Sensors

Triangulation and Light Barriers—The Basics

07 August 2017

Photoelectric sensors are used to detect objects by using light. Depending on the application, available space and specific requirements, a variety of sensors can be used. Detection is typically based on the light reflected off of the object. However, the source of the reflection can also be items in the background behind the object, surfaces outside of the object that reflect light and variability caused by color in the background.

By using triangulation—an accurate and reliable means of sensing objects using light and reflective properties—background suppression is used so that only the object, and nothing else beyond that point within a specific range, is considered.

When using the triangulation method, a sensor emits a laser or LED light beam that hits an object at an incidence angle, reflects from it and is detected. A triangle is formed between the laser source, the measured object and the detector. By measuring the exact location where the laser hits the detector, distance is calculated with simple geometry.

Triangulation can be used effectively, when sensing an object and there is open space behind the object, or when there is a bright color or material behind the object that could reflect light. Triangulation sensors are used in many applications effectively, providing for rapid measurement at a low cost.

How Background Suppression Works

With background suppression, photoelectric sensors consist of an emitter and receiver. The emitter sends out a beam of light for the receiver to sense. With the diffused and retro reflective type of sensors, both the emitter and receiver are placed in the same housing and are configured for light to be reflected back to the sensor.

Figure 1. Receiver R1 receives light reflected from target (T1), and R2 receives light reflected back from the background (T2). The transition from R1 to R2 determines the sensing distance.Figure 1. Receiver R1 receives light reflected from target (T1), and R2 receives light reflected back from the background (T2). The transition from R1 to R2 determines the sensing distance.

With standard diffuse sensors, sensitivity sets the suppression point. In this case, user adjustment sets it. There are three means of using background suppression:

  • Fixed range
  • Mechanically adjustable
  • Electronically adjustable

Fixed Range

Users who do not want to make adjustments at all use the fixed-range method, which is the most tamper-proof and economical of the three background suppression possibilities. In this case, receivers R1 and R2 are fixed relative to the lens position, making the suppression point fixed.

Mechanically Adjustable

An older, but valid design method for users requiring a precise setting of the suppression point is the mechanically adjustable method. Receivers R1 and R2 are fixed, but an internal lens and gear mechanism allows users to set the suppression point. The lens is moved to change the angle. Sensor specification provides the suppression point range and sensing range at the minimum and maximum suppression point, beyond which, accuracy cannot be guaranteed.

Electronically Adjustable

Similar in performance and sensor specification to mechanically adjustable designs, this method is preferred for applications where precise setting of the suppression point is required. An algorithm is used and receivers R1 and R2 can be either a position-sensitive diode (PSD) or a multi-pixel array.

The Challenges

Depending on the application, types of objects to be detected, distance between the light and the object and when miniature sensors must be used, background suppression can be problematic. For example, background suppression is often not reliable for clear and transparent objects, including glass, water and liquid surfaces, as these surfaces provide poor reflection of the emitted light. In some applications, the sensor head requires a minimum width making it difficult to use for sensing objects through narrow openings.

For other applications, the most reliable way to detect objects optically is with a light barrier rather than using reflective methods. With light barriers, anything that disrupts the light is detected regardless of color, shape or surface finishing.

Today’s highly integrated machines and fast processes require precise and compact sensors that can perform where it was previously deemed impossible to do so. With Baumer SmartReflect® sensors, no place is too cramped and no application is too difficult to handle.

The Baumer Approach

SmartReflect sensors by Baumer eliminate the reflector, while ensuring highly reliable object detection. The SmartReflect sensor features light barrier detection. When a light barrier is constructed, anything that disrupts that barrier triggers the sensor. SmartReflect teaches the sensor to reflect the light beam off of any object regardless of color (black, white, stainless steel), coatings, transparency or irregular shapes, using only the target. Any object that comes between the target and the sensor is detected—no matter where it directs light.

Figure 2. SmartReflect light barriers feature a closed light beam set up between the sensor and the machine. The sensor switches whenever an object interrupts the light beam.Figure 2. SmartReflect light barriers feature a closed light beam set up between the sensor and the machine. The sensor switches whenever an object interrupts the light beam.

SmartReflect is the only sensor in its class to detect transparent objects of any shape or structure up to a distance of 1 meter with a response time of 0.25 ms. An IO-Link interface provides for rapid format changes, easy installation and a variety of housing designs, even for use in demanding environments and those where cleaning agents could damage a reflector.

SmartReflect sensors can be used in extremely limited spaces or for ranges up to 2 m in such applications as packaging, material handling, graphics and a variety of industrial settings, or to recognize transparent objects over a detection range of up to 800 mm. They provide reliability and greater process safety by eliminating reflectors as a potential weak point, while also reducing operating costs.

Summary

A variety of optical sensors are available from Baumer that target the unique requirements of specific applications, environments, sizes and economic considerations. Whether light barriers or background suppression make the most sense for your application, Baumer offers advanced solutions that meet a variety of performance goals.

To contact the author of this article, email jonathan.fuller@ieeeglobalspec.com


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