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Pepperl+Fuchs Blog

How Do I Use a Cable Pull?

Posted by Sean Miller on Tue, Aug 20, 2013

Types of rotary encoders that work with cable pulls

Cable pulls are devices used to measure the distance traveled on a piece of equipment. You connect cable pulls to a rotary encoder that rotates around a shaft. The encoder sends output signals to a controller. With the data gathered from these signals, you can track the linear distance that the object connected to the cable pull is moving. How accurate this measurement is depends on the pulse count of the rotary encoder.

We offer cable pull models in two sizes. The cable length dictates which drum size and housing size to use. Use a larger housing to fit the longer cable lengths. Cable lengths are designated by the last two digits following SL30. Cable lengths of 1, 2, and 3 meters use the 80 mm x 80 mm housing size. Longer cable lengths use a housing size of 130 mm x 130 mm and up. The specifications list our cable pulls as having an IP65 degree of protection. However, this degree of protection applies only when you have an encoder attached to a cable pull, since there is a seal inside the cable pull. Figure 1 shows a rotary encoder connected to a cable pull.

Cable pull connected to a 58 mm encoder

Figure 1: One of our cable pulls connected to a 58 mm encoder

The cable pulls offered by Pepperl+Fuchs are designed to fit 58 mm diameter encoders such as the RVI58 Series as well as any 58 mm diameter absolute encoder. The X1 and X2 options call out mating connections to a synchro flange or a clamping flange. It is important that you select the correct mounting option based on the encoder mounting keys listed on the product data sheets. For example, an RVI58N-032K1R61N-01024 requires an SL cable pull with the X1 option.

The cable is physically connected to a ring that can be mounted to something that is round and fits inside the 20 mm diameter pull. Some applications use the end of a steel rod with a threaded end to secure the pulley loop. You can use any linear moving object that fits through a pulley loop. Ideally, you should use a smooth, rounded shaft with a way to keep the pulley loop from separating, since minor movements in a non-linear axis can cause accuracy issues. Basically, the pulley should be able to rotate slightly when being pulled along. Figure 2 shows a pulley end being pulled out by a screwdriver—the pulley allows for side to side movement, which means that you don’t have to pull it out in a perfectly straight line.

A screwdriver is used to pull the cable

Figure 2: As an example, a screwdriver used to pull the cable

The encoder ultimately determines the resolution of the linear travel on the cable length. So a cable with a very high resolution encoder will have greater resolution than a low pulse count model. You can calculate the resolution by taking the drum diameter and dividing it by the steps per revolution of the encoder. The drum diameter is listed on our data sheet and varies depending on the cable length of the specific model. For example, an encoder with a pulse count of 5000 PPR (pulses per revolution) coupled with an SL3005-X1/GS130 will yield a resolution of (334.1 mm divided by 5000) 0.067 mm linear resolution. This means that you will get a pulse for every 0.067 mm of travel on the cable pull. Resolution is an important factor when selecting an encoder and a cable pull for an application. Our cable pulls are capable of achieving great accuracy with a linear error of 0.1% to 0.5%.

Here's another blog on cable pulls: What Are Cable Pulls?

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