A draw-wire position sensor is also known as a string pot, cable extension transducer, string potentiometer, or yo-yo sensor. A draw-wire sensor is a transducer that can detect and measure linear position and velocity. It is composed of a measuring cable, drum, spring, and rotational sensor. They are generally durable, easy to set up, and inexpensive. Draw-wire sensors are relatively simpler in design as well.
How do they work?
The measuring cable inside the sensor is wrapped around the drum. The drum has threads that ensure proper unwrapping & retraction of the cable. The stainless cable has a wire clip attached at the open side of the sensor, and the other end is connected to the drum. The wire clip is attached to the object of interest. While the object moves, the stainless cable is unwrapped or retracted at the drum.
The spring inside the sensor keeps a constant tension applied on the cable to avoid sagging of the cable. They are very handy while taking horizontal measurements. The drum is attached to a rotation sensor. When the drum rotates, due to the movement of the object, the sensor detects this motion. The sensor then outputs a position signal proportional to the motion of the drum.
Pros & Cons
Draw-wire sensors can measure distances even up to 50000m. They have a simple design, yet are durable and reliable. The compact size of the sensor allows it to be deployed in areas inaccessible otherwise. A draw-wire sensor can produce digital and analog output, as required. An integrated digital circuitry can get this job done, without setting up an external system.
However, the draw-wire sensor reading precision is open to discussion. The length of the wire is affected by either wind or gravity, there is a risk of sag. Also, the precision of the sensor is lesser than the precision of the string potentiometer. Temperature is yet another factor that affects both the length and the resistance of the cable used.
Even with their simple design and not so great precision, they are used in a wide range of products and industries. They are used in hospital beds to detect the motion of patients. Automotive and aerospace testing is incomplete without draw-wire sensors. They are also used in hydraulic cylinder position measurement as well. Apart from these, they are also deployed in factory automation, structural testing, industrial machinery, and robotics as well.