Sick SSI Absolute Encoder Problem

Hi Guys

I am using SICK absolute encoder ATM 60 with an SSI to Parallel Converter (AD-SSIG-PA) to connect to a Mitsubishi Q02HPLC. The set up works fine initially but after a few weeks the reading from the converter to the PLC is just spurious data. This problem is resolved by powering off the converter and encoder. The set up works fine again for a few weeks before going AWOL again.

The length of serial cable to the converter is about 15m. The cable is not one piece; it is split and joined at a junction box. The wiring has been checked several times and appears ok.

Anyone seen a problem similar to this. I don't think the encoder or converter box is faulty - because we have 2 similar set ups and it occurs on both.

Any suggestions would be greatly appreciated.

kontenda said:

Hi Guys

I am using SICK absolute encoder ATM 60 with an SSI to Parallel Converter (AD-SSIG-PA) to connect to a Mitsubishi Q02HPLC. The set up works fine initially but after a few weeks the reading from the converter to the PLC is just spurious data. This problem is resolved by powering off the converter and encoder. The set up works fine again for a few weeks before going AWOL again.

The length of serial cable to the converter is about 15m. The cable is not one piece; it is split and joined at a junction box. The wiring has been checked several times and appears ok.

Anyone seen a problem similar to this. I don't think the encoder or converter box is faulty - because we have 2 similar set ups and it occurs on both.

Any suggestions would be greatly appreciated.

Click to expand...

We/our customers have had very good look with the Sick SSI absolute encoders. I would suspect the parallel part of the connection. Are the parallel lines differential? If not then you should keep the parallel lines as short as possible. Can you run the SSI output from the encoder both serial to parallel converter boxes and compare? I know this can work in some cases because some of our customers will use the same encoder to drive multiples of our product. Be sure to use different power supplies for each converter box.

Off hand I would be very suspect of the serial to parallel converter boxes or the power supply to them. If the power supply to them is not good then you can exchange the converters and still get the same errors when the power glitches. I am also suspect int that the converter box does not correct itself. A bad SSI position may cause a jump in position but the next good reading should restore the position back to the true position. It doesn't make sense to me that the converter box would not be able to take the next good SSI reading and correctly convert it unless its interal state machine is flawed or has been tweaked by some bad power spikes.

Thanks Peter,


I can't run the SSI to both converters because the customer is using the installation - and the problem occurs randomly - once every few weeks.

I think you are right that the problem is in the converter boxes - as once the power is reset the encoder works fine - and remembers its correct position.

It doesn't make sense to me either - but I will check the power and cabling further - it surprises me that after a spike the converter would remain giving bad readings until the power is reset. The parallel cable is quite short as the converter is beside the PLC.

We asked SICK when noise could be a factor (as the control panel has a couple of inverters installed as well) - but they told us they have no past experience of problems with noise.