Applying mechanical limit switches
Mechanical limit switches obey the first rule of engineering: Keep it simple. That's why they're still the best control option in many applications.
Mechanical switches are slower than purely electronic sensors but they can switch high inductance loads, even to 10 A. They're also very precise in terms of accuracy and repeatability.
Push rollers (normally actuated by direct thrust) can also be actuated by the vertical component of a cam. Side forces in these systems can cause binding and should be minimized.
Product manager Henry Menke, of Balluff, Inc., Florence, Ky., adds, "Wherever possible, machine designers are moving to non-contact sensors to eliminate the effects of mechanical wear. However, mechanical switches offer a wider range of operating voltages in ac and dc. They also carry heavier load currents to directly-actuate solenoids, relays, contactors, incandescent lights, heating elements, motors, and other power-hungry loads."
Actuation is probably to blame if a limit switch doesn't indicate position or
control properly. Why? Limit switch failures are normally mechanical in nature.
Since they have no internal electronics, mechanical switches do not emit electromagnetic
noise — and are immune to such effects, even at extremely high levels.
Contrast this with solid-state devices: They require continuous external monitoring
to continuously verify their operational status. Knutowski explains, "With little
to no solid-state components, mechanical switches are not affected by electrical
noise and there is little mystery to the cause when they do happen to fail."
Overtravel control: It's crucial to limit overtravel's force on the most delicate part involved — switch contacts. One method is to use external controls to keep the overtravel itself within basic switch specifications (usually 0.006 to 0.008 in.). Another straightforward means of controlling overtravel is with a physical stop on the switch.
Actuator position: When designing an external actuator mechanism, the amount of time the switch stays in its free position (vs the actuated position) is influential. If possible, switches should be wired so they're in the free position longer. This makes for longer switch life. If an application requires the switch be held in the operated overtraveled position, a "helper leaf" can be used under the switch actuator.
The most common rotary switches include one or more rotating cams shaped to actuate associated switches over portions of the cam's rotation. Several switches can be arranged in parallel to actuate a sequence of control events or actions.
Actuating force: During basic switch operation, actuation force should always be applied perpendicular to the switch actuator. Another important value is differential — the distance between the operating and release points. It is usually limited by a maximum value.
Special cam considerations: Rotary limit switches should never act directly on switch actuators because this shortens switch life by distorting switches where direct interaction takes place. To prevent distortion, an auxiliary leaf can be used betweenthe cam and switch actuator.-This way, the leaf is subjected to side motion while the switch actuator is only subjected to vertical motion.
Two more tips: Cam rise for most applications should be around 45° and the dimension between high and low cam diameters should be the total distance from the free to the minimum overtravel position.
Safety danceOne of the most important mechanical switch applications is personnel safety. Manufacturing cells often feature human operators working alongside robots, either serving up raw materials and components, or working in concert with them. To prevent any possibility of operator injury, safety standards require that robot controls be networked with operatorsensing devices such as safety light curtains, gate switches, and floor mats. Mechanical safety limit switches on dynamic limiting devices increase work-cell productivity by allowing robots to continue production while guaranteeing operator safety should the robot controller software malfunction and send it into human-occupied areas. Balluff Inc. (Florence, Ky.) develops these mechanical limit-switch solutions with a number of manufacturers for use on robots. When equipped with positive-opening contacts, they are adaptable to control-reliable circuitry that is single-point-fault tolerant per ANSI requirements. Henry Menke of Balluff Inc. explains, "Positive-opening designs guarantee electrical operation, even in the event that electrical overload causes internal contact welding." Multiple-position limit switches, multi-channel cam drums, and complementary cam sets allow diverse redundant operation of up to three independent zones. Diverse complementary operation means that for 360° of rotation, two complementary angle cams — for example, 90° and 270° — operate two independent mechanical switches in a diverse and redundant manner. Programmable unitsThe addition of a microprocessor can turn an ordinary limit switch into a programmable limit switch, opening the door to many new sensing and control features. Many programmable limit switches employ noncontact position-sensing transducers for position feedback. But as Knutowski explains, "Many new sensor technologies (optical, inductive, capacitive, and sonar, for example) are functionally dependent on the applications in which they're installed. Too, the roughness, color, angle, density, and material composition of the target can all affect the performance of these technologies — making their application difficult when they must work consistently over a required time period. On the other hand, heavyduty mechanical limit switches are much more easily applied." They can interface with many control-logic types including sinking, sourcing, and even TTL. Whether mechanical or purely electrical, programmable switches usually work in tandem with a controller that compares sensed position to programmed setpoints — and determines if outputs should be on or off. This signal can be used to control anything from simple relays and logic devices to high-current solenoids and PLCs. Gil Guajardo, product manager of safety products at Omron Electronics, LLC (Schaumburg, Ill.) explains, "When designers want to use safety switches with an existing relay, mechanical limit switches are most suitable, since non-contact types often require their own special safety relay." |
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