PHOTO ELECTRIC SENSORS

PHOTO ELECTRIC SENSORS

John   02:54:00

A photoelectric sensor is another type of position sensing device. Photoelectric sensors, similar to the ones shown below, use a modulated light beam that is either broken or reflected by the target.

The control consists of an emitter (light source), a receiver to detect the emitted light, and associated electronics that evaluate and amplify the detected signal causing the photoelectric’s output switch to change state.
Modulated light increases the sensing range while reducing the effect of ambient light. Modulated light is pulsed at a specific frequency between 5 and 30 KHz. The photoelectric sensor is able to distinguish the modulated light from ambient light. Light sources used by these sensors range in the light spectrum from visible green to invisible infrared. Light-emitting diode (LED) sources are typically used.

CLEARANCE

It is possible that two photoelectric devices operating in close proximity to each other can cause interference. The problem may be rectified with alignment or covers. The following clearances between sensors are given as a starting point. In some cases it may be necessary to increase the distance between sensors.

 EXCESS GAIN

Many environments, particularly industrial applications, include dust, dirt, smoke, moisture, or other airborne contaminants. A sensor operating in an environment that contains these contaminants requires more light to operate properly. There are six grades of contamination:
1. Clean Air (Ideal condition, climate controlled or sterile)
2. Slight Contamination (Indoor, nonindustrial areas, office
buildings)
3. Low Contamination (Warehouse, light industry, material
handling operations)
4. Moderate Contamination (Milling operations, high
humidity, steam)
5. High Contamination (Heavy particle laden air, extreme
wash down environments, grain elevators)
6. Extreme/Severe Contamination (Coal bins, residue on lens)

Excess gain represents the amount of light emitted by the transmitter in excess of the amount required to operate the receiver. In clean environments an excess gain equal to or greater than 1 is usually sufficient to operate the sensor’s receiver. If, for example, an environment contained enough airborne contaminants to absorb 50% of the light emitted by the transmitter, a minimum excess gain of 2 would be required to operate the sensor’s receiver. Excess gain is plotted on a logarithmic chart. The example shown below is an excess gain chart for an M12 thru-beam sensor. If the required sensing distance is 1 m there is an excess gain of 30. This means there is 30 times more light than
required in clean air hitting the receiver. Excess gain decreases as sensing distance increases. Keep in mind that the sensing distance for thru-beam sensors is from the transmitter to the receiver and the sensing distance for reflective sensors is from the transmitter to the target.

SWITCHING ZONE
Photoelectric sensors have a switching zone. The switching zone is based on the beam pattern and diameter of the light from the sensor’s emitter. The receiver will operate when a target enters this area.

SCAN TECHNIQUES

A scan technique is a method used by photoelectric sensors to detect an object (target). In part, the best technique to use depends on the target. Some targets are opaque and others are highly reflective. In some cases it is necessary to detect a change in color. Scanning distance is also a factor in selecting a scan technique. Some techniques work well at greater distances while others work better when the target is closer to the sensor.

THROUGH BEAM

 Separate emitter and receiver units are required for a thru-beam sensor. The units are aligned in a way that the greatest possible amount of pulsed light from the transmitter reaches the
receiver. An object (target) placed in the path of the light beam blocks the light to the receiver, causing the receiver’s output to change state. When the target no longer blocks the light path
the receiver’s output returns to its normal state. Thru-beam is suitable for detection of opaque or reflective objects. It cannot be used to detect transparent objects. In addition, vibration can cause alignment problems. The high excess gain of thru-beam sensors make them suitable for
environments with airborne contaminants. The maximum sensing range is 300 feet.

THROUGH BEAM EFFECTIVE BEAM

The effective beam of a photoelectric sensor is the region of the beam’s diameter where a target is detected. The effective beam on a thru-beam sensor is the diameter of the emitter and receiver lens. The effective beam extends from the emitter lens to the receiver lens. The minimum size of the target should equal the diameter of the lens.

REFLECTIVE OR RETROEFLECTIVE SCAN

Reflective and retroreflective scan are two names for the same  technique. The emitter and receiver are in one unit. Light from the emitter is transmitted in a straight line to a reflector and returns to the receiver. A normal or a corner-cube reflector can be used. When a target blocks the light path the output of the sensor changes state. When the target no longer blocks the light path the sensor returns to its normal state. The maximum sensing range is 35 feet.

RETROREFLECTIVE SCAN EFFECTIVE BEAM

The effective beam is tapered from the sensor’s lens to the edges of the reflector. The minimum size of the target should equal the size of the reflector.

REFLECTORS

Reflectors are ordered separately from sensors. Reflectors come in various sizes and can be round or rectangular in shape or reflective tape. The sensing distance is specified with a particular reflector. Reflective tape should not be used with polarized retroreflective sensors

RETROREFLECTIVE SCAN AND SHINY OBJECTS

Retroreflective scan sensors may not be able to detect shiny  objects. Shiny objects reflect light back to the sensor. The sensor is unable to differentiate between light reflected from a shiny object and light reflected from a reflector.

POLARIZED RETROREFLECTIVE SCAN

A variation of retroreflective scan is polarized retroreflective  scan. Polarizing filters are placed in front of the emitter and receiver lenses. The polarizing filter projects the emitter’s beam in one plane only. This light is said to be polarized. A corner-cube reflector must be used to rotate the light reflected back to the receiver. The polarizing filter on the receiver allows rotated light to pass through to the receiver. In comparison to retroreflective scan, polarized retroreflective scan works well when trying to detect shiny objects.

DIFFUSE SCAN
The emitter and receiver are in one unit. Light from the emitter strikes the target and the reflected light is diffused from the surface at all angles. If the receiver receives enough reflected
light the output will switch states. When no light is reflected back to the receiver the output returns to its original state. In diffuse scanning the emitter is placed perpendicular to the target. The receiver will be at some angle in order to receive some of the scattered (diffuse) reflection. Only a small amount of light will reach the receiver, therefore, this technique has an effective range of about 40”

DIFFUSE SCAN CORRECTION FACTOR

The specified sensing range of diffuse sensors is achieved by  using a matte white paper. The following correction values may be applied to other surfaces. These values are guidelines only
and some trial and error may be necessary to get correct operation.
Test Card (Matte White) 100%
White Paper 80%
Gray PVC 57%
Printed Newspaper 60%
Lightly Colored Wood 73%
Cork 65%
White Plastic 70%
Black Plastic 22%
Neoprene, Black 20%
Automobile Tires 15%
Aluminum, Untreated 200%
Aluminum, Black Anodized 150%
Aluminum, Matte (Brushed Finish) 120%
Stainless Steel, Polished 230%

DIFFUSE SCAN WITH BACKGROUND SUPPRESSION

Diffuse scan with background suppression is used to detect objects up to a certain distance. Objects beyond the specified distance are ignored. Background suppression is accomplished
with a position sensor detector (PSD). Reflected light from the target hits the PSD at different angles, depending on the distance of the target. The greater the distance the narrower the angle of the reflected light.

DIFFUSE SCAN EFFECTIVE BEAM

The effective beam is equal to the size of the target when located in the beam pattern.

 OPERATING MODES

There are two operating modes: dark operate (DO) and light operate (LO). Dark operate is an operating mode in which the load is energized when light from the emitter is absent from the
receiver.

Light operate is an operating mode in which the load is energized when light from the emitter reaches the receiver.

FIBER  OPTIC
Fiber optics is not a scan technique, but another method for transmitting light. Fiber optic sensors use an emitter, receiver, and a flexible cable packed with tiny fibers that transmit light. Depending on the sensor there may be a separate cable for the emitter and receiver, or it may use a single cable. When a single cable is used, the emitter and receiver use various methods to distribute emitter and transmitter fibers within a cable. Glass fibers are used when the emitter source is infrared light. Plastic
fibers are used when the emitter source is visible light.

Fiber optics can be used with thru-beam, retroreflective scan, or diffuse scan sensors. In thru beam light is emitted and received with individual cables. In retroreflective and diffuse scan light is emitted and received with the same cable (bifurcated). Fiber optics is ideal for small sensing areas or small objects. Fiber optics have a shorter sensing range due to light losses in the fiber optic cables.

LASERS

Lasers are sometimes used as sensor light sources. Siemens uses Class 2 lasers which have a maximum radiant power of 1 mW. Class 2 lasers require no protective measures and a laser
protection officer is not required. However, a warning notice must be displayed when laser sensors are used. Laser sensors are available in thru-beam, diffuse scan, and diffuse scan with background suppression versions. Lasers have a high intensity visible light, which makes setup and adjustment
easy. Laser technology allows for detection of extremely small objects at a distance. The Siemens L18 sensor, for example, will detect an object of 0.03 mm at a distance of 80 cm. Examples
of laser sensor applications include exact positioning, speed detection, or checking thread thickness of 0.1 mm and over

ULTRASONIC PROXIMITY SENSORS

ULTRASONIC PROXIMITY SENSORS

John   02:14:00

Ultrasonic proximity sensors use a transducer to send and receive high frequency sound signals. When a target enters the beam the sound is reflected back to the switch, causing it to energize or deenergize the output circuit.

PIEZOELECTRIC DISK

A piezoelectric ceramic disk is mounted in the sensor surface. It can transmit and receive high-frequency pulses. A highfrequency voltage is applied to the disk, causing it to vibrate at the same frequency. The vibrating disk produces high-frequency sound waves. When transmitted pulses strike a sound-reflecting object, echoes are produced. The duration of the reflected pulse is evaluated at the transducer. When the target enters the preset operating range, the output of the switch changes state.
When the target leaves the preset operating range, the output returns to its original state.

The emitted pulse is actually a set of 30 pulses at an amplitude of 200 Kvolts. The echo can be in microvolts.

BLIND ZONE

A blind zone exists directly in front of the sensor. Depending on the sensor the blind zone is from 6 to 80 cm. An object placed in the blind zone will produce an unstable output.

RANGE DEFINITION

The time interval between the transmitted signal and the echo is directly proportional to the distance between the object and sensor. The operating range can be adjusted in terms of its width and position within the sensing range. The upper limit can be adjusted on all sensors. The lower limit can be adjusted only with certain versions. Objects beyond the upper limit do not produce a change at the output of the sensor. This is known as “blanking out the background” . On some sensors, a blocking range also exists. This is between the lower limit and the blind zone. An object in the blocking range prevents identification of a target in the operating range. There is a signal output assigned to both the operating range and the output range.

RADIATION PATTERN

The radiation pattern of an ultrasonic sensor consists of a main cone and several neighboring cones. The approximate angle of the main cone is 5°.

FREE ZONES

Free zones must be maintained around the sensor to allow for neighboring cones. The following examples show the free area required for different situations.

OPERATING MODES

Sonar sensors can be setup to operate in several different modes: diffuse, reflex, and thru-beam.

DIFFUSE MODE

This is the standard mode of operation. Objects, traveling in anydirection into the operating range of the sound cone, will cause the sensor output to switch states. This mode of operation is similar to a proximity sensor.

REFLEX MODE

The reflex mode uses a reflector located in the preset operating range. The operating range is adjusted for the reflector. The pulses are bounced off the reflector and the echo pulses are returned to the sensor. When a target blocks the echo pulses the output is activated. Typically used in applications where the target is not a good sound absorber.

THROUGH BEAM MODE

Thru-beam sensors consist of a transmitter, which emits ultrasonic pulses, and a receiver. If the beam between the transmitter and the receiver is interrupted the output of the
receiver switches state.

ENVIRONMENTAL INFLUENCES

Sound travel time can be affected by physical properties of the air. This, in turn, can affect the preset operating distance of the sensor.

CAPACITIVE PROXIMITY SENSORS

CAPACITIVE PROXIMITY SENSORS

John   01:48:00

Capacitive proximity sensors are similar to inductive proximity sensors. The main difference between the two types is that capacitive proximity sensors produce an electrostatic field
instead of an electromagnetic field. Capacitive proximity switches will sense metal as well as nonmetallic materials such as paper, glass, liquids, and cloth.

The sensing surface of a capacitive sensor is formed by two concentrically shaped metal electrodes of an unwound capacitor. When an object nears the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit. As a result, the oscillator
begins oscillating. The trigger circuit reads the oscillator’s amplitude and when it reaches a specific level the output state of the sensor changes. As the target moves away from the sensor the oscillator’s amplitude decreases, switching the sensor output back to its original state.

STANDARD TARGET AND DIELECTRIC CONSTANT
Standard targets are specified for each capacitive sensor. The  standard target is usually defined as metal and/or water. Capacitive sensors depend on the dielectric constant of the target. The larger the dielectric number of a material the easier it is to detect. The following graph shows the relationship of the dielectric constant of a target and the sensor’s ability to detect the material based on the rated sensing distance (Sr).

The following table shows the dielectric constants of some materials. If, for example, a capacitive sensor has a rated sensing distance of 10 mm and the target is alcohol, the effective sensing distance (Sr) is approximately 85% of the rated distance, or 8.5 mm.

DETECTING THROUGH BARRIER
One application for capacitive proximity sensors is level detection through a barrier. For example, water has a much higher dielectric than plastic. This gives the sensor the ability to “see through” the plastic and detect the water.

SHIELDING

All capacitive sensors are shielded. These sensors will detect conductive material such as copper, aluminum, or conductive fluids, and nonconductive material such as glass, plastic, cloth, and paper. Shielded sensors can be flush mounted without adversely affecting their sensing characteristics. Care must be taken to ensure that this type of sensor is used in a dry environment. Liquid on the sensing
surface could cause the sensor to operate.

SENSORS, INDUCTIVE PROXIMITY SENSORS

SENSORS, INDUCTIVE PROXIMITY SENSORS

John   03:09:00

The sensor incorporates an electromagnetic coil which is used  to detect the presence of a conductive metal object. The sensor will ignore the presence of an object if it is not metal.

Siemens inductive proximity sensors are operated using an Eddy Current Killed Oscillator (ECKO) principle. This type of sensor consists of four elements: coil, oscillator, trigger circuit, and an output. The oscillator is an inductive capacitive tuned circuit that creates a radio frequency. The electromagnetic field produced by the oscillator is emitted from the coil away from the face of the sensor. The circuit has just enough feedback from the field to keep the oscillator going.

When a metal target enters the field, eddy currents circulate within the target. This causes a load on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the sensor the eddy currents increase, increasing the load on the oscillator and further decreasing the amplitude
of the field. The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (on or off). As the target moves away from the sensor, the oscillator’s amplitude increases. At a predetermined level the trigger switches the output state of the sensor back to its normal condition (on or off).

OPERATING VOLTAGE

inductive proximity sensors include AC, DC, and AC/ DC (universal voltage) models. The basic operating voltage ranges are from 10 to 30 VDC, 15 to 34 VDC, 10 to 65 VDC, 20 to 320 VDC, and 20 to 265 VAC.

DIRECT CURRENT DEVICES

Direct current models are typically three-wire devices (two-wire also available) requiring a separate power supply. The sensor is connected between the positive and negative sides of the power supply. The load is connected between the sensor and one side of the power supply. The specific polarity of the connection depends on the sensor model. In the following example the load is connected between the negative side of the power supply and the sensor.

OUTPUT CONFIGURATION

Three-wire, DC proximity sensor can either be PNP (sourcing) or NPN (sinking). This refers to the type of transistor used in the output switching of the transistor. The following drawing illustrates the output stage of a PNP sensor. The load is connected between the output (A) and the negative side of the power supply (L-). A PNP transistor switches the load to the positive side of the power supply (L+). When the transistor switches on, a complete path of current flow exists from L- through the load to L+. This is also referred to as current sourcing since in this configuration conventional
current is (+ to -) sourced to the load. This terminology is often confusing to new users of sensors since electron current flow (- to +) is from the load into the sensor when the PNP transistor turns on.

The following drawing illustrates the output of an NPN sensor. The load is connected between the output (A) and the positive side of the power supply (L+). An NPN transistor switches the load to the negative side of the power supply (L-). This is also referred to as current sinking since the direction of conventional current is into the sensor when the transistor turns on. Again, the flow of electron current is in the opposite direction.

NORMALLY OPEN, NORMALLY CLOSED

Outputs are considered normally open (NO) or Normally Closed (NC) (NC) based on the condition of the transistor when a target is absent. If, for example, the PNP output is off when the target is absent then it is a normally open device. If the PNP output is on when the target is absent it is a normally closed device.

COMPLEMENTARY

Transistor devices can also be complementary (four-wire). A complementary output is defined as having both normally open and normally closed contacts in the same sensor.

SHIELDING
Proximity sensors contain coils that are wound in ferrite cores. They can be shielded or unshielded. Unshielded sensors usually have a greater sensing distance than shielded sensors.

SHIELDED PROXIMITY SENSORS

The ferrite core concentrates the radiated field in the direction of use. A shielded proximity sensor has a metal ring placed around the core to restrict the lateral radiation of the field. Shielded proximity sensors can be flush mounted in metal. A metal-free space is recommended above and around the
sensor’s sensing surface. Refer to the sensor catalog for this specification. If there is a metal surface opposite the proximity sensor it must be at least three times the rated sensing distance of the sensor from the sensing surface.

UNSHIELDED PROXIMITY SENSORS

An unshielded proximity sensor does not have a metal ring  around the core to restrict lateral radiation of the field. Unshielded sensors cannot be flush mounted in metal. There
must be an area around the sensing surface that is metal free. An area of at least three times the diameter of the sensing surface must be cleared around the sensing surface of the sensor. In addition, the sensor must be mounted so that the metal surface of the mounting area is at least two times the
sensing distance from the sensing face. If there is a metal surface opposite of the proximity sensor it must be at least three times the rated sensing distance of the sensor from the sensing surface.

MOUNTING MULTIPLE SENSORS

Care must be taken when using multiple sensors. When two or more sensors are mounted adjacent to or opposite one another, interference or cross-talk can occur producing false outputs. The
following guidelines can generally be used to minimize interference.
• Opposite shielded sensors should be separated by at least four times the rated sensing range
• Opposite unshielded sensors should be separated by at least six times the rated sensing range
• Adjacent shielded sensors should be separated by at least two times the diameter of the sensor face
• Adjacent unshielded sensors should be separated by at least three times the diameter of the sensor face.

STANDARD TARGET

A standard target is defined as having a flat, smooth surface, made of mild steel that is 1 mm (0.04”) thick. Steel is available in various grades. Mild steel is composed of a higher content of
iron and carbon. The standard target used with shielded sensors has sides equal to the diameter of the sensing face. The standard target used with unshielded sensors has sides equal to the diameter of the sensing face or three times the rated operating range,whichever is greater. If the target is larger than the standard target, the sensing range does not change. However, if the target is smaller or irregular
shaped the sensing distance (Sn) decreases. The smaller the area of the target the closer it must be to the sensing face to be detected.

TARGETING SIZE CORRECTION FACTOR

A correction factor can be applied when targets are smaller than Correction Factor the standard target. To determine the sensing distance for a target that is smaller than the standard target (Snew), multiply
the rated sensing distance (Srated) times the correction factor (T). If, for example, a shielded sensor has a rated sensing distance of 1 mm and the target is half the size of the standard target, the new sensing distance is 0.83 mm (1 mm x 0.83).
Snew = Srated x T
Snew = 1 mm x 0.83
Snew = 0.83 mm

TARGET THICKNESS

Thickness of the target is another factor that should be considered. The sensing distance is constant for the standard target. However, for nonferrous targets such as brass, aluminum, and copper a phenomenon known as “skin effect” occurs. Sensing distance decreases as the target thickness
increases. If the target is other than the standard target a correction factor must be applied for the thickness of the target.

TARGET MATERIAL

The target material also has an effect on the sensing distance. When the material is other than mild steel correction factors need to be applied.

RATED OPERATING DISTANCE

The rated sensing distance (Sn) is a theoretical value which does not take into account such things as manufacturing tolerances, operating temperature, and supply voltage. In some applications the sensor may recognize a target that is outside of the rated sensing distance. In other applications the target may not be recognized until it is closer than the rated sensing distance. Several other terms must be considered when evaluating an application.
The effective operating distance (Sr) is measured at nominal supply voltage at an ambient temperature of 23°C ± 0.5°. It takes into account manufacturing tolerances. The effective
operating distance is ±10% of the rated operating distance. This means the target will be sensed between 0 and 90% of the rated sensing distance. Depending on the device, however, the
effective sensing distance can be as far out as 110% of the rated sensing distance.
The useful switching distance (Su) is the switching distance measured under specified temperature and voltage conditions. The useful switching distance is ±10% of the effective
operating distance. The guaranteed operating distance (Sa) is any switching distance for which an operation of the proximity switch within specific permissible operating conditions is guaranteed. The
guaranteed operating distance is between 0 and 81% of the rated operating distance.

RESPONSE CHARACTEERISTICS

Proximity switches respond to an object only when it is in a defined area in front of the switch’s sensing face. The point at which the proximity switch recognizes an incoming target is the
operating point. The point at which an outgoing target causes the device to switch back to its normal state is called the release point. The area between these two points is called the hysteresis zone.

SENSORS, LIMIT SWITCHES

SENSORS, LIMIT SWITCHES

John   02:52:00

One type of feedback frequently needed by industrial-control systems is the position of one or more components of the operation being controlled. Sensors are devices used to provide information on the presence or absence of an object.

sensors include limit switches, photoelectric , inductive, capacitive, and ultrasonic sensors. These products are packaged in various configurations to meet virtually any requirement found in commercial and industrial applications.

Limit switches use a mechanical actuator input, requiring the sensor to change its output when an object is physically touching the switch. Sensors, such as photoelectric, inductive,
capacitive, and ultrasonic, change their output when an object is present, but not touching the sensor.
In addition to the advantages and disadvantages of each of these sensor types, different sensor technologies are better suited for certain applications. The following table lists the sensor technologies that will be discussed in this course.

CONTACT ARRANGEMENT

Contacts are available in several configurations. They may be normally open (NO), normally closed (NC), or a combination of normally open and normally closed contacts. Circuit symbols are used to indicate an open or closed path of current flow. Contacts are shown as normally open (NO) or
normally closed (NC). The standard method of showing a contact is by indicating the circuit condition it produces when the contact actuating device is in the deenergized or
nonoperated state. For the purpose of explanation in this text a contact or device shown in a state opposite of its normal state will be highlighted. Highlighted symbols used to indicate the opposite state of a contact or device are not legitimate symbols. They are used here for illustrative purposes only.

 Mechanical limit switches, which will be covered in the next section, use a different set of symbols. Highlighted symbols are used for illustrative purposes only.

LIMIT SWITCHES

A typical limit switch consists of a switch body and an operating head. The switch body includes electrical contacts to energize and deenergize a circuit. The operating head incorporates some
type of lever arm or plunger, referred to as an actuator. The standard limit switch is a mechanical device that uses physical contact to detect the presence of an object (target).
When the target comes in contact with the actuator, the actuator is rotated from its normal position to the operating position. This mechanical operation activates contacts within the switch body.

PRINCIPLE OF OPERATION

A number of terms must be understood to understand how a mechanical limit switch operates.
The free position is the position of the actuator when no external force is applied.
Pretravel is the distance or angle traveled in moving the actuator from the free position to the operating position. The operating position is where contacts in the limit switch
change from their normal state (NO or NC) to their operated state. Overtravel is the distance the actuator can travel safely beyond the operating point.
Differential travel is the distance traveled between the operating position and the release position.
The release position is where the contacts change from their operated state to their normal state.
Release travel is the distance traveled from the release position to the free position.

MOMENTARY OPERATION

One type of actuator operation is momentary. When the target comes in contact with the actuator, it rotates the actuator from the free position, through the pretravel area, to the operating position. At this point the electrical contacts in the switch body change state. A spring returns the actuator lever and electrical contacts to their free position when the actuator is no longer in contact with the target.

MAINTAINED OPERATION

In many applications it is desirable to have the actuator lever and electrical contacts remain in their operated state after the actuator is no longer in contact with the target. This is referred to as maintained operation. With maintained operation the actuator lever and contacts return to their free position when a force is applied to the actuator in the opposite direction. A forkstyle actuator is typically used for this application.

ACTUATOR

Several types of actuators are available for limit switches, some of which are shown below. There are also variations of actuator types. Actuators shown here are to provide you with a basic knowledge of various types available. The type of actuator selected depends on the application.

ROLLERS
The standard roller is used for most rotary lever applications. It is available in various lengths. When the length of the roller lever is unknown, adjustable length levers are available.

FORK

The fork style actuator must be physically reset after each operation and is ideally suited for transverse movement control.

MOUNTING CONSIDERATIONS

Limit switches should be mounted in locations which will prevent false operations by normal movements of machine components and machine operators. An important aspect of limit switch mounting is cam design. Improper cam design can lead to premature switch failure.
For lever arm actuators it is always desirable to have the cam force perpendicular to the lever arm. For applications in which the cam is traveling at speeds less than 100 feet per minute a cam lever angle of 30 degrees is recommended.

OVERRIDING AND NON OVERRIDING CAM

In overriding cam applications it is necessary to angle the trailing edge of the cam in order to prevent the lever arm from snapping back. Snapping back of the lever arm can cause shock loads on the switch which will reduce the life of the switch.
Non-Overriding cams are cams which will not overtravel the actuating mechanism.