Solid Waste & Recycling

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Robotic sorting

The picks-per-minute illusion


Eric Camirand is CEO of Waste Robotics.

Recycling or waste sorting is about recognizing precisely recyclable objects and extracting them as rapidly as possible to put them in the right bin.

Human pickers are fantastic waste sorting machines. Their eyes see colour and have a stereoscopic vision system that can capture 24 images per second. Their brain can process all these images, learn to recognize any target object, calculate the trajectory of an object on a moving conveyor.

Furthermore the brain can develop a complex grappling strategy and command their two arms and 10 fingers to perform highly adapted and coordinated mechanical moves to optimize the extraction.

The problem with human pickers is that they are increasingly rare, expensive and inconsistent. They get tired, sick, distracted, intoxicated, bored, careless: you name it!

As an MRF operator, if they show up for work, you will have to pay them no matter the fluctuations in their daily sorting performance. So what’s the solution to all the world’s aches? Of course, A.I. ROBOTS!

Inexpensive, fast, precise, reliable and consistent waste sorting machines.

“As a MRF operator, that’s exactly what I need!” The remaining question is: “Can this robot really out perform my human pickers?” To answer this question we need to define sorting performance.

In current and future recycling market conditions, the quality of sorting is paramount – “Quality wins!” What would be the point of sorting at light speed if quality of sorting is poor and end-products are cross-contaminated with noone to buy them?

Identical HDPE & PP lids

In the pictures above, humans or A.I. would be hard pressed to tell, without investigating further, which product is made of which resin. Only proven technologies such as Near-Infrared (NIR) or Hyperspectral (HYS) scanning can precisely differentiate these products. In the current market conditions if “Quality wins”, the primary quality of a sorting robot should be precision of sorting, not speed.

By the way, what is the precision of a human picker? 70 percent? 95 percent? Your guess is as good as mine. It depends on the picker, the products being targeted, the time of day, the weather outside or if his team won yesterday…

Of course we need these robots to be fast, but at what speed does the robot outperform the human? 35 picks-per-minute? 80 picks-per-min? The faster the better right?

Wait a minute.

Human picks-per-minute vary greatly. You have guys that can use a scooping technique who can probably deliver up to 100 picks-per-minute. But, can they do this day-in day-out? No.

Somehow the industry seems to agree on a 40 picks-per-minute average. Is this a true average sustainable day-in day-out? Maybe. Nevertheless, let’s assume this as a basic metric to measure human sorting speed. By the way, we are talking about MRF light weight items, not C&D material.

So how many picks can your robot do? Well…in theory and according to the robot manufacturer’s specification sheet it can do 80 picks-per-minute. The work of two human pickers.

Really? No.

It is not that simple. What the robot manufacturer’s specification sheet is telling you is that it can accomplish up to 80 picks-per-minute on an optimal trajectory, well within the robot work area. On a random set of trajectories moving to the outer edge of the work area, as necessary in waste sorting, the performance will go down significantly. Sure these robots can move crazy fast but not every move is as fast as the next.

The picking sequence has a greater impact on the number of picks-per-minute performed by the robot. On a 150-feet-per-minute (FPM) conveyor the items will traverse the robot work area (4four feet) in 1.6 seconds. If several items show up at the same time it is unlikely they will all get picked up no matter the speed of the robot.

The important metric in picking efficiency is robot work area which converts on a moving conveyor to contact time with the material. Just like humans, the more hands you have on deck the more sorting you will perform. No matter the picks-per-minute claimed by the robot supplier no one robot can do it all.

Slower multi-robots working in cooperation, will deliver more Overall Equipment Effectiveness (OEE, www.oee.com) than the world fastest robot. Beware of multi-robots that do not really work in cooperation but that just divide the belt into two half belts therefore not  significantly increasing the work area or contact time. They surely increase the number of picks performed but they will never beat truly cooperating full-belt robots that can pass “missed picks” to the robots downstream.

Finally, grappling efficiency is also a very important metric to measure OEE. You can have the most sophisticated vision system, the fastest robot but if you can’t extract the product from the belt you are dead in the water. Keep in mind that in dirty environment, “suction sucks”.

An educated sorting robot buyer, will look beyond the picks-per-minute claims and try to truly measure OEE of the contemplated robotic sorting system by looking at recognition technologies, robot speed, work area and grappling efficiency of the proposed system.

In a follow up article we will go beyond Overall Equipment Effectiveness (OEE) and dive into the measure of Return on Investment (ROI) for a robotic sorting system. To be continued…