During year 2022 I inherited the PLC program written by our other colleagues for some industrial machines, on which I was supposed to make additions and improvements.The PLC is in ET00SP format with CPU 1510, where in addition to the machine logic that was written entirely in ladder only, I was asked to add six V90 drives, (which I chose to integrate as technological objects), and some temperature regulatorswith PID function.

The six servo drives are all positioners, with five of these making a short, cyclic, bi-directional path, while the sixth drive is a uni-directional, position-feedback speed control (a rotary table).

After all the required additions and modifications, during the first commissioning phase there were many problems on two of the six V90 servos, problems that greatly slowed down the progress of the work.

After many checks, lost many hours of work, after various accusations of incompetence, it turned out that on one of the V90 servos, the encoder, of the absolute type, had a defect, in just one point, only if it turned in one direction. On the other hand, it worked fine.

Well, after changing the servo motor everything seemed to be fixed. The machine finally takes shape and begins to behave well. It was then sent to the final customer who began to use it.

After the first ten days of work, the customer begins to notice defects in the machine which, again after several investigations, showed that the rotary table controlled by one of the six servomotors was mechanically in a different position from that measured by the encoder. Given the previous past experience, I hypothesize that the servomotor combined with the rotary table, (identical to the first defective result), is equally defective.

I involve Siemens again, more inspections was made, I write program lines and lines to monitor the situation and try to hide the existing problem until it is resolved, I change the firmware to the servo drives, I have all the mechanical joints disassembled and checked, I change cables. Nothing, all useless: after having installed additional diagnostic equipment, I always and in all cases noticed that the encoder position of the servomotor drifted slowly by a few degrees with respect to the actual mechanical position of the rotary table, occasionally, after a few minutes or a few hours ofwork without any rules.

In the end (without boring the forum too much), after about nine months of investigations, one day that I was online with the system, I realize that a calculation that I had added as a diagnostic tool that should have given a certain result, gave a different and greater one. But not major at random; the result I saw on the monitor was as if the calculation had been done twice in the same scanning cycle of the program. Incredible? No! Wrong code? No! I had already seen and told a similar story in this post. In this post I was recounting an experience that seemed strange, experienced in a different way, but which after this second experience made me realize that they are the result of a similar phenomenon.

Without altering my program, I verified that the CPU in question had firmware 2.9.2 which, from the Siemens site, had a very short life.

In fact, in less than six months version 2.9.4 had been released, (which I had never considered before).

Now that CPU has firmware 2.9.4 the machine works fine.

I don't expect answers, it was just a story of past experience a little different from the usual requests of how to do this or that.Good job everyone.

 I don't expect nothing more than tell my story.

After reading the post of 0_Einstein, I can say that he get one of possible points of view.

I just use the forum for, as I wrote, not only ask, but talk about what is our work.

qwazee

Hello Mauro.

Your post is valuable. It warns of problems baked into the devices we use. We are quick to assume 'operator' error while there exists the unthinkable case where it is not under our control. Always a good idea to have some 'sanity tests' (as named by a forum VIP).

I can only imagine how much reputational damage this can cause. And of lesser importance the money spent on site visits and renting outside help.

Reading your post, I just cannot shake off the question.. "What is the purpose is of this posting?" Especially as the topic you linked to is a topic were I volunteered assistance. A conclusion was reached there, though not a one we would want.

Mauro, what influence do you want this topic to have?