WhoshouldIsee Tracks

13th June 2022

What is a linear position sensor?

What is a linear position sensor?

What is a linear position sensor?

They are all around us – in our car suspension systems as we drive over speed bumps, in the trains we sit in as they move around curved tracks and in the ‘fly-by-wire’ aircraft that adjust their ailerons and elevators as they start to descend. They are in the factory machines that we work with every day to package and manufacture products, and in medical equipment that keeps us alive. What are these inconspicuous but crucial devices? They are linear position sensors which are helping to embed digital technologies in the next industrial revolution.

Linear position sensors and custom electronic circuitry

Linear position sensors measure the linear distance between an object and a point of reference, as well as speed and changes in position. They do this by converting linear displacement into an extra low voltage, in either an analog or digital signal. That electrical output is generated by exploiting different phenomena in nature such as magnetic fields, solar energy and electrical resistance. Excitation circuitry in the sensor creates a stimulus and signal conditioning circuitry then selects and amplifies the electrical response. Sensors then digitise the signal using an Analogue-to-Digital Converter (ADC), the digital result is then passed to a Central Processing Unit (CPU).

The Fourth Industrial Revolution

The Fourth Industrial Revolution or Industry 4.0 is well underway and adopting next generation linear position sensors that employ Application Specific Integrated Circuits (ASICs) to deliver the speed, accuracy and cost savings to industrial automation. Linear position sensors detect the location of machinery and equipment parts so they can be tracked and automated. They provide a digital signal via Ethernet connectivity to accurately report the target position in real-time with sufficient resolution. Sensor electronics are a key part of the evolution in automation, having direct influence on speed, accuracy and overall form factor.

What are the basic types of linear position sensor?

Linear position sensors consist primarily of two types: contact and non-contact, depending on whether they require physical contact to measure. Contact sensors tend to wear or degrade over time due to the constant friction for example of the wiper in a linear potentiometer measuring electrical resistance. In contrast, noncontact sensors use magnetic fields, solar energy and lasers to sense changes in position so they are less prone to wear and tear and can tolerate higher levels of vibration.

Linear encoders

A linear encoder is a sensor, transducer or readhead combined with a scale that encodes position. The sensor moves along the scale which it ‘sees’ using optical, magnetic, inductive, capacitive or eddy current technologies. It then converts the encoded position into an analog or digital signal, which is then decoded into position by a digital readout (DRO) or motion controller.

A linear encoder can be either incremental or absolute. What does this mean? Incremental encoders use a simpler method of determining movement by counting the number of pulses and then using that number to compute the position. With an absolute encoder, the output signal generated by the device creates a unique set of digital bits that correspond to a specific position of the object being measured. So incremental encoders measure the relative movement against some point of reference, whereas absolute encoders measure the position directly using a unique signal code that precisely reflects the position.

Linear encoders are used in two main areas of application: measurement and motion systems.

Measurement applications include coordinate-measuring machines (CMM), laser scanners, callipers, gear measurement, tension testers, and digital readouts (DROs).

Servo controlled motion systems provide accurate, high-speed movement in robotics, machine tools, pick-and-place PCB assembly equipment, semiconductors handling and test equipment, wire bonders, printers and digital presses. Humans design these intricate circuit boards but cannot match the speed and accuracy of robots and assembly equipment.

Linear potentiometers

In a linear potentiometer a wiper moves along a resistor as the equipment moves through the full length of stroke, providing a variable resistance related directly to position. Signal conditioners then convert this reading into other electrical output levels. Linear potentiometers are a tried and tested sensing method that are simple, inexpensive, and easy to work with. Although they are a fundamental method for detecting equipment position, as a physical device subject to constant mechanical wear they deteriorate over time and need to be replaced. They may not be resistant to liquids and contaminants, and the form factor must be large enough to accommodate the fully extended and retracted rod stroke, which can be limited in use due to installation space constraints.

Linear potentiometers can measure spring travel in bicycles and motorcycles so as to determine the optimal spring fork for challenging terrains, especially in motorsports. Imagine a mountain biker in Utah, USA speeding through wooded forests, Aspen Pine lands, bubbling streams and then across open prairie land, bouncing easily off paths covered with roots and stones thanks to the valuable data these sensors provide in the design of mountain bikes.

Aside from sport, more mundane but crucial applications include agricultural machinery, increasingly automated, moving across vast open fields. Lumbering combine harvesters use linear potentiometers for the wheel angle measurement in their steering systems as they move back and forth.

Linear potentiometers can also measure the stroke movement of both hydraulic and pneumatic cylinders by installing them directly inside the cylinder itself. The sensor compares the target stroke with the actual stroke to ensure process safety. Reliable measurement data is collected even at high pressure.

Being one of the most versatile and widely used sensors in industry, linear potentiometers are used in a whole range of applications including the control of motors and actuators, robotics, industrial machines, audio equipment volume control, automotive engine control systems, calibration for precision test equipment and medical equipment.

Hall effect sensors

A Hall effect sensor detects the presence and magnitude of a magnetic field using the Hall effect: namely when a conductor flowing with electrical current is plunged into a perpendicular magnetic field a voltage (the Hall voltage) is generated. It was discovered by the American physicist Edwin Hall in 1879. The output voltage of a Hall sensor is directly proportional to the strength of the magnetic field. Exploiting Hall Effect technology enables the sensors to be non-contact resulting in highly precise measurements and an exceptionally long mechanical life.

Hall sensors are used in proximity sensing, positioning, speed detection, and current sensing applications. Combined with threshold detection a Hall Effect Sensor can act as a binary switch. Frequently seen in industrial applications such as pneumatic cylinders, they are also used in consumer equipment for example detecting missing paper and open covers in computer printers. They can also measure filament thickness in the 3D printers as used in rapid prototyping, among other things.

Hall sensors time the speed of wheels and shafts in internal combustion engine ignition timing, tachometers and anti-lock braking systems and detect the position of the permanent magnet in brushless DC electric motors.

How can Live Electronics support manufacturers with linear position sensors?

Here at Live Electronics we are continually looking for the best products at the best price to help with modern technologies such as linear position sensors. Our linear position sensors from ZF use Hall Effect technology which allows the sensors to be non-contact resulting in highly precise measurements and an exceptionally long mechanical life.

FAQ's

Linear position sensors measure the linear distance between an object and a point of reference, as well as changes in position. They are used for detecting the location of machinery and equipment parts so they can be tracked and automated.

Linear position sensors exploit different phenomena in nature such as magnetic fields, solar energy and electrical resistance to convert the displacement of moving machinery into an electrical output.

There are many types of linear position sensor, however the linear potentiometer is widespread being an inexpensive and easy to use piece of equipment. The Hall effect sensor however is becoming the most common due to its non-contact nature, resulting in highly precise measurements and an exceptionally long mechanical life.

A linear potentiometer can be tested using a voltmeter. With a linear potentiometer fully extended, the voltmeter should display the maximum volts of the sensor in DC. With the linear potentiometer fully retracted, the voltmeter should display zero volts DC. The Hall effect sensor can be tested in a similar way in that the voltage changes from 0V to 5V (the high voltage can be changed based on the set up but usually this is between 1-5V, with 5V being the most common)