There is a kind of alarm that we can run into every now and then in the workshop, called Siemens alarm F31112. The first time many people see it, their intuition thinks that the communication cable between the encoder and the drive is broken, or the plug is loose. As a result, checking down the route of communication faults, changing a round of cables and modules, and tossing around for half a day, finally they discover it wasn't that case at all—the problem lies on the encoder itself, and it is actively shouting "help" to the drive.
F31112 is Not Communication Broken, It is the Encoder Telling You "Something Is Wrong Inside Me"
Siemens' official definition given to F31112 is a bit tongue-twisting, called "F31112 – Error Bit Set in the Serial Protocol".
Sounds exhausting to listen to, right? Actually, saying it in human words is extremely easy to understand. Between the drive and the encoder, they are every minute and second "making phone calls" through that line; the encoder not only has to tell the drive data like current position and speed, but will along the way send a "health status code" of its own. If the encoder, when doing internal self-checking, finds which electronic component is not right, internal storage has an error, or self-diagnosis didn't pass the check, it will actively raise a "small flag" inside this health status code, which is setting the error bit on. This equals clearly and plainly telling the drive: "Old brother, our communication isn't broken, but my own body bones developed a little problem, you see what to do." The drive receives this signal, and immediately generates the F31112 alarm, locking up the current axis.
Therefore, here is a place especially easy to confuse: F31112 does not mean the signal is lost; quite the contrary, the signal was properly transmitted over, but sandwiched inside is a "critically ill notice" from the encoder.
Running Into This Alarm, What State Will the Machine Generally Be In
On site as long as F31112 pops out, most equipment directly goes on strike. Sometimes as soon as powering on, before starting to work, the servo enable cannot even get up; sometimes while doing work, suddenly it emergency-stops, and that axis can no longer move; and some machining centers or automation equipment, directly cannot even finish returning to zero, with the screen reporting "reference point establishment failed". Because after the drive receives the encoder reporting an error, it doesn't dare to trust the position fed back from it anymore, and simply pinches off the pulse output. At this time, whether you run automatically or shake manually, don't think about making that axis move a bit more. Multi-axis equipment, sometimes only one axis reports, and other axes can still do work as usual.
Why on Earth Does the Encoder Actively Raise the White Flag
Many customers, as soon as they see F31112, firmly believe that the encoder must be completely resting in peace. In reality, the encoder itself having problems is indeed the most common situation, but it really isn't the only root. The encoder itself is truly broken, internal chips aging, sampling making mistakes, storage abnormal. Current Siemens encoders, inside all have self-diagnosis functions, like a personal old Chinese medicine doctor. Once it feels out its own pulse is not right, for example sensor chip is no good, position sampling error is too big, or purely it has been years and electronic components aged, it will first give itself a diagnosis, and then tell the drive through the error bit. At this kind of time, even if the communication cable outside is brand new brand new, the drive as usual reports F31112. If the equipment has been used for many years, or stays in a high temperature and high humidity environment for a long time, the probability of this kind of fault is particularly high. Some Siemens servo motors, the encoder is integrated behind the butt, running into this situation, often one can only replace the encoder or even the whole motor.
However, don't only stare at the encoder body either. Sometimes it is that connecting wire or plug causing trouble. Although F31112 is not communication interruption, if the communication quality turns rotten, it can likewise force the encoder into an abnormal state. For example, the encoder cable followed the drag chain running back and forth for several years, inside some core wires are already snapped; the plug wasn't locked tight, pins oxidized; or during processing coolant and oil stains seeped into the joint, these will all make the signal quality drop heavily. Many machine tool encoder cables swing back and forth in the drag chain all year long, used for three to five years, at the place of the drag chain exit it is easiest to start aging. Looking at the outer skin nothing is wrong, peeling open inside the copper wires might have broken several strands. If you have a spare encoder line on hand, changing it on to try is often the fastest way.
There is also a situation easy to be ignored, which is shielding and grounding not done well, getting messed up by interference. What the encoder transmits are all high-precision feedback signals, particularly delicate. If the shielding layer is not grounded solidly, or the cabinet grounding itself is messy, outside electromagnetic interference can drill into the signal loop to make trouble. Especially nearby if there are high-power inverters, welding robots doing work, high-frequency starting equipment, beside big current busbars, or inside the cabinet densely crowded a pile of servos, under this kind of environment it is easier for things to go wrong. If when originally routing lines you didn't follow Siemens EMC rules, and bundled the encoder signal line and power line together, then the probability of F31112 coming out to drop by as a guest is much higher. So reaching the site, checking if the shielding layer from head to tail is grounded well, if the ground wire is reliable or not, and if the signal line dodged the power line, this step is quite key.
Still another thing is the encoder's power supply itself is unstable. Although Siemens officially didn't list the power supply problem as the main cause of F31112, we in maintenance indeed encountered quite a few, just because the road of power supplied to the encoder had a low side voltage, the plug contact was not good, or the terminal block inside the cabinet was loose, making the encoder fail the self-check. If the voltage keeps swaying, the encoder might frequently enter an error state, constantly sending error bits to the drive. So, before spending big money to change the encoder, first take a meter to measure if the power supplied to it is stable or not, and along the way confirm the whole power supply loop.
Reaching the Site, How Do We Generally Trace the Vine to Touch This Melon
Running into F31112, we won't come up and tear down the drive right away. A better route is to first save the alarm log, figure out whether this alarm comes as soon as powering on, or randomly pops while the machine is running, and at the same time glance to see if it came together with other encoder alarms or communication alarms. Then, focus on checking that line of the encoder, to see if there is wear, if the plug is loose or not, and if the pins turned black or not. Next, go through the shielding and grounding inside the cabinet once, to see if they meet requirements. If these are all normal, then measure if the voltage supplying power to the encoder is steady and solid. Only after excluding all these external suspects, finally it is the turn to suspect the encoder itself, taking a good part to replace and try a bit. Walking according to this order can save a lot of wrong ways; don't drill into a horn's point at the beginning thinking the drive is broken.
When Alarmed Can We First Reset and Hard-Carry to Do Work
Generally we don't recommend doing so. The encoder is the position benchmark of the servo system; it has told you itself is not right, and you still force it to continue pointing the road for you, then the position deviation will only get bigger and bigger. Lightly the processing size runs off, heavily directly crashes the machine, synchronization between axes gets messed up, and even gets the equipment scrapped. So the
SINAMICS drive directly locks up that axis, which is precisely protecting the whole machine.
Normally Keep a Few More Minds Open, F31112 Can Less Add Trouble for You
Many encoder troubles are ground out slowly; normally doing some simple maintenance can let it be used for many more years. For example, regularly take a look to see if the encoder cable has skin peeling or wearing open; keep plugs clean, don't let coolant seep in; cabinet grounding and EMC line routing must be standardized; signal lines shouldn't crowd together with power lines for a long time. For those machine tables running under full load for long times, best to list the servo system check into annual maintenance. Discovering the encoder state is not right early is much more money-saving than it suddenly stopping machine catching you off guard.
Checked a Round Myself and Still Have No Clue, When Should We Move the Rescue Troops
If you have already confirmed the encoder cable, power supply, and grounding have no problem, and F31112 still repeatedly comes, or even after changing the drive it is still the same old look, then basically it can be decided that the encoder or servo motor itself has problems. At this time blindly changing parts has no meaning anymore; best to use a professional test bench to put the encoder, servo motor, and the drive's feedback interface together to do a comprehensive physical check, so that you can more accurately drag out the source of the fault, and don't throw away wronged money for nothing.
When all is said and done, Siemens alarm F31112 is not common communication interruption, but the encoder itself through serial communication, actively confessing "something is wrong inside me" to the drive. If confirmed it is the encoder's internal trouble, fix what should be fixed and change what should be changed; handling it early not only can let the equipment return to normal as soon as possible, but also dodge more serious mechanical faults and production losses caused by abnormal position feedback.
