Don't rush to dismantle the hard disk as soon as you hear the alarm, as this is likely to be a waste of money. When we arrive at the site, we usually have a plan in mind and follow the following approach:
First: check when the alarm first appeared. This is particularly important. Does it trigger as soon as the power is turned on? Or does it trigger after the machine has started running? Is it triggered during high-speed movement? Or is it triggered only when a specific position is reached? Does it trigger more frequently after the machine has warmed up? These details can directly help you determine the direction of the problem. For example, if it triggers as soon as the machine moves, it's likely that the encoder cable is broken; if it triggers after the machine has warmed up, it could be a loose connection or aging electronics; and if the error occurs completely randomly, you should focus on checking for interference or a loose connection at the plug.
Next, instead of rushing to disassemble major components, thoroughly check the encoder cable first. This is really the most common source of failure. Pay close attention to areas where the cable bends frequently, the drag chain section, and where the cable exits from the motor - check for oil corrosion or anything pressed against it. Many wires break internally and are not recognizable from the outside. In this case, it's best to twist the wire to test it, or use the continuity mode of a multimeter. If you have a good replacement wire on hand, swapping it is the fastest way to confirm if the problem is in the wire.
Next, unplug the CN2 connector and inspect it. This step often provides immediate results. Check the pins inside for bends, green oxide or grease buildup, and make sure the connector isn't loose. Often, simply spraying it with cleaner, wiping it clean and re-inserting it securely will make the alarm go away.
Then, don't forget about grounding and interference. Many people don't even consider this. Encoder signals are actually very susceptible to interference. Especially in environments with lots of inverters, bundled power and signal wires, messed up grounding, and welders causing serious interference, all of which can completely disrupt encoder communications. In one case we encountered, a customer replaced two drives without success; eventually, we realized that the encoder cable had been running parallel to the power line for years, causing excessive interference. The problem was resolved immediately after rewiring.
If you've checked the cables, replaced the connectors and eliminated the interference and the problem persists, it's time to start suspecting the encoder itself. Common symptoms include motor vibration, incorrect zero return and erratic speed feedback. If an oscilloscope is available, examining the feedback waveform is the most direct way to diagnose the problem. Even if you don't have an oscilloscope, the diagnosis is simple: replace the motor with the same model, or just replace the encoder - it's usually much faster than taking the time to make slow measurements.
As a last resort, if nothing else works, suspect the driver. Don't suspect the driver straight away to begin with, as the actual percentage of Alarm 20 cases caused by faults in the driver itself is not high. Unless you have replaced cables, swapped motors, checked connectors and eliminated interference and the alarm persists, then you should consider whether the encoder interface on the driver is damaged, the communications circuitry is faulty, or the motherboard is faulty.
