Introduction

When selecting proximity switches beware that by using this term without any further qualifications you may be getting any number of different switch/sensors . A vast array of sensors fall into the generic category of "proximity sensors" which vary considerably in construction, operation and performance. Only one common characteristic exists between these elements and that is the concept of no physical contact between the switch mechanism and the trigger system.

 Proximity sensors may be mechanical environmentally sealed, hermetically sealed reed type and non-arcing solid state types. This article will describe the most common proximity switch types, typical uses for each and the characteristics that should be considered when installing them. Finally, after reviewing the alternatives, it is believed that solid state inductive sensing systems have evolved to the point that they present the most cost-effective alternative for the process industries when reliability and performance is a consideration.

Mechanical Proximity Switches

These switches are tripped by a magnet biasing the switch to one position or the other. The mechanism is mechanical in nature and may be constructed so that the magnet systems are integrated into the body of the switch holding the "rocker arm" in the closed or open position. When a ferrous target comes near the magnetic force lines the bias is changed causing the switch to trip. The elements are available in SPDT and DPDT configurations which make them suited for relay ladder logic and dual circuit operation.

 These high power proximity switches typically operate at voltages ranging from 20 VDC up to 240 VAC with currents as high as 15 amps. Most mechanical proximity switches are environmentally sealed so contamination should not enter the switch contact chamber allowing them to function in severe atmospheres in both high power and low power (computer input) circuits.

 Although they are sealed, they are typically not hermetically sealed and therefore require costly seal fittings in the conduit system in hazardous areas.

In recent years these switches have begun to fall out of favor because of their high cost and relatively imprecise triggering and release points. Also, as computer inputs have become the norm, the need for high power switching is unnecessary in most of today's applications.

Reed Proximity Switches

Used extensively in the 60's for all types of relay circuits these glass encapsulated switches have become popular again as a preferred choice to the less reliable snap-acting mechanical switches for computer input circuits in the process industries. Reed switches received their name because of the "reed" elements which are cantilever suspended in a hermetically sealed glass capsule. When magnetic flux lines intercept the reed elements they become magnetized and snap together completing the electrical circuit. Since the electrical contacts are contained in an inert environment they do not oxidize or corrode making them desirable for low power switching.

 Typically reed switches are designed and constructed for specific voltage and current applications. For example tungsten contact reeds are best suited for wattages ranging from a minimum of 1 watt to a maximum of 100 watts with voltages up to 250 VAC. They are not recommended in 24 VDC computer input circuits because of their propensity to tarnish rapidly in most but the nearly perfect inert environments resulting in failure. Ruthenium and Rhodium contact reeds are designed to operate well in low energy and low voltage applications (120 VAC and 24 VDC computer inputs). However, special protective circuitry must be employed with the reed elements in 120 VAC industrial signal circuits to guard against commonly occurring current surges due to cable capacitive discharge.

 The glass to metal hermetic seal on the reed capsule, although eliminating the need for seal fittings in industrial environments, is also susceptible to cracking and/or breakage. Should the hermetic seal be compromised the advantages of the reed are no longer present and, in the case of tungsten contacts in low energy circuits (computer inputs), may result in rapid failure.

Solid-State Proximity Switches

As the name implies there are no moving parts in the solid state proximity switching system. The primary sensing mechanism for the sensors may be inductive, capacitive or Hall effect. Capacitive sensors are triggered by changes in the capacitance caused by proximity to another object whether this be metal, skin, or fabric. Hall effect sensors are triggered by a magnet influencing the Hall effect element. Inductive sensors are triggered by sensing the eddy current effect when a metal target is in close proximity.

 Most solid state proximity switches used in the process industrial environment are the inductive sensing type. Hall effect sensors tend to be power hungry and if used in a two wire system have prohibitively high leakage current (this concept will be explained later) usually 4 to 15 milliamps. Capacitive type, with their ability to be triggered by nearly any material, may not be desirable when the target should only be a metal object and not be inadvertently triggered by a human hand or other nonmetallic substances. They are also susceptible to changes in the surrounding air which changes the dielectric constant i.e. humidity, contaminants etc.

 Inductive Proximity Switches

Inductive proximity sensors operate with the presence of a conductive metal target. They are robust, very reliable and have precise triggering characteristics with high repeatability and ideal dead-band characteristics. And, when using some of the latest technology, may be designed for near universal adaptability to most industrial circuits.

There are also significant differences between various inductive proximity sensors as discussed below:

Three wire Inductive Proximity Switches in DC circuits

When used in DC circuits three-wire sensors typically have a power wire, a return wire and a signal wire. The power and return wires supply power to the circuit and when metal is present in the target region the circuit will connect the signal wire to the load if operating normally open. If operating normally closed the signal will be disconnected from the load.

 These three wire DC sensors are further segregated into sourcing or sinking circuits. Sinking circuits (NPN) have the load connected between the power and signal wires and when energized the switch "sinks" the power from the load to the return. A sourcing circuit (PNP) has the load attached between the signal and return wires and when energized will "source" power to the load.

Two-wire Proximity Switches

Another variety of inductive proximity sensors has the signal wire and power wires combined so there are two instead of three wires attached to the switch. In the "off" state enough current must flow through the circuit to keep the sensor active. This off state current is called leakage current and typically may range from 1 to 2 milliamps. When the switch is operated it will conduct the load circuit current.

It is critical to determine if the leakage current levels will create a problem. If it is too high it will not allow the load to switch to the off state. For example in a computer input circuit the parameters may be as follows:

• Off Threshold < 1.0 milliamp

• On Threshold > 2.0 milliamps

If the leakage current is less than 1.0 milliamp the computer will go to the off state. If the leakage current exceeds 2.0 milliamps it will remain on. Between those two states we can't predict what will happen. So to follow conservative design practice the leakage current must remain well below 1.0 milliamp.

 AC & DC Proximity Switches

These inductive switches will function in both AC and DC applications and are typically two wire devices. As with the AC or DC circuits leakage current is a consideration and tends, as a general rule, to run lower in 24 VDC circuits than 120 VAC circuits.

Namur Sensors

For intrinsically safe applications a "Namur" standard was developed which specifies the sensing output without the "switching"function. If used in an intrinsically safe circuit, when a target is present the current drops to less than 1 milliamp. When the sensor is unoperated (target not present) the current level stays above 3 milliamps.

 Since the Namur sensor's circuitry is minimal there is a significant cost savings over standard solid state proximity switches. However, it must be used with intrinsically safe repeater relay switches. The repeater relay, which takes the place of a zener barrier, acts as the switching system and the barrier. It monitors the output of the Namur sensor located in the hazardous area and changes state consistent with the Namur sensor current levels.

Technology Leads the Way

Fortunately, there are now available inductive solid state sensing systems for the process industries that offer the best of all worlds. Newly developed inductive technology is available for use broadly throughout processing plants.

In the North American marketplace, with broad usage of both AC and DC voltages for computer inputs, two-wire inductive sensors have been developed which can directly replace far less reliable mechanical switches. For example, StoneL Valve Communications has introduced a two-wire AC/DC inductive solid state sensing system for either normally open or normally closed operation with leakage current under 0.150 milliamps, thus ensuring computer compatibility.

 In European hazardous environments, Namur sensors are extensively used and are available in a variety of physical packages. Due to the simplicity of circuitry they tend to be relatively economical.

 With the refinements in solid state sensing technology and recent improvements in cost per sensor, inductive sensor systems should be strongly considered for computer input switching. If reliability is a factor in making that decision, solid state inductive sensors are the right choice. 

This Article has been submitted by:

 Ross Kunz

Product Development Manager

StoneL Valve Communications

One StoneL Drive

Fergus Falls, MN 56537 USA

E-Mail: rkunz@stonel.com

Phone : (218) 739-5774 x18

Fax : (218) 739-5776