photoelectric diffuse sensor Archives

Standard sensors and equipment won’t survive for very long in automated welding environments where high temperatures, flying sparks and weld spatter can quickly damage them. Here are some questions to consider when choosing the sensors that best fit such harsh conditions:

- How close do you need to be to the part?
- Can you use a photoelectric sensor from a distance?
- What kind of heat are the sensors going to see?
- Will the sensors be subject to weld large weld fields?
- Will the sensors be subject to weld spatter?
- Will the sensor interfere with the welding process?

Some solutions include using:

- A PTFE weld spatter resistant and weld field immune sensor
- A high-temperature sensor
- A photoelectric diffuse sensor with a glass face for better resistance to weld spatter, while staying as far away as possible from the MIG welding application

Problem, solution

A recent customer was going through two sensors out of four every six hours. These sensors were subject to a lot of heat as they were part of the tooling that was holding the part being welded. So basically, it became a heat sink.

The best solution to this was to add water jackets to the tooling to help cool the area that was being welded. This is typically done in high-temperature welding applications or short cycle times that generate a lot of heat.

- Solution 1 was to use a 160 Deg C temp sensor to see if the life span would last much longer.
- Solution 2 was to use a plunger prob mount to get more distance from the weld area.

Using both solutions was the best solution. This increased the life to one week of running before it was necessary to replace the sensor. Still better than two every 6 hours.

Taking the above factors into consideration can make for a happy weld cell if time and care are put into the design of the system. It’s not always easy to get the right solution as some parts are so small or must be placed in tight areas. That’s why there are so many choices.

Following these guidelines will help significantly.

Some photoelectric applications require not only knowing if the object is present or not but exactly where the object is while providing a continuous or dynamic value representative of the objects location. For instance, if a robot is stacking a product is the stack at the correct height or how many additional pieces can be placed on the stack, how large is the coil or roll diameter of a product, and how high is the level or how much further can the product move before it is in position. Distance sensors can provide this dynamic information and in some case provide a digital output as well for alarms.

These sensors are normally based on diffuse sensing technology. However, in some cases retro-reflective technology is used for extremely long sensing distances. As with diffuse sensors there is only one device to mount and wire. However, due to the technology required for the higher resolutions, lensing, electronics and outputs these devices are typically much more expensive than a discrete diffuse sensor.

Similar to a diffuse sensor the distance sensor emits a pulsed light that strikes an object and a certain amount of light is reflected back to the sensor’s receiver. The sensor then generates an analog output signal that is proportional to the distance to the target. The technology that is utilized within the sensor to determine the distance is either Time of Flight or Triangulation.

Time of Flight sensors are more immune to target color and texture than light intensity based system because of the time component. These devices measure greater distances than the triangulation method however there is a sacrifice in resolution.

Triangulation sensors emit a pulsed light towards the target object. The light is then reflected back to the receiver. When the light reaches the sensor it will strike the photosensing diode at some angle. The distance between the sensor and the target determines the angle in which the light strikes the receiver. The closer the target is the sensor the greater the angle.
Triangulation based sensors being dependent on the amount of reflected light are more susceptible to target characteristics such as color and texture. These sensors are characterized by short to mid-range sensing distance however they provide higher resolutions than TOF sensors.

Output signals are either 0…10 volts, 1…10 volts or 4…20mA each of which has their pros and cons. Voltage outputs, 0 – 10 or 1- 10 volts, are easier to test and there is typically a broader offering of interface devices. However voltage outputs are more susceptible to noise from motors, solenoids or other coils and voltage drops of the wire. In addition generally voltage output cable runs should be less than 50 feet. Also since 0 volts is an acceptable output value broken wires, device failures, or power failures can go undetected.

Current outputs, 4 – 20 mA, provide the best noise immunity, are not affected by voltage drop and the cables lengths can exceed 50 feet. Since the sensor will be providing 4mA at zero distance its lowest possible signal, if the sensor should fail, the cable damaged or a power failure the interface device can detect the absence of the signal and notify an operator. Current outputs are more difficult to test and in some cases are affected by temperature variations.

For more information about photoelectric sensors, request your copy of Balluff’s Photoelectric Handbook.