How do parcel centers sort packages?

1. Every night, trucks bring thousands of parcels to a parcel center. They tumble onto a belt in a jumble. A faster belt pulls them apart, so they travel in a single line with a gap between each one.
2. Every parcel has a label. People read the address. Machines read the barcode: black and white stripes that stand for a number. No other parcel has the same number.
3. The belt carries each parcel through a scan tunnel. Cameras look from the top and the sides, because the label could be facing any way. In a blink, they find the barcode and read its number.
4. A computer looks up the number in a list. The list says which town the parcel is going to, and which slide leads to that town's truck. Now the computer knows where to push this parcel off.
5. The parcel rides on. Just as it reaches its slide, a paddle swings out and nudges it off the belt. Down it slides into the cart for its town. The paddle swings back before the next parcel comes.
6. By morning, each cart holds parcels for one town. They are loaded onto that town's truck, and the trucks set off. A parcel that could not be read goes to a person, who sorts it by hand.

A sorting line from start to finish. Tap each part to see what it does to a parcel.

• Unloading belt. Parcels from the truck are put onto this belt. They arrive in a jumble, some side by side and some touching.
• Spacing belt. This short belt runs faster than the one before it. Each parcel speeds up as it rolls on, so it pulls ahead of the one behind. That makes a gap.
• Scan tunnel. Cameras all around the belt take pictures of each parcel as it passes. Software finds the barcode in the pictures and reads its number.
• Weigh and measure. A scale built into the belt weighs each parcel without stopping it. Light beams measure how long, wide and tall it is.
• Sorting computer. The computer looks up each parcel's number and chooses a slide. It knows how fast the belt moves, so it knows when the parcel will reach that slide.
• Diverter. A diverter moves a parcel off the main belt. This one is a paddle that swings out. Others use tilting trays or little wheels that pop up and steer the parcel sideways.
• Slide. A smooth slide, called a chute, lets the parcel glide down by gravity. Each chute leads to one town's cart.
• Roll cage. A cart with wire sides, called a roll cage, collects all the parcels for one town. When it is full, it is rolled onto that town's truck.

Watch the parcels land in their carts. Then make the gap smaller and smaller. What goes wrong, and why?

Experiment: What happens when the parcels ride very close together?

1. Start with a big gap. Check that every parcel lands in the cart with its own picture.
2. Slide the gap down until the parcels almost touch.
3. Watch the paddle after it pushes a parcel. What does it hit next?

What we found: With a good gap, each paddle pushes one parcel and swings back in time. When the parcels are too close, the paddle is still out when the next parcel arrives, so that one is pushed down the wrong slide.

Turn on JavaScript to try it yourself.

Each parcel must reach the truck with its picture. Tap the switches to set the path before the parcel gets there. Deliver five parcels.

A parcel center is a huge building where parcels change trucks. Trucks arrive in the evening from many places, and by morning every parcel must be on the right truck for the next part of its trip. People could never read that many labels in time, so machines do most of the sorting.

First, the parcels have to line up. They tumble out of the trucks in a jumble, so a faster belt pulls them apart into a single line with gaps between them. Then each parcel rides through a scan tunnel. Cameras look at it from all around, find the barcode on the label and read its number.

A computer looks that number up and learns where the parcel is going. Each town has its own slide, called a chute. The computer knows how fast the belt is moving, so it can work out exactly when the parcel will pass its chute. At that moment a paddle swings out and nudges the parcel off the belt, and it slides down into a cart.

The gaps matter. A paddle needs a moment to push and swing back. If the next parcel is too close, the paddle hits it as well and sends it to the wrong town. So a sorter is always balancing two wishes: pack the parcels close to sort more of them, but leave enough room for the paddles. The line on this page is a simplified model of a real one.

Key words

Barcode

A pattern of black and white stripes that stands for a number a machine can read.

Conveyor

A moving belt, or a row of rollers, that carries things along.

Diverter

The part of a sorter that moves a parcel off the main belt, such as a paddle, a tilting tray or pop-up wheels.

Chute

A smooth slide that lets parcels glide down to a cart or a bag.

Gap

The empty space between one parcel and the next on the belt.

Throughput

How many parcels a sorter handles in a given time, for example parcels per hour.

Check your thinking

1. How does the sorting computer find out where a parcel is going?

   Show the answer

   Cameras read the barcode, and the computer looks the number up. The barcode is a number. The computer keeps a list that matches numbers to towns and slides.

2. Why do parcels need a gap between them on a sorting belt?

   Show the answer

   So the paddle can swing back before the next parcel arrives. A paddle takes a moment to push and return. A parcel that arrives too soon gets pushed the wrong way.

3. Why does a scan tunnel have cameras on several sides?

   Show the answer

   The label could be on the top or on any side of the parcel. Parcels land on the belt any way up, so the cameras must be able to see every side.

4. A parcel's label is torn and cannot be read. What should the sorter do?

   Show the answer

   Send it to a person, who reads the label and sorts it by hand. A guess could send the parcel to the wrong town. A person can read what the cameras could not.

5. Parcels are 0.6 meters long with a 0.9 meter gap, and the belt runs at 1.5 meters per second. About how many parcels pass in one hour?

   Show the answer

   About 3,600. Front to front, each parcel takes up 0.6 + 0.9 = 1.5 meters. At 1.5 meters per second, one parcel passes every second, and there are 3,600 seconds in an hour.