Field Devices

Sensors

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.

S iemens S ensors                             Siemens 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. Each type of sensor will be discussed in detail. At the end of the course an application guide is provided to help determine the right sensor for a given application.

Technologies 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.

Sensor	Advantages	Disadvantages	Applications
L imit S w itch	•High Current   Capability •Low Cost •Familiar "Low-   Tech" Sensing	•Requires Physical   Contact with   Target •Very Slow   Response •Contact Bounce	•Interlocking •Basic End-of  Travel Sensing
Photoelectric	•Senses all Kinds of   Materials •Long Life •Longest Sensing   Range •Very Fast   Response Time	•Lens Subject to   Contamination •Sensing Range   Affected by Color   and Reflectivity   of Target	•Packaging •Material   Handling •Parts Detection
Inductive	•Resistant to Harsh   Environments •Very Predictable •Long Life •Easy to Install	•Distance   Limitations	•Industrial and   Machines •Machine Tool •Senses Metal  Only Targets
Capacitive	•Detects Through   Some Containers •Can Detect   Non-Metallic   Targets	•Very Sensitive to   Extreme   Environmental   Changes	•Level Sensing
Ultrasonic	•Senses all   Materials	•Resolution •Repeatability •Sensitive to   Temperature   Changes	•Anti-Collision •Doors •Web Brake •Level Control

Contact A rrangement 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.

         Circuit Example                                 In the following diagram a mechanical limit switch (LS1) has

been placed in series with a Run/Stop contact and the “M” contactor coil. The Run/Stop contact is in the Run condition and the motor is running a process. This could be a conveyor or some other device. Note that the “M” contacts and the “Run/ Stop” are shown highlighted, indicating they are normally open contacts in the closed position. LS1 is a normally closed contact of the mechanical limit switch.

When an object makes contact with the mechanical limit switch the LS1 contacts will change state. In this example the normally closed contacts of LS1 open. The mechanical limit switch symbol is highlighted. The “M” contactor coil is deenergized, returning the normally open contacts of the “M” contactor to their normal position, stopping the motor and the process.

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.

S nap-A ction Contacts There are two types of contacts, snap-action and slow-break. Snap-action contacts open or close by a snap action regardless of the actuator speed. When force is applied to the actuator in the direction of travel, pressure builds up in the snap spring. When the actuator reaches the operating position of travel, a set of moveable contacts accelerates from its normal position towards a set of fixed contacts.

As force is removed from the actuator it returns to its free position. When the actuator reaches the release position the spring mechanism accelerates the moveable contact back to its original state.

Since the opening or closing of the contacts is not dependent on the speed of the actuator, snap-action contacts are particularly suited for low actuator speed applications. Snapaction contacts are the most commonly used type of contact.

S low -Break Contacts Switches with slow-break contacts have moveable contacts that are located in a slide and move directly with the actuator. This ensures the moveable contacts are forced directly by the actuator. Slow-break contacts can either be break-before-make or make-before-break.

In slow-break switches with break-before-make contacts, the normally closed contact opens before the normally open contact closes. This allows the interruption of one function before continuation of another function in a control sequence.

In slow-break switches with make-before-break contacts, the normally open contact closes before the normally closed contact opens. This allows the initiation of one function before the interruption of another function.

                                                                                                                               

                                                                                                                                 

Contact State		Break-Before-Make		Make-Before-Break
		NO	NC	NO		NC
Free Position		Open	Closed	Open		Closed
Transition		Open	Open	Closed		Closed
Operated State		Closed	Open	Closed		Open
	Contact A rrangements				There are two basic contact configurations used in limit switches: single-pole, double-throw (SPDT) and double-pole, double-throw (DPDT). This terminology may be confusing if compared to similar terminology for other switch or relay contacts, so it is best just to remember the following points. The single-pole, double-throw contact arrangement consists of one normally open (NO) and one normally closed (NC) contact. The double-pole, double-throw (DPDT) contact arrangement consists of two normally open (NO) and two normally closed (NC) contacts. There are some differences in the symbology used in the North American and International style limit switches. These are illustrated below.