A draw wire, also known as a string pot or cable extension transducer, works by converting linear movement into a rotary signal that can be measured. As the wire is extended, it rotates a spool connected to a sensor. This sensor then generates a signal proportional to the length of the extended wire, providing precise linear position or speed measurements.
Here's a more detailed breakdown:
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The Wire: A precision-wound, flexible wire (typically stainless steel or nylon-coated) is attached to the moving object being measured.
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The Spool: This wire is spooled around a cylindrical drum or spool located inside the sensor housing.
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The Rotary Sensor: The spool's rotation is directly coupled to a rotary sensor. Common sensor types include:
- Potentiometer: As the spool rotates, it changes the resistance in the potentiometer circuit, creating a voltage signal proportional to the wire's extension.
- Encoder (Incremental or Absolute): An encoder generates a digital signal (pulses for incremental, unique code for absolute) for each increment of rotation. This digital signal precisely indicates the wire's extension.
- Resolver: A resolver provides an analog signal representing the angular position of the spool. This signal is then converted to a linear displacement measurement.
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The Signal Output: The rotary sensor's output (voltage, digital pulses, or analog signal) is then processed and transmitted as a signal representing the linear displacement. This signal can be analog (e.g., 4-20mA, 0-10V) or digital (e.g., SSI, CANbus, Ethernet).
In essence, the draw wire mechanism converts linear motion into rotary motion, which is then sensed and converted into an electrical signal.
Key Advantages of Draw Wire Sensors:
- Accuracy: They offer precise measurement capabilities.
- Cost-Effectiveness: Generally, they provide a good balance of performance and price.
- Ease of Installation: Relatively straightforward to install in various applications.
- Versatility: Suitable for a wide range of applications, including industrial automation, robotics, and automotive testing.
