SICK Wire Draw Absolute Encoder Checklist: CMM Metrology and Starrett Angle Finder

At my company, I'm the quality/compliance manager for an industrial automation team. I review every sensor-related deliverable before it reaches the factory floor—roughly 200 items a year. I reject maybe 6% on first pass, usually because the mounting hardware is wrong, not because the sensor itself is dead. If you're looking up SICK wire draw absolute encoders, you're probably doing incoming inspection, installing one, or trying to figure out why an existing one reads erratically. This checklist works for all three.

Before you search 'sick encoder price' and get confused: the brand name is SICK, all caps. That's the name. I'll use it as a brand name only. There are six steps below. The last one is the one most people skip. It saves you the most time later.

The short version: a wire draw encoder only knows what you mount it on. Check the mount first.

Step 1: Confirm the part number and the actual specifications

Before you even look at price, confirm the part number matches your order. A SICK wire draw absolute encoder can look identical from the outside while having different measuring lengths, output protocols, and resolutions. I've seen an SSI version and an IO-Link version of the same housing get mixed up in the same parts drawer. The diagnostic messages didn't make sense until we checked the label.

On the price question: SICK encoder price depends a lot on measuring range and output. In Q3 2024, we paid about $680 for a 1.25 m wire draw absolute encoder with SSI output. Maybe I'm misremembering by a few dollars; don't quote me on that. Prices as of Q3 2024, and you should verify current pricing. The point is not the exact number. The point is that you cannot compare quotes until you have the exact part number.

Action item: write down the part number, measuring range, output, and resolution from the datasheet. Keep it attached to the unit.

Step 2: Inspect the wire and the exit path

Unbox it and look at the wire before mounting anything. A wire draw encoder stores a steel cable on a drum. If the cable has a kink, a burr, or a sharp bend at the exit, it will repeat that defect at the same spot on every cycle. What most people don't realize is that wire draw encoders are sensitive to the exit angle. The wire should be pulled straight along the direction of movement. If you let it rub against the edge of a hole or a bracket, you'll introduce an error that looks like sensor drift but is really mechanical friction.

Three things to check before power-up: wire direction, wire condition, mounting angle. In that order.

Also, do not pull the wire past the maximum extension. There is a mechanical stop inside. Pulling past it once can stretch or damage the drum. Not ideal, but workable if you catch it early. Worse than expected if you don't.

Step 3: Check the mounting angle with a Starrett angle finder

Mounting is where accuracy goes to die. If your application uses an angled bracket or a sloped surface, verify the angle before locking the encoder down. A small angular error creates a cosine error that scales with travel. The fix is a simple angle finder.

If you've ever googled how to use a Starrett angle finder, here's the short version: place the base against your reference surface, open the blade until it sits flush against the angled surface, lock the knob, and read the angle. On a digital model, the display shows the angle directly. On a dial model, use the scale that matches the direction you're measuring. If the finder has a sliding bevel, zero it against your reference surface first.

It took me about four years and one 0.8 mm error on a 2 m travel system to understand that this step matters more than the encoder's resolution. A high-resolution encoder in a bad mount gives you very precise wrong answers. The old 'more resolution means more accuracy' thinking comes from an era when mounting errors were harder to measure. Today, a CMM shows you that the mount is often the limit.

Step 4: Verify the fixture with a metrology CMM

If you're building a test fixture or a machine base, don't trust the machined surface just because it's new. Use a CMM to verify the datum. Now, a metrology CMM might sound like overkill for a sensor mounting job. But in our shop, it's the only way to separate encoder error from fixture error.

We use a bridge CMM calibrated according to ISO 10360-2. Its quoted volumetric accuracy is around plus or minus 2.5 µm. That's enough to check a mounting face that needs to be flat within 0.05 mm over 300 mm. What I mean is: measure the mounting face as a datum, then measure the line of wire travel relative to that datum. If the wire exit is angled more than about 1 degree, you will see it in the CMM report before you see it in the position data. That's the whole point.

Scope limitation: if you don't have a CMM, a surface plate and a dial indicator can catch the same gross errors. But for a repeatable fixture, the CMM is faster and easier to document.

Step 5: Run the position test and test the absolute behavior

With the encoder mounted, power it up, set the zero point, and move to known positions. We use five positions at 0, 25, 50, 75, and 100 percent of travel. Compare the output to a calibrated reference. For our 1.25 m model, the datasheet lists linearity as plus or minus 0.3 mm. We saw about plus or minus 0.12 mm in the middle and a little more at the ends. Acceptable.

Test the absolute part too. An absolute encoder should report the same position after a power cycle without homing. Disconnect power, reconnect it, and check that the output didn't shift. If it changes, either the battery is dead, the power-up sequence has a problem, or the unit isn't as absolute as you expected.

I have mixed feelings about this step. It takes setup time and a reference. On the other hand, the one time we skipped it, a unit passed our bench test but lost its zero after every power cycle. That was a $900 mistake in labor and downtime before we noticed. The test costs about 10 minutes.

Step 6: Document with a photo log

The step everyone skips: documentation. Take a photo of the part number label, the mounting arrangement, the CMM report, and the position test results. Save it with the serial number. Do it before the unit goes into service.

In Q1 2024, we had a batch of 12 wire draw encoders where one showed a 0.2 mm offset after 500 cycles. The vendor said it was within spec. I pulled the log and showed them that the other 11 were within plus or minus 0.05 mm. They replaced the one without much argument. Without the log, it would have been an afternoon of back-and-forth and zero resolution.

Common mistakes

• Over-tightening the mounting screws can distort the housing. Use the torque values in the SICK operating instructions.
• Routing the wire around a radius that's too tight. Give it a straight exit path.
• Using the encoder's wire as a mechanical stop. It's a measuring device, not a limiter.
• Ignoring temperature effects. Wire draw encoders are mechanical; thermal expansion affects both the wire and the mounting.

One more thing on cost: SICK encoder price is a small part of the total cost of an accurate measurement system. A $500 encoder in a $50 mount is a $5,000 problem if it stops a production line. When you're under a deadline, the premium for a documented, verifiable part is worth it. The uncertain alternative is not worth it.