Vision and Discrete Sensors

Choosing between discrete, continuous position sensors for an application

Discrete and continuous position sensors offer many benefits for manufacturers, but they offer different benefits depending on the application.

Courtesy: Balluff

 

Learning Objectives

  • Learn the difference between discrete and continuous position sensors.
  • Understand which applications they’re best-suited for and see examples of how they help manufacturers and end users.

Discrete sensor insights

  • Continuous position sensors offer real-time feedback, ideal for high-precision applications requiring closed-loop servo control, ensuring precise cylinder movement.
  • Discrete position sensors detect specific positions, vital for applications like clamping, welding, and gauging, providing essential control points.

The role of smart cylinders — hydraulic or pneumatic cylinders with integrated position detection capability — has increased as manufacturers improve efficiency through automation. Smart cylinders can use continuous or discrete position sensing, providing manufacturers with options, but also questions as to which is best for their particular application and setting.

Continuous position sensors provide near real-time position feedback throughout the entire stroke of the cylinder making them the ideal choice for applications at the higher end of the control spectrum. They are commonly used when the application calls for closed-loop servo control and where the cylinder’s position, speed, acceleration and deceleration must be controlled.

Discrete position sensors are equipped with a switching functionality at one or more designated positions along the cylinder’s stroke. These positions are often set to detect fully retracted and extended positions, but one can also be used to detect mid-stroke position.

To determine which sensor is correct requires reviewing the application and a determination of how precisely the movement of the cylinder needs to be controlled. Some hydraulic cylinder applications may require no position sensing at all and can rely on manual control or external switches and stops. For these scenarios, smart cylinders can still enhance efficiency through their hydraulic or pneumatic capabilities.

Figure 1: High-pressure inductive sensors installed in hydraulic cylinder.
Figure 1: High-pressure inductive sensors installed in hydraulic cylinder. Courtesy: Balluff

Discrete position sensing

Many applications require only the beginning and end of the cylinder stroke be detected so the cylinder can be commanded to reverse direction. These applications are ideal for discrete position sensing.

Several types of sensors are used for discrete position detection, but one of the most common is high-pressure inductive proximity sensors, which are installed into the end caps of the cylinder. The sensors detect the piston as it reaches the end of the cylinder stroke in either direction.

These sensors are designed to withstand the full pressure of the hydraulic system. Inductive sensors are reliable because they operate without any form of mechanical contact and are unaffected by oil temperature or viscosity changes.

Discrete position sensing applications examples include:

  • Hydraulic clamps: In manufacturing processes that involve the clamping of workpieces, discrete sensors are essential for ensuring the clamps are fully engaged, guaranteeing workpiece stability during operations.

  • Welding operations: In welding applications, discrete position sensors play a critical role in detecting open and closed positions, ensuring precise control over the welding process and the quality of welds.

  • Compactors and balers: Hydraulic compactors and balers rely on discrete position sensors to detect the end of the cylinder stroke, preventing over-compression and safeguarding equipment integrity.

  • Pneumatic clamps and grippers: Pneumatically-actuated clamps and grippers often use discrete sensors to indicate fully extended and fully retracted positions, and in some cases, intermediate positions as well. This ensures components are accurately positioned during handling and assembly.

  • Gauging and sizing: In certain industrial applications, multiple discrete sensors are employed in grippers to gauge and size workpieces, contributing to quality control and precision.

Magnetically-actuated switches for discrete position sensing

It is quite common for pneumatically-actuated clamps and grippers to use discrete sensors to indicate fully extended and fully retracted positions, and in many cases, in-between positions as well. There are even applications where multiple discrete sensors are used in grippers for gauging and sizing work pieces.

The most common method of providing discrete position in an air cylinder is using externally-mounted switches that react to a magnet installed around the circumference of the piston. Magnetically-actuated switches can sense the field of a magnet embedded in the cylinder’s piston through the cylinder’s aluminum body.

There are several different operating principles used in magnetically-actuated switches, ranging from simple, low-cost reed switches and hall-effect switches to significantly more reliable sensors that use magnetoresistive technology. One of the big advantages of magnetoresistive sensors is they will reliably detect radial and axial magnetic fields, making them ideal replacements for reed or hall-effect switches.

Figure 2: Magnetically-actuated sensor installed into a cylinder C-slot.
Figure 2: Magnetically-actuated sensor installed into a cylinder C-slot. Courtesy: Balluff

Continuous position sensing

Continuous position sensors play a crucial role in industrial manufacturing, especially when precise control over a hydraulic cylinder’s position, speed acceleration and deceleration is required. These sensors are the cornerstone of closed-loop servo control systems, which are extensively used in a wide range of industrial applications to optimize processes and ensure accuracy.

Examples of discrete position sensing applications include:

  • Sawmills: In the lumber industry, continuous position sensors are indispensable for controlling the precise positioning of cutting blades and conveyor systems. This ensures each piece of lumber is cut to the exact dimensions required, minimizing waste and maximizing production efficiency.

  • Steel processing: Steel mills rely on closed-loop servo control to maintain precise alignment of rollers and other machinery involved in the steel production process. Continuous position sensors help regulate the position and movement of heavy steel coils, guaranteeing the quality of the final product.

  • Tire manufacturing: Tire production demands extreme precision in shaping and curing processes. Continuous position sensors are utilized to control the movements of molds and curing presses, ensuring the uniformity and durability of tires produced in high volumes.

  • Off-highway equipment: In the construction and agriculture sectors, off-highway equipment such as bulldozers, excavators and agricultural machinery employ hydraulic cylinders with continuous position sensors to enable precise control of various functions. This technology enhances operator safety and equipment efficiency.

Magnetostrictive linear position sensors

Among the various continuous position sensing technologies, magnetostrictive linear position sensors are one of the most commonly used due to their reliability and accuracy. They detect the position of a magnet attached to the piston and provide a continuous, absolute position signal. They provide several advantages.

They are rated to withstand the full pressure of the hydraulic system, making them well suited for demanding industrial environments. Magnetostrictive technology offers the advantage of being completely non-contact, meaning it requires no mechanical contact between the sensor and the moving cylinder and is not subject to wear and performance degradation. In addition, numerous electrical interface options are available, from simple analog (0 to 10V or 4-20mA) to advanced industrial fieldbus interfaces. These options cater to a wide range of industrial automation requirements.

Continuous position sensors can also be used in pneumatic cylinders. While closed-loop servo control with pneumatics is not as common as it is with hydraulics, there are situations where pneumatic cylinders require continuous position sensing capability. For example, low-pressure pneumatic cylinders are sometimes used as measurement probes, or touch probes, where the cylinder rod is extended until it touches a part to be measured or gaged. In these situations, it is beneficial to be able to get continuous position feedback, especially when there is variability in the measured part.

The role of smart cylinders with integrated position detection capabilities is pivotal in today’s manufacturing landscape. Whether it’s continuous position sensing for high-precision tasks or discrete position sensing for basic detection needs, smart cylinders contribute to increased automation, improved efficiency and enhanced control across various industrial applications. The choice between continuous and discrete sensing depends on the specific demands of the application, with many sensors available to help drive innovation and efficiency in their operations.

Scott Rosenberger is the product marketing manager for positioning sensors at Balluff. Edited by Chris Vavra, web content manager, CFE Media and Technology, [email protected].

Keywords: discrete sensors, linear position sensors

CONSIDER THIS

What are your biggest considerations when choosing a discrete sensor?

Written by

Scott Rosenberger

Scott Rosenberger is the product marketing manager for positioning sensors at Balluff America. He has more than 30 years of experience in the industrial automation field. In addition to an extremely strong technical product background, he has extensive experience in product management, product marketing, training and product support.