Presence sensing is the process of using a touch or non-contact sensing device to determine whether an object is present or absent. An electrical output signal that can be utilized to control machinery or procedures is then generated by the sensors.
Mechanical limit switches are contact sensing devices that are frequently employed in industrial applications to detect the presence or position of items. The device’s operation serves as the basis for the term limit switch. The actuator of the switch is eventually moved to the “limit” where the electrical contacts change state as an object (or target) makes contact with the switch operator.
Electrical contacts are opened (in a typically closed circuit) or closed through this mechanical movement (in a normally open circuit). photoelectric, capacitive, and inductive proximity
Know The basics of LIMIT SWTICH
Advantages and Features of Limit Switches
Very accurate in terms of precision and repeatability.
It is practically universally applicable to industrial settings and is highly accurate and repeatable.
Use a minimal amount of electricity
Is able to switch loads with a high inductance.
Can manage numerous loads
Disadvantages of Limit Switches
· Generally restricted to equipment operating at relatively low speeds. · Must make direct contact with target. · Moving mechanical parts will wear out.
Limit Switch Terminology
Pretravel — the distance or angle that the actuator must go through to trip the contacts
Operating Point — position of the actuator at which the contacts snap to the operated position
Release Point — the position of the actuator at which the contacts return to their original state
Differential — distance (degrees) between contacts trip and contacts reset
Over travel — movement of the actuator beyond the contacts trip point
Initial Position — position of actuator when no external force is ap-plied to the actuator
Some other important terms associated with limit switches: Operating force (torque) — force required to move the actuating element
Minimum return force (torque) — minimum force required to return actuator to its initial position
Total Travel — the maximum allowable distance the actuating ele-ment can travel
Repeat Accuracy — ability of a switch to repeat its characteristics precisely from one operation to the next operation
Mounting Considerations of Limit Switches
Limit switches should be installed in areas where regular operator or machine component motions won’t cause false operations.
By conduit connections, liquids can occasionally enter limit switches’ interiors through condensation or seepage into conduit junctions. Switches shouldn’t be placed at the low point of lengthy conduit runs and any connection points that are exposed to fluids need to be tightly sealed.
Limit switches ought to be installed in areas where, during typical operation, machining chips shouldn’t collect.
Limit switches should be firmly mounted in accessible locations with enough clearance for straightforward maintenance and replacement as needed. Face the maintenance access point when installing cover plates.
Limit switches should be positioned facing down if there is a chance of liquid infiltration. Gravity will aid in preventing liquids from accessing functioning head seals. In the event that such mounting is ineffective, the user should look into submersible duty limit switches.
Electrical Ratings of Limit Switches
Electrical ratings are generally described in two ways. 1. Inductive 2. Resistive
Inductive Rating — indicates the inductive load from devices such as starters, contactors, solenoid and relay coils that the contacts can make or break. Inductive ratings typically have three components:
Make — load that the switch can handle when contacts close. This is associated with inrush currents and is a short duration rating, typically two cycles.
Break — load that the switch can handle when the contacts open. In most cases this should be considered the maximum continuous switch current.
Continuous — load that the switch can handle without making or breaking the load.
Resistive Rating — Resistive loads only that the contacts can make or break. For AC control circuits inductive ratings are less than the continuous or resistive ratings. When the contacts break an inductive circuit, the current in the circuit has a tendency to continue in the same direction due to the inductance of the load.
An arc will then occur across the contacts which will tend to heat and burn them. Therefore in order to compensate for the extra heat and wear, the maximum allowable inductive current is always less than the resistive current to achieve equal contact life.
For AC inductive loads the make current is often 10 times the break current. This is due to the fact that inductive coil loads often have inrush currents that are ten times the sealed (break) current.
Mechanical limit switches offer high repeatability, precision, and
True
False
Answer: a. True
One weakness of limit switches is that they must make physical/mechanical contact with an object to be actuated.
True
False
Answer: a. True
Limit switches are a large group of products that can be implemented in many different ways to provide a sensing solution for a variety of applications.
