【SPECIAL EDITION】 In-depth Explanation of Positioning Touch Switch Selection and Terminology

What is a Positioning Touch Switch?

【 Demonstration Video 】Application Examples of Positioning Touch Switches

First, please watch the video (3 min. 20 sec.) of an application example of positioning touch switches.
The video introduces a "workpiece transfer robot" incorporating a positioning touch switch.

The “Workpiece Transfer Robot” uses switches to perform the following operations:

・Clamping the workpiece and confirming that it is seated.
・Positioning of jig
・Checking the position of the table

Specifications of Touch Switches

Operating Forms (NO and NC)

There are two types of signal formats output from the contact:
Normally open (NO) 　and 　Normally closed (NC)

Bearings (metal and ball bearings)

There are two types of bearings: Metal and Ball bearings.
The type of bearing you may select depends on the way the contact-point is applied to the workpiece.

What is a Contact (Ball and Flat type)

The "contact" is the part at the tip of the switch that touches the sensing workpiece.
It is also called a "contact probe".
When the contact-probes are pushed in and the internal contacts touch each other or separate, current is energized and a signal is output.

The contact shape can be roughly selected from "ball" and "flat".
Selecting the appropriate one for the shape of workpiece will stabilize the accuracy.

● Ball type:
Select "Ball type" contacts if the object has a flat shape.

● Flat type:
  When the object is spherical, convex or curved, please select "flat" type contacts.

What is Waterproof Construction (IP Protection Rating)?

“IP protection rating” refers to the level of dustproof and waterproof.

Many of METROL's sensors, including high-precision touch switches, have a high waterproof rating of IP67. Therefore, our sensors can be used in adverse environments with coolants and chips splashed on the machine tools.

[Glossary] Touch Switch Term 

	Term	Unit	Definition
Position	Free position	mm	The position of a contact when no force is applied to it
	Operating position	mm	The position where the signal switches when the contact-probe is pushed in
	Return position	mm	The position where the switched signal returns
	Operating limit position	mm	The position when the contact-probe is pushed all the way out
Operation	Stroke	mm	Overall movement
	Pre-travel(PT)	mm	Movement up to the point of operation
	Overtravel(OT)	mm	The amount of movement that can be pushed in after the signal is switched. Stroke minus pre-travel
Force	Contact force	N	Force required to switch the signal
	Return force	N	Force required to move the signal from the operating limit position to the return position (spring force)
Precision	Repeatability	mm	Difference in positional variation of the signal operating point
	Hysteresis	mm	The difference in distance from the point of sensor operation to the point of signal return
	Temperature drift	mm	The range of variation of the signal operating point due to changes in environmental temperature

[TERMINOLOGY] Structure of Positioning Touch Switches

Position at Which The Signal Switches

● Free position
 The position of the contact when nothing is touching the contact-probe.

●  Operating position
 The position where the signal switches when the contact is pushed in. 

● Operation Limit Position
 The position where the contact is fully pushed in and cannot be pushed in any further.  

●Return position 
 The point at which the switched signal returns, i.e., from the operating position to the free position.
For METROL's sensors, the operating position = return position, and there is no hysteresis.

Operation of Touch Switches 

●  Stroke
Stroke refers to the "overall movement".
It is the maximum amount (mm) that can be pushed into a contact, and is the distance traveled from the free position to the operation limit position.

●  Pre-travel
  Pre-travel is the amount by which the contact is pushed in before the signal is switched.
 This is the distance from the free position to the operating point, also known as slack.

 

Normally open (NO) type always has pre-travel.
Normally closed (NC) types are available with or without pre-travel.
 
 With pre-travel, the full pre-travel distance must be pushed in until the signal is switched.
 It prevents false reactions due to vibration, shock, or the use of heavy contacts.
 On the other hand, without pre-travel, the signal switches immediately when the contact is pushed in.
 Since the amount of push-in is minimal, the force load on the sensing object can be reduced.

● Overtravel 
This is the amount of further push-in from the point where the signal switches.
It is the distance from the stroke minus the pre-travel.
The larger the overtravel, the more likely it is that the sensing object will not break the sensor.

 Some sensors are equipped with an "overtravel signal".
An "overtravel signal" is a signal to prevent the contact from being pushed in too far.

Force that Operates the Touch Switch

●  Contact force
This is the force required to push the contact to the position where the signal is switched (unit: N).
The contact force increases according to the amount of contact push-in.
When using a heavy contact, please pay attention to the following points.

 ・When vertical mounting is used horizontally:
    The contact force is increased by the weight of the movable part.

・When a horizontal mounting is used vertically:
    The contact force is reduced by the weight of the movable part. Please be careful of return failure.

● Return Force 
  This is the force required to move the switch from the operation limit position to the return position.
  In the case of our positioning touch switches, no force needs to be applied because the switch returns by the force of the internal spring.

 [Terminology Explained] Precision of Touch Switches

Repeatability

 The difference in the variation of the signal operating point.
 The difference between the maximum and minimum values is indicated by a range (R) based on the variation of the operating position when the sensing workpiece is placed perpendicular to the contact and pressed 30 times consecutively. (Our definition)

 METROL's positioning touch switches have high repeatability and small signal operating point variation, enabling OK/NG recognition of objects with narrow tolerances.

Difference/Hysteresis

 The difference in distance between the operating position and the return position.
 METROL's contact-type sensors have no hysteresis.
In other words, the movement position and the return position are equal. The lack of hysteresis enables detection of fine motion of the workpiece.

On the other hand, electrical sensors (limit switches, microswitches, proximity sensors, optical sensors, etc.) are subject to differences. The inside of the hysteresis zone is a dead zone and cannot be detected, making it unsuitable for detecting minute movements of the workpiece.

 Displacement between the operating position and the return position may be caused by deflection of the bracket (the pillar that holds the sensor in place) due to contact force, in addition to the switch itself.
 For the type with large contact force, please pay attention to the rigidity of the bracket.

Temperature Drift

When the temperature of the sensor changes, the internal components expand and contract, causing a shift in the operating point.
METROL positioning touch switches themselves have no temperature drift.
Since it does not use electronic components, it has a simple structure with no heat source.

Sensors that use amplifiers will experience temperature drift.

Thank you for your attention to the above, an explanation of the basic structure of positioning touch switches and terminologies, which can be used when selecting sensors fit for your needs.

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