Sensors are our world. Imagine having to determine if a part is in place using what was available in the 1980s—mechanical switches. But the 1980s also brought us electronic sensing with photocells and proximity switches. This wasn’t new technology, but we didn’t have an interface system to use them with like a programmable logic controller (PLC).
Sensing encompasses the world. Any physical attribute can be measured and sensed. High-speed processes can be implemented because of the devices that are used to sense.
I have spoken before about my first application of a lumber sorter, and it bears to speak of it again.
I needed to monitor the speed of a chain carrying various lengths of lumber to be cut to length. There was a large drive sprocket that controlled the chain, and it was a variable speed motor, although I can’t remember how.
I used a 120 Vac proximity switch to detect the large teeth on the gear that mechanically drove the chain. If we assume it had 24 teeth, then the leading and falling edge would happen 48 times. At a certain lugs/minute setpoint, the saws that cut the boards to length would need to operate—come down, cut and then retract—at a certain interval based on speed.
The system operated perfectly at 50 lugs/minute, but at 51 it lost its way. Target speed was 72 lugs/minute. What was the problem?
The ac proximity sensing device was the problem. In reading the spec for the device, the transition time between part present and part absent (leading/trailing edge) was 25 milliseconds. This is the part when I’m sure you are smirking, but remember it was the 1980s. A dc-powered sensor had a transition time of less than 2 milliseconds—10 times faster. Problem solved.
Sensing is taken for granted in lots of applications. Reaction time is important in all non-contact applications. Mechanical switches are typically not used in anything high speed and more for end-of-travel indication and safety.
Nothing is more consequential than a missed signal in a safety system. Whether the sensors are mechanical or non-contact, they can’t fail. It is very difficult to detect that an arm has fallen off a limit device, so this would not be a good choice for a safety application. A safety rated non-contact device which is securely positioned on/in the process would be the better choice.
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So, the application should determine the right presence sensing device. In our modern world, anything can be sensed. The advent of Ethernet and the PLC/PAC processors and associated I/O suggests that we have options to sense a part.
One application I had worked on a few years ago was a bag stuffing, or poucher, machine. The machine was supplied by a third party, and I had to integrate it into the system. It was for a DNA test kit, so the swab had to be placed in a waiting pouch, and then the pouch was sealed when the swab had been detected inside the pouch.
How is that accomplished with 1980s technology? It was nothing short of a miracle.
In the modern day, vision systems communicate with the PLC over Ethernet using a common protocol called Ethernet/IP.
The level of intelligence that this vision system had was astounding. Once we trained the vision controller to see what the part looked like, it searched the pouch on demand for the outline. The orientation of the swab was immaterial—the vision system found it and gave an all-clear signal to the control system that the pouch can be sealed and released.
Throughput was a major consideration since DNA test kits were a hot item, and, to service all of the order requests, the throughput approached 60 swabs/minute.
There was no other way to accommodate this requirement other than a high-speed vision system.
Proximity, photocell, light curtains and vision can all be used for part sensing. The PLC systems allow for high-speed sensors to be employed for high-speed systems. Relay panels cannot play in the same sandbox.
In every application, everything is based on time and position. The devices you choose for the application have to use its core benefit to reliably operate within the parameters of design and operational specifications.
We are in a global market with access to everything. Spare parts and inventory are a consideration for vendor selection.
