At present, the robot system which provides power for the warehouse distribution center is more attractive while the UAV is concerned. Tens of thousands of mobile robots on the ground have been put into operation and are delivering packages. These systems were originally built by Kiva systems in Boston. The author found an old version of Kiva robot, opened its bright orange shell, and demonstrated the excellent engineering design inside. This article shares Kiva's hard work.
Inside the distribution centers of large e-commerce companies like Amazon and staples, large numbers of people walk miles a day, picking up goods from shelves and putting them in boxes. The process is expensive, inefficient and error prone. Before Kiva, the most advanced and convenient tool was bicycle. After working for Webvan for some time, Kiva founder Mick Mountz realized that if the shelves can be moved to the front of people instead of people moving to the shelves, the picking efficiency will be greatly improved. In 2003, Mick, in collaboration with Pete Wurman and raffaelo D'andrea, founded Kiva systems, which fundamentally changed the way people and goods moved in warehouses.
The system developed by Kiva consists of five key parts:
(1) Mobile and very flexible customized Kiva shelves;
(2) 2D code grid laid on the floor;
(3) Intelligent packaging station with scale, laser and sensor;
(4) Orange robot that can quickly lift shelves and move them on the floor;
(5) The most important part: a complex and powerful software system that connects all the hardware together.
Although all of these are amazing and necessary for the normal operation of the system, this paper focuses on the orange robot running on the warehouse floor.
2、 Structure and mechanism
In principle, Kiva robots are very simple. It reads QR or DataMatrix QR codes laid on the floor every 40 inches and receives action commands from the cloud in its brain. After the robot reaches the shelf, it uses a clever lifting mechanism to rotate itself to lift a ball screw mechanism to lift the shelf from the ground. Robots look simple, but consider the following: shelves can weigh up to 1000 pounds, distribution centers have tens of thousands of shelves, hundreds of robots and dozens of packaging stations. A small collision or shelf fall can cause thousands of dollars in damage. This is not a toy robot.
Each side of the robot shell has a set of infrared sensors and a pneumatic obstacle avoidance device for detecting and avoiding collision. There is also a charging port (Kiva can park and charge automatically) and a series of LED status indicators. Another main part is to lift the shelf off the ground with X-shaped lifting mechanism.
The robot has three independent moving parts: two driving wheels and a motor for shelf lifting. When the lifting motor rotates, both driving wheels rotate in the opposite direction, so when the robot rotates, the lifting mechanism and the shelf appear to be stationary. This reduces the number of drive components and avoids complex hydraulic or shear lifting mechanisms.
At the top of the elevator is a large aluminum casting (Figure 1). At the same time, the main large structural parts of the robot are made of cast aluminum. All castings are made of 319 universal alloy. Each part undergoes a series of finishing after casting, such as finishing surface and threaded hole. Similar processes are often used in many high-quality aluminum parts such as automobile engine block, hydraulic actuator, etc.
Each infrared sensor has built-in filter logic and communicates through serial bus. From Figure 2, you can see the wireless control module as well as the hoist motor and the big gear. At the back of the robot, there are four lead-acid batteries and high current wires. Both the front and rear housings are molded by ABS vacuum injection molding (Fig. 3). The vacuum forming machine and CNC equipment used to make these parts are very large. This part is very complex and expensive.
The front and back of the robot are curved surfaces. It is very difficult to develop a full-length collision sensor on the whole curved surface. Kiva engineer proposed a simple and cost-effective solution. They use vinyl / rubber tubing to connect simple pressure sensors (Figure 4), and once a change in pressure is detected, the robot stops all movement. The black part in the right figure is used to receive pressure data, connect all infrared sensors, and then connect to the main controller as a whole.
The lifting mechanism adopts customized ball screw, which is the most complex and expensive component. The ball screw is externally connected with standard nylon pinion and drive motor (Figure 5). The motor is manufactured by Pittman (AMETEK) and has a torque of 27 in LBS and stall power of nearly 1kW. The right angle two-way reducer is manufactured by brother of Japan and can handle 407 in lbs of torque at a deceleration ratio of 25:1 at 72 rpm. The combined unit price of this motor and reducer is about $1000 (the batch may be lower). After removing the elevator and electronics, the robot can be turned over to better view the drive system (Figure 5, the main logic board is hidden under the steel plate panel). With the same two motors and gearboxes on the back, the custom cast Kiva wheels (Figure 6) may be designed to guarantee high durability on the concrete floor. The two ends are two pairs of double wheel 360 degree rotating casters. Such a drive system supports in situ rotation. The main body is composed of three cast aluminum parts (Figure 7), which are connected by simple U-clip pins, thus realizing simple and passive double suspension movement. They are all made of 319 aluminum alloy and machined after casting. As production increases, Kiva's engineers may turn to low-pressure aluminum molds with steel tools. Note the fins at the top of the lower right front casting (Figure 8): the large transistors of the motor controller lean against the back of these fins for maximum heat efficiency, which is a very delicate design.
3、 Electronic components
Even though Kiva has little decision-making power (its actions are controlled by servers in the cloud), the robot needs some very formal electronic equipment in the process of lifting and controlling thousands of pounds of goods. The system is powered by four 12V, 28ah lead-acid batteries in series (48V DC, see Figure 9). Two of the batteries are also fitted with custom thermal sensors under the brackets to ensure they do not overheat. When the power is low, the robot will return to the charging station offline for charging. The design of the charging port allows a large offset in the interface matching process.
One of the main components of Kiva system is the dual camera imaging module installed inside the lifting mechanism. A camera scans down the floor to identify the QR code on the warehouse floor. Another camera scans up the bottom of the shelf. Each camera is illuminated by six embedded red LEDs, all of which, like the rest of the robot, are custom-made (Figure 10).
The image processing board is between the two imagers, which is equipped with ADI adsp-bf548 multimedia processor. The processor carries out two-dimensional code detection and transmits the results through high-speed serial. The main logic board connects all the components together (Figure 11). The motor control circuit is powered by 48V battery, and the independent power filter rail supplies power for logic / communication / CPU. The driver of the three-phase BLDCM is completely customized and driven by the latticelfxp6c FPGA control chip hidden under the daughter board. Each of the three driver circuits has a current sensor, six full bridge high current FETs (cooled by the body structure), input of the motor rotary encoder, and a large 4-pin connector. The daughter board summarizes the system functions by coordinating the wireless module, imaging unit, emergency stop, infrared / pressure sensor, power management and motor driver. The MCU is a 32-bit Freescale mpc5123 with a 400MHz clock and may run PowerPC Linux (on a bare machine). The two Ethernet ports are connected to the wireless module and the firmware blink / external hardwire connection, but they are switched by mircel ksz8993. The only ready-made circuit on the whole robot is the communication module: soekris engineering net4526 router, which has a single winstron neveb cm9 wireless module with dual antenna configuration and is connected to the main logic board through Ethernet.
4、 Wonderful lifting parts
This generation of Kiva robots has many fascinating and well executed aspects, one of which is the lifting mechanism (Figure 12). This part of the organization undertakes the heavy lifting task of Kiva. It must be able to support a thousand pounds of load, and at the same time, it is required to be completely parallel to the ground when lifting, and the ball screw can perfectly meet such requirements. Most ball screws have a diameter of 1 inch or 2 inches, whereas the diameter used here is 11 inches (Note: 11 inches = 279mm), and they are hollow.
The inner and outer shell of the ball screw is made of aluminum alloy, and like the main parts of the chassis, it needs several auxiliary machining in the manufacturing process. Both parts are coated with Teflon impregnated anodized film. This treatment is rare, but it does well in providing lubricity and rust resistance. The inner ring ("ball nut") is equipped with an injection molding ball return mechanism, which can recover nylon / POM balls. During the actual rotation lifting process, the outer ring ("screw") is engaged with the inner ball bearing and the outer pinion.
Due to the complexity of the structure, it is difficult to accurately estimate the cost of the lifting mechanism. According to the requirements of the part size, post-treatment, coating, durability and special assembly process, the author estimates that the price is about $1000.
It is true that the content of this article is a small part of the overall solution, but it is a very important part.
In the field of hardware start-up, we spend a lot of time thinking and discussing the consumption robots in our daily life, such as the robots that can cook food, arrange meeting time, and clean the floor. But for now, robots like Kiva have had a greater impact on our daily lives. With the continuous development of e-commerce, Kiva technology will become the driving force to change the warehouse and distribution center, which is crucial to meet our breakthrough aspirations. Kiva is one of the few companies that cleverly integrate complex hardware and software into a seamless solution, and the company has built a system that can significantly change the way we buy, sell and live.