When it comes to measuring linear distances of more than a few feet, or working in harsh environments, linear encoders are not always able to meet the application requirements. Many optical linear encoders use a clear linear strip with black lines precisely placed on them by a process using photo lithography. I know that the encoder manufacturing company that I worked for was limited to making linear strips to lengths of 34 inches maximum because of the size limits imposed by the photo lithography process.
So what is the answer to the problem of needing to make linear measurements greater than a few feet, or when needing a linear encoder to work in harsh environments? Introducing the draw wire position sensor! This is a special kind of encoder that turns linear motion into rotary motion.
The draw wire position sensor is known by other names such as draw wire encoder, string encoder, linear string encoder, draw wire sensor, draw wire displacement transducer, draw wire transducer, string pot, string potentiometer, and many other similar names. From here on, this article will refer to them by the shorter name of string encoder.
This article will delve into the string encoder, and explain what they are, how they work, and how they are used in real world applications.
A Cross Between A Rotary Encoder And A Tape Measure
To answer the question, “What is a string encoder?”, I would have you think of it as a cross between a rotary encoder and a tape measure. If you can imagine in your mind mounting a carpenter’s tape measure onto the shaft of a rotary encoder, you will get a good feel for how a string encoder works.
As you pull out the tape of the tape measure, the shaft on the rotary encoder would rotate. The linear motion of you pulling on the tape would be converted into rotary motion as the encoder shaft rotates. Correspondingly the encoder would output electrical signals of one type or another depending on the type of rotary encoder being used. For example, if it is an incremental encoder, it would output a fixed number of 2-channel A/B quadrature square waves for each revolution of the shaft.
And we all know what happens when you let go of the tape of the tape measure. The spring-loaded action inside the tape measure causes the tape to be rapidly reeled back in. This would make the shaft on the rotary encoder turn in the opposite direction, and again depending on the type of encoder being used, it would output electrical signals that correspond to this motion.
The Spring Loaded Draw Wire Mechanism
Here is a drawing that illustrates the component parts of a spring-loaded string encoder mechanism.
There is the wire that may be made of a flexible metal wire rope or plastic coated flexible wire rope or even just plastic or other materials. As the wire is pulled out, it causes a constant diameter spool to rotate. This in turn makes the shaft of the rotational sensor or encoder turn with it.
The constant diameter spool is designed to move right to left on the shaft as the wire reels out, and left to right on the shaft as the wire reels back in. This keeps the wire lined up perfectly with the exit hole in the mechanism housing as it reels out and in and eliminates the measurement error that would be caused if the wire angle to the exit hole was not held constant at zero degrees.
Then there is the high torque power spring. The spring keeps tension on the wire as it is reeled out, and also causes the draw wire to be reeled back in when the motion has reversed.
The Rotational Sensor
The rotational sensor can be a number of different types of devices depending on the requirements of the application. These would include incremental encoders, multi-turn absolute encoders, and multi-turn potentiometers.
Shown above is an example of a string encoder that incorporates an incremental encoder that outputs 2-channels A and B square waves in quadrature, and a third channel Z (index). It has a measuring range of zero to 2 meters (2000 mm). It is also available with the digital display option shown below.
A 1 meter (1000 mm) measuring range encoder is also available with the A, B, and Z outputs. Or the digital display and encoder can be purchased together as a pair.
The digital display with inputs for A, B, and Z encoder signals and with a 4-to-20 mA output is also available separately.
If a longer measuring length is needed on the draw wire, there is an encoder with zero to 3 meters (3000 mm) measuring range.
The rotational sensor can be an absolute encoder. Shown below is an example where the rotational sensor is a multi-turn absolute encoder with Modbus RTU RS485 serial outputs.
Then if an analog output such as zero to 5,000 ohms, or zero to +5 volts, or 4-to-20 mA is what is needed in an application, those options are available. Shown below is what the one with the 4-to-20 mA output and a measurement range of zero to 1 meter (1000 mm) looks like. It is also available in a 500 mm measurement range version.
Other analog versions available include a zero to +5-volt output version with zero to 1000 mm measurement range, and a zero to 5,000 ohms output version with a zero to 500 mm measurement range.
Draw Wire Length Determines The Maximum Range Of Motion
Of course what determines the maximum travel or range of motion for the string encoder is the length of the wire.
Some common lengths available include 500 mm, 1000 mm, 2000 mm, and 3000 mm. The longest draw wire length that I have personally read about is 50 meters in length!
Synchronous Jacking Systems And Other Applications
Let’s now take a look at some typical applications for string encoders.
Imagine having to lift something, and maybe it is very heavy, and at the same time having to keep the object being lifted perfectly level. What is known as a synchronous jacking system could be just the solution for such a situation.
A synchronous jacking system is made up of a number of hydraulic jacks that are computer controlled so that they all work together to keep the object being lifted level. A string encoder would be installed on each jack so that the computer can monitor and control the height of each hydraulic jack very precisely.
String encoders are used in medical equipment applications as well. For example, the string encoder gives position feedback information to the computer that is controlling the position of the patient table within the MRI or CAT scan machine. The horizontal position of the table that the patient is lying on can be accurately monitored and controlled as it moves back and forth and in and out of the examination tube.
String encoders can be used on forklifts, cranes, hydraulic lifts in automobile related applications, and many other lifting situations to provide height display, monitoring, and computer control. We have only scratched the surface here in terms of possible applications, but I think that you can see that these string encoders are very useful devices.
A list of all the products that I have mentioned in this article can be found by clicking here to go to a product review page. From the product review page you will be able to click on specific products to get more specifications, mechanical drawings, cable wire color to signal name listings, and prices.
Please leave your comments and questions below.
Maybe you have some great ideas for putting string encoders to practical use. I would love to hear about them.
Also, if you are needing some technical support regarding using string encoders in your application, don’t hesitate to get back to me about that.