Ocado is the world’s largest online-only grocery retailer, shipping more than two million items every day to customers around the UK. Having an efficient warehousing system is obviously vital to the success of the business, so anything that can be done to improve that is worth investigating.
Dr David Sharp, Head of Ocado Technology 10X, Ocado’s advanced research unit, is tasked with helping to make those improvements by identifying future technology that might impact the business – be that machine intelligence, Internet of Things applications or smart kitchens.
Ocado has two massive warehouses in Dordon, North Warwickshire and Hatfield, Hertfordshire. Between the two, there are 25km of conveyor belts carrying crates full of produce from which pickers extract groceries to fulfil customer orders. Ocado has established what is believed to be the fastest way of shopping in the world.
Typically, people buy 50 items and spend about £110 per order. Ocado can pick an item every six seconds and complete an entire average order in five minutes. It is tasked with delivering each order within one hour and has a 93% success rate.
However, the conveyor belts take up a lot of space, so Ocado wanted to create a more compact warehouse system. To do this it is developing two new warehouses; one in Andover, Hampshire, which is reasonably small; and a very big one in Erith, near Dartford in Kent. Andover will go live this year and Erith at the end of 2017.
‘We can reduce space by stacking crates up to 21 crates high,’ says Sharp. ‘But then we have to find a way to enable people to pick the groceries out of them. So we have developed a platform grid with autonomous robots storing and retrieving crates around the grid at a speed of 9mph. Each one is as heavy as two people, so we need a good structural design to support them.’
Command & Control
The problem was that the robots needed to move quickly around the grid to ensure orders are fulfilled at the desired speed. Sharp says: ‘We have 1,000 robots and each one must be able to talk to a controller and receive messages back 10 times a second, so the command and control information must be very reliable.
‘But we are also operating in a warehouse full of metal, so that is a very harsh radio environment. We needed a way to control them and prevent them from banging into each other. So we looked for a wireless technology that could handle the requirements.’
Ocado turned to Cambridge Consultants (CC) to find a suitable point to multipoint wireless technology solution. CC created a system based on 4G technology, but operating in the unlicensed 5GHz band. The solution can support 1,000 mobile machine control modules (MCM) – or robots – from a single base station capable of handling over 10 times the density that is usually possible.
It provides a guaranteed connection 10 times a second to each of the 1,000 machines per access point – and all within a 50m radius. It can potentially handle 20 times the number of movements.
‘One simplification is if we have a base station on one side of the warehouse, the radio signals from the robots nearer the base station will sound much louder than the ones further away – it’s the classic near/far problem,’ explains Sharp.
‘So the cleverness in CC’s solution is how it simplifies 4G to fit this application and overcome our particular problems. In addition, 4G is generally licensed cellular spectrum that doesn’t normally work in 5GHz unlicensed spectrum, so CC has had to make it work in that frequency band,’ he points out.
The core wireless system comprises the fixed base station and the robots or MCMs. Both base stations and robots connect to an external controller via Ethernet. The base stations connect into a site-wide Ethernet network. The robots connect to an on-board industrial PC on the mobile machine. Between these two IP ports, the base stations and robots provide a transparent communications pipe.
The radio system is arranged as a number of fixed base stations, each of which is operating in a star formation connecting to a large number of robots. The robots can be moving at up to 4ms-1 and can have a maximum range of up to 150m.
The system can incorporate multiple base stations, which can be used in the same vicinity, and these use a base station controller (BSC) to co-ordinate the operations. The BSC manages system configuration and reporting data between the site network and the base stations.
The system controller originates control commands via the base station to the robots and accepts position and reporting/status responses from each robot also via the base station. The hardware system is combined with Ocado’s end-to-end eCommerce, fulfilment and logistics software platform running in the cloud to form what is now referred to as the Ocado Smart Platform (OSP).
Tim Ensor, commercial director, Cambridge Consultants, explains that the key issue was to come up with a radio control system to manage the robots moving around the warehouse at high speed. The system had to co-ordinate thousands of these fast-moving machines to within a fraction of a second to maximise warehouse efficiency.
‘The place we started with was posing the problem of how could we provide a system that is scalable and can talk to 1,000 robots 10 times a second. We looked at lasers, cameras and Wi-Fi, but we quickly decided broadband mobile was the place to start – and our team has a lot of expertise in this space.
‘Our solution is more like halfway to 5G where the low latency aspect is one of the big challenges. We are dealing with a very large numbers of devices with a very low latency and a very high reliability requirement,’ explains Ensor.
‘The idea was to take the building blocks freely available in the market for 4G and 4G-enabled devices, break that down and build something that would meet Ocado’s requirements. We wrote our own baseband layer to divide up the time and frequency slots to be able to service all the robots at the same time.
‘We then built the access layer looking at how to co-ordinate the base stations in the warehouse and how to manage all the simultaneous transmissions. It was a case of aligning robots to frequency channels, to base station planning and management, and being able to move frequencies without interrupting transmissions, especially if interference issues are being experienced,’ explains Ensor.
‘We had to build the software stack from the baseband up to the control layer to handle what we believe is the most densely packed number of mobile terminals per square foot in the world,’ he adds.
Licence-free spectrum issues
Once all this had been worked out, CC had to move the finished solution into licence-free spectrum operating in a harsh radio environment. The equipment has a 150m range requirement (although it will do more than that).
‘The robots know the time to one micro second and the internal time is synchronised to less than 1ms,’ says Ensor. ‘That helps us co-ordinate all the robots, so that there are lots of deliberate near misses and no hits. The synchronisation and timing of transmissions must therefore be very accurate, very fast and very tightly controlled.’
Ensor adds that the system is able to simultaneously support narrowband (3kbps to every robot) and wideband modes (394kbps to between one and four selected robots) to enable real-time control. The wideband channels can be used for diagnostics (as well as uploading the long-term performance data stored on each robot), so if one robot goes wrong there is software to ensure the others keep away from it.
‘We have rescue robots to go in and pick up the faulty robot. It is like an air traffic control issue. This compares with the old conveyor belt system where if something goes wrong the conveyor belt stops and everything slows down,’ he says.
Sharp adds that operating in unlicensed spectrum does add a further complication. ‘One of the rules of the game in unlicensed 5GHz spectrum is that the system has to monitor for other users in the band. If it detects an aircraft radar or other interfering signals you have to be able to automatically switch spectrum channels without any service interruption or else all the robots would slam on the breaks.’
Ocado is the intellectual property holder of the solution and has filed a number of patents for the technology. Sharp says: ‘We are marketing the solution as the Ocado Smart Platform (OSP). We are also looking to generate revenue from the solution by making it available to other retailers around the world to run their own on-line offerings by providing a very attractive commercial model.
‘They can build their grocery sorting grid, but only buy a few robots. Then, as the business grows, they can buy more robots to meet rising demand, but the fees they pay Ocado are proportionate to the size of their business.’ As the system operates in licence-free spectrum it can be quickly deployed anywhere in the world, which should accelerate take-up.
Sharp adds: ‘In time we expect it to have applications beyond the grocery sector. It could also be used to control semi-autonomous vehicles in factories, construction sites, mining, military and airports, for example. What we have here is a wireless story and a wireless-enabled business transformation story.’
Key parameters of the Ocado Smart Platform
• Operating range between BS and MCM is up to 150m
• Maximum number of connections of MCMs per BS is 1,000 with a maximum latency of 100ms
• The system is able to simultaneously support narrowband and wideband modes to communicate with each mobile machine
• Narrowband channel provides 394kbps to every mobile machine
• Wideband channel provides 394kbps to between 1 and 4 selected machines
• Aggregate data rate per BS is ~9Mbps bidirectional which can be flexibly allocated to support a number of machines and channel widths
• Up to 24 BSs may be operating simultaneously, depending on the size of the site
• The system works in the frequency band 5470MHz to 5275MHz licence exempt
• 10MGHz nominal channel bandwidth
• 4ms-1 maximum speed of mobile machines
• For robust operation the system uses a dual channel receiver to implement maximum likelihood equalisation in both uplink and downlink
• The MCM works in conjunction with a PC controller board which runs part of the upper layer of the radio stack. This PC board connects directly to the MCM via Ethernet. The PC board, in turn, connects to the machineries that are under control
• The system uses time division duplex (TDD) frame to split uplink and downlink connectivity. The uplink sub-frame is 10ms and the downlink sub-frame is also 10ms, making a complete frame 20ms. This frame rate can be adjusted for different up/downlink ratios
• The system uses frequency division multiple access (FDMA) to divide the RF channel into smaller blocks and time division multiple access (TDMA) to further split the time into time slots. Each MCM is then allocated two frequency blocks and a time slot as the physical resource used to support the wireless communication
• The BS transmits a broadcast channel at the start of every frame. This broadcast channel is received by every MCM and is used for system control messages
• The narrowband mode uses an instantaneous 450kHz of bandwidth which frequency hops over the entire 9MHz occupied bandwidth with one hop every 100ms with a transmit duty cycle of 0.5%. The narrowband mode carries command and control data for an individual MCM
• The wideband mode uses the entire occupied bandwidth of 9MHz at 10% transmit duty cycle. This mode is used to allow a faster data transfer from an MCM for uploading long-term performance data stored on the MCM. Only a few MCMs (maximum of 4) can be allocated the wideband mode at any one time as each wideband connection is equivalent to 400 narrowband connections.
• In narrowband mode up to 20 devices may be simultaneously in communication in a total occupied bandwidth of 20 x 450 kHz = 9MHz
• The downlink modulation method is OFDM and the uplink modulation method is also OFDMA
• To comply with regulation, the system monitors for other users in the band. If radar or other interfering signals are detected, the system will automatically change channel.
• The BS supports synchronous Ethernet – the local clock generators can be locked to the received bit rate of either of the Ethernet ports
• The BS station supports IEEE 1588v2 precision time protocol (PTP). The local clock generators use PTP messages received over either of the Ethernet ports to allow multiple BSs to operate with locked air-interface timing.
• Operating temperature of BS and MCM is 0oC to +45oC.
Source: Cambridge Consultants