 2:58 PM (10 Replies)

Rating (4)

Thanks 3

Result pages:: |; 1 | 2; |

11 Entries

Entries per page: 10 | all

10/13/2016 2:58 PM	Rate (0)
fimbulwinter Experienced Member Joined: 1/29/2015 Last visit: 10/11/2022 Posts: 14 Rating: (0)	Hello everyone, I'm studying on the possibility of measuring PWM signals through HSC on my S7-1200 setup (specifically I have CPU 1212C DC/DC/DC). However, I'm still not sure if this is applicable and couldn't find a similar topic or a clear answer on Siemens forum. Here are my case and what I have done so far: - My initial question was: "Is it possible to send a PWM information to S7-1200 from Arduino PWM output?" - I know that while Arduino PWM output has the level of 5 VDC, a digital input of my S7-1200 setup has 24 VDC. Therefore, I constructed a very simple circuit with a BC546 to circumvent the mismatch of the voltage level for S7-1200. After checking by an oscilloscope, I finally got what I wanted: 24 V PWM. - After studying the necessary steps for HSCs provided on this forum (thanks to all contributors who have shared sample programs and detailed instructions), I chose HSC1 for the next action. I connected positive terminal to I0.0 (the input for single phase measurement for HSC1) and negative terminal to the 1M of digital inputs. After running my program in TIA Portal v13, I realized the counter value didn't change at all, always 0. I disconnected negative side from 1M terminal this time and still the same result. Moreover; when both terminals are connected, the voltage drops from 24V to 8 V at maximum duty cycle. I couldn't find an explanation to this situation. I realize that Arduino part is out of scope for this forum but I would really like to know if detecting PWM for S7-1200 by HSC is possible. If possible, how? If not, I would be glad to have a certain answer. I'm stuck at this point and hoping someone here might have thought on this issue long before. Thank you.

Suggestion To thank Quote Answer

10/13/2016 4:51 PM	Rate (3)
William_B Posts: 5225 Rating: (1165)	Hi. What you try to achieve should be possible. It is the same type idea as reading a 5V encoder. Do be careful that the ON pulse time is not too small. My real concern is however on the signal conditioning. TRUE is above 15Vdc and FALSE is below 5Vdc (..info from memory, actual figure may be different). Does the two systems share the same (-) common? Mixed voltages can give many headaches.. which you already have. Measure the output FALSE state from the Arduino in relation to the common for the PLC. First only try the TRUE / FALSE switching without the PWM. See if the normal input can read that. Also know that a signal board (SB) exists that make it possible for the PLC to read 5V signals. What are your thoughts? w
William_B Posts: 5225 Rating: (1165)
Suggestion To thank Quote Answer
This contribution was helpful to 2 thankful Users MicroMan fimbulwinter

10/13/2016 10:28 PM	Rate (0)
fimbulwinter Experienced Member Joined: 1/29/2015 Last visit: 10/11/2022 Posts: 14 Rating: (0)	Hello again, Thank you very much for your immediate reply. I've already checked the system manual for what are the extreme levels for logic 0 and 1. The values are exactly what you said. With 8 VDC I it's natural not to see a green LED at digital input for my case and I didn't see it. Of course this information doesn't explain the dramatic voltage drop and your concern is correct. Connecting all commons together seems to be a huge mistake and I never thought of that. This will be my first checkpoint tomorrow. I'll study this issue in the light of your suggestions about checking TRUE/FALSE conditions and voltage comparison. It's nice to hear that there's a signal board working with 5 V signals. At least it cancels out the need of an intermediate circuit. I don't have it now but I'll keep it in my mind for future. Until I read you reply, I was questioning the possibility of measuring PWM. It's also a pleasant surprise to get an advice from you because I studied your instructions for HSC in TIA Portal. I will keep this topic updated when I have concrete results. I have a question as a final word. William_B Do be careful that the ON pulse time is not too small. Is this about duty cycle of PWM or filter time for digital inputs?
	Last edited by: fimbulwinter at: 10/13/2016 10:43:27 PM Typo. Corrected using HSC instead of PWM.
Suggestion To thank Quote Answer
This contribution was helpful to 1 thankful Users William_B

10/14/2016 5:58 AM	Rate (1)
William_B Posts: 5225 Rating: (1165)	Hi. fimbulwinter William_B Do be careful that the ON pulse time is not too small. Is this about duty cycle of PWM or filter time for digital inputs? It is good to read your feedback. Thank you. I am glad that you are reading the manual and verifying the numbers. Please continue to do so as soon as you understand what to look for. The filter setting in the hardware configuration is important. I guess you have read this already in other posts. This was however not the little detail I aimed for. You have a PWM signal. Strictly speaking - in our technical jargon - the PWM is a fixed frequency with variable duty cycle. Where PTO (pulse train output) is a variable frequency at a fixed 50% duty cycle. Duty cycle is the width of the ON pulse. So if the pulse is 1s long (include ON and OFF) a 10% duty cycle has a 100ms ON cycle for each pulse. The HSC needs the signal to be ON for a bare minimum time. If you have a 50% duty cycle, then there is no issue. However if you have a PWM with 3% duty cycle running at 100kHz, you start asking for trouble. In short... if you are having a 50% duty cycle from your source, then there is no problem. As an extra tip... The HSC has an extended function instructions that can handle PWM to measure timings of the individual pulse.. if that was ever needed by you. Just out of interest - what type of project are you busy constructing? w
William_B Posts: 5225 Rating: (1165)
Suggestion To thank Quote Answer

10/17/2016 2:36 PM	Rate (0)
fimbulwinter Experienced Member Joined: 1/29/2015 Last visit: 10/11/2022 Posts: 14 Rating: (0)	Hello again, I found my mistake, rather than connecting grounds together it was because of poor choice of resistor in my circuit. I finally succeeded to get a voltage level of 19 VDC which is OK for logic 1. It may be a bit off topic; but I used a BC546 transistor, a 1k resistor on the base and a 10 k resistor on the collector on my circuit. I connected the PWM output to the base and a 24 VDC source to the collector to get a switching mechanism. Well, the output is an inverse of the input, but that's not too important for me now. But after spotting 2.5 mA condition for logic 1 ("min 15 VDC at 2.5 mA"), I decreased the the resistor value from 10k to 1 k on the collector. Finally this gave me 19.4 VDC (which was around 8 VDC last week) at max duty cycle on I0.0 and I managed to see a green LED. I'm a beginner at electronics so if you have a much wiser solution, I'm all ears. Maybe using a signal board to read 5 VDC signals and therefore getting rid of any intermediate circuit would be a smart move. On TIA Portal side the counter finally shows some values. With decreasing duty cycle it slows down, with increasing duty cycle it speeds up. This is reasonable of course but first I have to understand some aspects of HSC such as in what intervals does HSC count or what is the effect of input filter and some strange things such as why does the counter stop at max duty cycle. These will be my next steps. But I wanted to inform those who may be interested in this kind of topic. Thank you very much for letting me know your concerns and giving me the hope that this can be done, William_B. According to Arduino's web page, PWM frequency is at about 500 Hz. I guess this frequency level is too basic for S7-1200 regardless of duty cycle. Let's come to my story. I have studied S7-300 and S-120 with a control unit of CU320 to accomplish speed and position control. In near future this setup will be changed with new hardware, e.g. S7-1200 instead of S7-300. One day the question came to my mind: "Can I adjust the speed or the position of a motor in the setup with a remote handset which is basically for hobby motors?". I already know that there are HSCs and motion control is now integrated in TIA Portal but I have never used them. At a very basic level I began with using only Arduino to try to answer my question. If I count the pulses coming from Arduino within a certain period, I may be able to map it into a meaningful one in my program and set it as a reference point. I'll really appreciate if you have any suggestion.

Suggestion To thank Quote Answer

10/17/2016 4:38 PM	Rate (0)
William_B Posts: 5225 Rating: (1165)	fimbulwinter <snip> One day the question came to my mind: "Can I adjust the speed or the position of a motor in the setup with a remote handset which is basically for hobby motors?". I already know that there are HSCs and motion control is now integrated in TIA Portal but I have never used them. </snip> Hi. Thanks for the detailed feedback. You have now determined that reading feedback from an encoder will not be an issue. The next step is to do some motion tasks, perhaps. A pulse-and-direction signalling to a stepper motor's amplifier will probably be the easiest to start with. Configuring the motion control is not so much difficult.. just new (for you). Speed control by frequency pulse (as you are doing with your test) is not so easy to do for S7-1200 up to FW4.1. You need to use the jog function in motion control. FW4.2 with TIA Portal V14 however offer easier frequency pulse control without using motion control. There are examples available as well as documents for the motion control theme. Review the suggestions made in {LINK}. Hope you find these thoughts as a good kick-start. You are sure welcome to return and exchange some more thoughts with the Forum. Good luck. w
William_B Posts: 5225 Rating: (1165)
Suggestion To thank Quote Answer

10/18/2016 7:07 PM	Rate (0)
Jav Pinz Posts: 108 Rating: (27)	Hello Fimbulwinter fimbulwinter I'm a beginner at electronics so if you have a much wiser solution, I'm all ears. Maybe using a signal board to read 5 VDC signals and therefore getting rid of any intermediate circuit would be a smart move. One of the suggestion about the 5 Vdc, may be a Operational amplifier, wich is quite easy to use. You can test getting the 24 Vdc that you want, just need to connect it as the img attached, and applying the equation given, you can obtain your voltage. You can Use a LM358 or a 741. Just read the datasheet. Regards Javier Attachment OPAmp.png (215 Downloads)
Jav Pinz Posts: 108 Rating: (27)	Last edited by: Jav Pinz at: 10/18/2016 7:10:26 PM
Suggestion To thank Quote Answer

10/31/2016 9:37 AM	Rate (0)
fimbulwinter Experienced Member Joined: 1/29/2015 Last visit: 10/11/2022 Posts: 14 Rating: (0)	I'm really sorry for the late reply, William_B, I have already completed to control 2 servo motors under an architecture with S7-300 and CU320. Both speed and position control have been realized. However it's done using SIMATIC Manager not TIA Portal. I hope it won't be a problem under TIA Portal. If I'm not mistaken, your post is about stepper motors rather than servo motors. Nevertheless, it's worth studying from the point of stepper motors. Thank you very much for your all valuable comments and all the documents you have prepared. When the relevant hardwares are available for me, I will update this post. I don't know when, but eventually I will. Jav Pinz, thank you for your suggestion. I'll definitely try that one.

Suggestion To thank Quote Answer

10/31/2016 5:27 PM	Rate (0)
William_B Posts: 5225 Rating: (1165)	fimbulwinter If I'm not mistaken, your post is about stepper motors rather than servo motors. Nevertheless, it's worth studying from the point of stepper motors. I must be doing something wrong then. (wink) The servos in our machines are controlled with pulse and direction signals.
William_B Posts: 5225 Rating: (1165)
Suggestion To thank Quote Answer

10/31/2016 5:50 PM	Rate (0)
fimbulwinter Experienced Member Joined: 1/29/2015 Last visit: 10/11/2022 Posts: 14 Rating: (0)	No, I didn't mean that of course. What I did was create telegrams, then change appropriate bits and send a reference for speed or position control through them. The procedure seemed a bit different in that post.

Suggestion To thank Quote Answer

|

Share this page:

Share this page on...

Related topics by keywords:

Follow us on