Newnes Sensor Technology Handbook 2005 Yyepg Lotb Part 10 potx

For unshielded proximity switches mounted opposite to each other or side by side, the minimum allowable distances in Figure 15.1.45 apply: Figure 15.1.45: Minimum mounting distances for

Sr = Sn ± (.l0 × Sn)

Temperature drift tolerances must be also calculated Over a range of –25 to +70°C,

a sensing distance drift of +10% can be expected Over –25 to +85°C, the tolerance

Figure 15.1.41: Nominal sensing distance (Sn) versus usable sensing

distance (Su).

Once the usable sensing distance is determined, you need to figure in the actual plication conditions There are three factors to take into account:

ap-■ Target material,

■ Target size, and

■ Target presentation mode

The nominal sensing distance given in inductive proximity sensor specifications is determined with a target made of mild steel (in accordance with EN 60947-5-2) Whenever a target is a different metal, a correction needs to be made to the usable sensing distance (Su) The formula is:

New Su = Old Su × M

M = material correction factor

The standard target is a square of steel, 1mm (.04 in.) thick, with sides equal to sor diameter To determine the sensing distances for materials other than standard, a corresponding correction factor is used Some common materials and their correction factors are listed in Table 15.1.1.

sen-Table 15.1.1: Correction factors for non-standard target materials.

Mild steel targets of “standard” sizes are used to establish published sensing tances The standard size for each size and style of sensor usually is given in the manufacturer’s order guides If your desired target is the same size or larger than the standard target, no correction factor is necessary However, a smaller target affects sensing distance The surface area of the application target versus the surface area of the standard target provides the correction factor (See Table 15.1.2.) The formula is:New Su = Old Su × T

dis-T = target correction factor

Table 15.1.2: Correction factors for non-standard target sizes

Surface Percent of Standard:

rials with high dielectric constant to be sensed through the walls of containers made

of a material with a lower constant An example is the detection of salt (6) through a glass wall (3.7)

Each application should be tested The list of dielectric constants in Table 15.1.3 is provided to help determine the feasibility of the application

Table 15.1.3: Dielectric constants for different targets.

Constant

Acetone 19.5 Acrylic Resin 2.7–4.5

Ammonia 15–25 Aniline 6.9 Aqueous Solutions 50–80 Benzene 2.3 Carbon Dioxide 1.000~85 Carban Tetrachloride 2.2 Cement Powder 4

Chlorine Liquid 2.0 Ebonite 2.7–2.9 Epoxy Resin 2.5-6 Ethanol 24 Ethylene Glycol 38.7 FiredAsh 1.5–1.7

Shell Lime 1.2 Silicon Varnish 2.8–3.3 Soybean Oil 2.9–3.5 Styrene Resin 2.3-3.4

Wood Dry 2-6 Wood Wet 10–30

As shown in Figure 15.1.42, there are two target presentation modes Published sensing distances usually are determined by using the head-on mode of actuation The target can also approach the sensor in the slide-by mode However, the slide-by method reduces actual sensor-to-target distance by 20%

Figure 15.1.42: Targets may

be presented in head-on or slide-by mode.

Inductive proximity switches are available with a choice of switching functions mally open circuitry causes output current to flow when a target is detected; normally closed circuitry produces zero output current when a target is detected Changeover circuitry has two sensing outputs; one conducts when a target is detected while the other will not

Nor-Applicable Standards for Proximity Sensors

CENELEC (The European Committee for Electrotechnical Standardization), www.

cenelec.org

IEC (International Electrotechnical Commission), www.iec.ch/, especially IEC

60947-1 and IFC 60947-5-1, which explain the general rules relating to low-voltage switch and control gear for industrial use; IEC 529 rates the level of protection pro-

vided by enclosures, using an IP (International Protection) rating system

Description of protective classes (EN 60529) common to proximity sensors:

■ IP 65: Protection against ingress of dust and liquid

■ IP 67: Protection against limited immersion in water and dust ingress under predetermined pressure and time conditions (1 meter of water for 30 minutes minimum)

■ IP 68: Protection against the effects of continuous immersion in water

NEMA (National Electrical Manufacturer’s Association), www.nema.org NEMA

rates the protection level of enclosures as does IEC 529, but includes tests for

envi-ronmental conditions, such as rust, oil, etc that are not included in IEC 529

UL (Underwriters Laboratories), www.ul.com.

Interfacing and Design Information for Proximity Sensors

When applying capacitive sensors, it’s important to note that while shielded capacitive sensors may be flush-mounted, unshielded sensors require isolation—a material-free zone around the sensing face Materials immediately opposite both shielded and un-shielded sensors must be removed to avoid false actuation See Figure 15.1.43

Figure 15.1.43:

Unshielded proximity sensors

require isolation.

Device-to-device isolation is used when two or more sensors are mounted near each other to prevent cross talk and interference between the devices Mounting distance between shielded capacitive proximity sensors (center to center) should be at least the diameter of the sensing face Distance between unshielded sensors will vary and be three to four times the nominal sensing distance.

When shielded or unshielded sensors are facing each other, distance between sensing faces should be at least eight times the sensing distance To ensure that both shielded and unshielded proximity switches function properly, and to eliminate the possibility

of false signals from nearby metal objects, plan for minimum distances as shown in Figure 15.1.44

Figure 15.1.44: Minimum distances for proximity sensors.

For unshielded proximity switches mounted opposite to each other or side by side, the minimum allowable distances in Figure 15.1.45 apply:

Figure 15.1.45: Minimum mounting distances for unshielded sensors

The switching hysteresis (Figure 15.1.46) represents the difference between the switch ON and switch OFF points for axial or radial approach to a target and the sub-sequent retreat Usually it will be 3 to 15% of the real sensing distance (Sr)

To measure the maximum switching frequency, two tests (performed in accordance with

EN 60947-5-2) enable the maximum switching frequency

f = l/(tl + t2)

to be determined exactly from the duration of the “switch ON” period (tl) and the

“switch OFF” period (t2) (See Figure 15.1.47.)

■ Voltage drop (Vd)

Maximum voltage drop at the proximity switch if the output drops to zero

■ Residual voltage (Vr)

Voltage drop at the load if the sensing output is not conducting

■ Maximum load current (la)

Under nominal conditions, the output of the proximity switch cannot be driven

by a current greater than this value

■ Residual current (lr)

If the output is not conducting, Ir is the maximum current flowing through the load

■ Current consumption without load (lo)

Current consumption of the switch under nominal conditions without load

■ Standby delay (tv)

Period between the application of the operating voltage and the sensor

reaching the “ready” state It is determined by the transient behavior of the oscillator

■ Series and parallel circuitry

If required, inductive proximity switches can be connected in series or in parallel For series connection, the voltage drops Vd of two or more 3-wire switches (DC) or 2-wire switches (AC or DC) can be significant Care should be taken that the output voltage is large enough to drive the load With the NPN-version, the 3-wire switches must be con-nected to a common positive terminal With the PNP-version, connect the switches to a common negative terminal Series connection results in an AND function

Parallel connection of 2-wire switches (AC) and 3-wire switches (DC) with open lector outputs is possible The sum of the residual currents must be negligible enough

col-to prevent the load (the holding current of a relay or magnetic switch) from being vated For 3-wire switches with a collector resistor, it is recommended to decouple the sensing outputs with diodes An OR function is obtained by connecting the switches

acti-in parallel

Logic cards can be added to inductive proximity sensors They receive the ity sensor signal, amplify it and modify the output to respond in a particular way (as determined by time delay, pulse, or other logic) Besides operating output devices, the logic card output signal can be used as input to another card for customer logic This

proxim-is done most often with a modular control base

One-shot (pulsed) logic gives a single fixed pulse in response to a change at the sor This is often used as a leading edge detector for moving parts, where the first indication of presence requires a single operation to take place, but where the contin-ued presence will not cause recycling to occur.

sen-Maintained (latching) logic might be used to detect parts for manual reject The put is continuous until the operator resets After resets, the output will not trigger if the original target is still in front of the sensor

out-ON delay does not trigger immediately with a change at the sensor, but will ger only if the input signal exceeds a preset time delay For example, it can provide jam-up detection on a conveyor for parts feeding at specific intervals A slow down or stoppage downstream will cause a slower rate of passage, recognized as overloading

trig-or jam-up, and will cause an output to give warning trig-or shut down the equipment until the cause is eliminated A similar type provides an output which stays ON even when the cause is corrected, until manually reset by the operator

ON/OFF delay is used especially for jam-up detection on vibration feeders and veyors The ON delay detects a jam-up, and the OFF delay allows the needed time for the jam to clear the sensing area

con-Zero-speed detection provides shutdown for universal jam-up detection where the product may end up in front of the sensor for too long an interval, depending on whether the jam is upstream or downstream If the interval exceeds a preset time, the output turns OFF or shuts down the equipment

Photoelectric Sensors

Photoelectric sensors respond to the presence of all types of objects, be it large or small, transparent or opaque, shiny or dull, static or in motion They can sense targets from distances of a few millimeters up to 100 meters Photoelectric sensors use an emitter unit to produce a beam of light that is detected by a receiver When the beam

is broken, a “presence is detected

The emitter light source is a modulated, vibration-resistant LED This beam, which may be infrared, visible red or green, is switched at high currents for short time intervals so as to generate a high-energy pulse to provide long scanning distances or penetration in severe environments Pulsing also means low power consumption.The receiver contains a phototransistor that produces a signal when light falls upon

it A phototransistor is used because it has the best spectral match to the LED, a fast response, and is temperature stable By tuning the receiver circuitry to respond to a narrow band around the LED pulsing frequency, very high ambient light and noise

rejection can be achieved Tuning the receiver to respond only to a specific phase of the pulsed beam can further enhance this effect.

The availability of various fiber optic cables with sensing elements permits tric sensors to be used in many applications where space is limited or where there is a hazardous environment These sensors also are capable of sensing objects traveling at high speeds with the option of detection at up to 8 kHz if necessary

photoelec-Selecting and Specifying Photoelectric Sensors

There are different scanning techniques available for photoelectric controls

Figure 15.1.48: Retroreflective scanning.

Figure 15.1.49: Polarized scanning.

Retroreflective scanning uses an emitter and receiver housed in the same unit with the beam reaching the receiver via a reflector (Figure 15.1.48) Advantages are single side mounting, easy alignment and the ability to mount a reflector in spaces too small for

a receiver unit Reflectors are either acrylic discs or panels, or reflective tape cut to

a convenient size The larger the reflector, the more light reaches the receiver, giving longer scanning distances

Polarized scanning involves all the features of retroreflective scanning with the dition of a polarized lens (Figure 15.1.49) When the light wave hits the prismatic reflector, it is turned 90 degrees and, on return, allowed to pass through the receiving lens This prevents false reflections when detecting shiny surfaces

ad-To reliably activate retroreflective and polarized scanning techniques, approximately

80 percent of the effective beam needs to be blocked (See Figure 15.1.50.) The eter of the effective beam is the same as the reflector on one end and the lens of the photoelectric

diam-Figure 15.1.50: Effective beam for retro reflective and polarized scanning.

Figure 15.1.51: A polarized retro reflective photoelectric detects highly reflective objects.

Using polarized retroreflective photoelectrics, highly reflective objects (Figure

15.1.51) are detected for conveyor control Polarized controls respond only to ner-cubed reflectors and ignore light reflected from the target, ensuring that the target always blocks the beam

cor-In automated assembly, the proper orientation of parts can be controlled by rizing the reflectivity difference of the target sides With the microprocessor-based photoelectric in Figure 15.1.52, this is achieved by simply pushing an auto-tuning button

memo-With a through-scan technique (Figure 15.1.53), the emitter and receiver are separate and positioned opposite one another, so that the light from the emitter shines directly

on the receiver This scanning mode gives maximum reliability (little chance of false reflections to the receiver), high penetration in contaminated environments, and long scanning distances When installing adjacent through-scan systems, the emitter of one should be positioned next to the receiver of the next, to avoid one system detecting light from the other

Figure 15.1.52: A based photoelectric memorizes reflectivity differences on target.

microprocessor-Figure 15.1.53: Through scanning.

Figure 15.1.54: Long distance, harsh duty photoelectrics withstand

outdoor environments to solve such applications as traffic control at

toll ways and automatic security gates.

To reliably activate through scanning, approximately 80 percent of the effective beam needs to be blocked The diameter of the effective beam is the same size as the emitter and receiver lenses as shown in Figure 15.1.55.

In diffuse scanning, the emitter and receiver share the same housing, and the emitted beam is reflected to the receiver directly from the target (Figure 15.1.56) This mode

is used in cases where it is impractical to use a reflector, due to space considerations

or when detection of a specific target is required Because the reflected light is diffuse,

a cleaner environment is necessary and scanning distances are shorter The maximum scan distance of a diffuse-scan sensor is rated to a 10 × 10 cm white card If the actual target is less reflective than a white card, the scan distance will be reduced If the tar-get is more reflective, the distance will be increased

Figure 15.1.55: Effective beam for through scanning.

Figure 15.1.56: Diffuse scanning.

Diffuse with background suppression is a special variety of diffuse scan Using dual receivers and adjustable optics, targets can be reliably detected while backgrounds directly behind the targets are ignored (Figure 15.1.58) They can be very useful when dark-colored objects are placed in front of highly reflective backgrounds (stainless steel, white conveyors, etc.)

A convergent beam is another special variety of diffuse scan Special lenses converge the beams to a fixed focal point in front of the control (Figure 15.1.59) Convergent beams are useful for product positioning and ignoring background reflections Con-vergent beams using visible red or green light produce a concentrated, small light spot on the target that can be used to detect color marks Targets are detected within the “sensing window” of convergent beam controls This window will increase with targets of higher reflectivity and decrease with targets of lower reflectivity.

Figure 15.1.57: Polarized and diffuse photoelectrics with time delays are

used to detect both the presence and the height of the target to control

wrapping on this palletizing and wrapping machine.

Figure 15.1.58: Diffuse scanning with background suppression.

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SOD A

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SOD A

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Figure 15.1.59:

Convergent beam scanning

Figure 15.1.60: A visible red and green convergent beam photoelectric provides a small beam spot that enables accurate detection of color marks used in packaging.

Figure 15.1.61: Fiber optic photoelectrics.

Fiber optic photoelectric sensors use either through scan or diffuse scan fiber optic cables (Figure 15.1.61) These cables allow sensing in very space-restricted areas and provide detection of very small targets Cables are available with either plastic

or glass fibers that the user can cut to length Glass and stainless steel cables provide rugged protection and high-temperature capability Many different cable end tips help solve many different applications

Figure 15.1.62: A photoelectric sensor uses a small diameter diffuse scan fiber optic cable to detect electronic component lead wires.

The specified scanning distance for a photoelectric sensor is the guaranteed minimum operating distance in a clean environment For retroreflective units, this distance is that obtained using a reflector of 100 percent efficiency For diffuse units, this dis-tance is that obtained using white Kodak paper with specified dimensions, usually

10 × 10 cm Use of other materials affects the diffuse scanning distance as follows:

■ Kodak white paper, 100%

■ Aluminum, 120−150%

■ Brown Kraft paper, 60−70%

Response time is the time between optical change of the system and the output ing to ON or OFF

chang-Frequency of operation is measured in cycles per second (Hz) and is calculated by:Frequency of Operation = 1

(Response time ON + Response time OFF)

Interfacing and Design Information for Photoelectric Sensors

Photoelectric sensors have light and dark operation (LO/DO) modes In LO, the put is ON when light is incident on the receiver and OFF when there is no light at the receiver; in DO, the output is ON when there is no light incident on the receiver and OFF when there is light at the receiver

out-Today, many photoelectric sensors have self-diagnostic LED indicators and outputs Most are equipped with LED indicators that provide early warning of malfunctions due to misalignment or contaminants on the lens surface, Generally, the LEDs indi-cate a stable light or unstable light condition (see Figure 15.1.63)

Figure 15.1.63: LEDs indicate stable and unstable light conditions.

Stable light: The Green LED illuminates to show that the photoelectric is receiving at least 1.5 times the minimum operating light level of the sensor (normal operation)

Unstable light: The Green LED changes to Red (or turns OFF) to show that the toelectric is receiving an amount of light less than 50% extra but still greater than the minimum operating point The sensor is still operating but marginally

pho-Certain photoelectric sensors also are equipped with an additional wire that provides

a remote self-diagnostic output This output activates when the sensor is operating in the unstable light condition This signal can be connected to a PLC or directly to an alarm circuit to inform an user at a remote location about an unstable sensor Adjust-ment to the sensor (cleaning the lens, realignment, etc.) can then be made to prevent downtime

Some newer photoelectrics have LED indicators that provide information on the both the “dark” conditions as well as the “light” conditions On these sensors, the Green LED indicates whether the sensor is operating in a stable dark or unstable dark condi-tion in addition to stable and unstable light (Figure 15.1.64)

Stable dark: The Green LED illuminates to show that the emitted light beam is fully blocked from the receiving element of the photoelectric (normal operation)

Unstable dark: The Green LED turns OFF to show that some light is still reaching the photoelectric receiver It is not a level high enough to operate the sensor, but it is

a marginal condition If the marginal condition continues for one full cycle of tion, the Green LED will flicker and activate a remote self diagnostic output

opera-Figure 15.1.64: LEDs also can indicate stable or unstable darkness.

Latest and Future Developments

Position sensors indicate the precise location of an object, a defined target or even

a human being to control a surrounding process or improve its effectiveness New electronic parts have improved the overall characteristics of sensors, and more func-tionality is being added at the sensor level Diagnostic functions and easy-to-use calibration features are improving control systems and reducing installation time.Communication modes are increasingly important in determining the right sensing technology for an application, and the ability of manufacturers to offer a combination

of technologies is a major advantage The focus is and will be on the application and how to best solve it Sensing technology is the enabler and, therefore, the emphasis should not be on the technology itself but on the most effective way to meet the needs

pos-References and Resources

“Hall Effect Sensing and Application,” Honeywell, Inc