Enjoy a video about the pool float car…
Pool Float Car – Introduction
What’s better than the ability to pull up to class lounging on a pool float?
Enjoy lounging on a pool float, but want to use it outside a swimming pool?
Or perhaps you just want some cool swag utility vehicle (SUV) for fun?
If any of those descriptions above suits you, I’ve got you covered!
The goal of this post is to explain this project and how I plan on accomplishing it!
Mechanics
This pool float car is extremely simple in construction. It consists of a three-wheeled design for cost reduction and simplicity.
A frame will be made of wood (for easier machinability). On top of this frame, the pool float will be attached.
Two fixed-direction wheels support the front of the mobile. These motors will not be powered.
One wheel upon a steering mechanism will be present in the back of the mobile. This wheel will be a hub motor, similar to those commonly seen on commonly-available electric scooters such as the Xiaomi M365. Steering will be implemented by mounting this motor on the rotor of a turntable. This turntable will be turned via a stepper motor.
The stator of the electric steering system will be mounted to the bottom of the rear of the mobile’s frame. This stator will contain a stepper motor with a small spur gear, which mates with a large spur gear manufactured directly upon the rotor of the steering system. A potentiometer will be added to allow the computer to calibrate and center the steering at boot.
The design of the steering system allows the mobile to turn in place with a steering angle of 90 degrees! Do doughnuts without even trying!
Drive Wheel and Steering Electronics
The main SoC used in this design will be the ESP32 (showin in the middle), a dual-core microcontroller with the Xtensa architecture. This microcontroller will be connected to the controller via a wired signal connection (blue solid arrow) and a wireless link (dashed blue arrow).
The SoC can read the battery voltage for safe low-voltage cutoff and high-voltage regen cutoff. The SoC will be communicate power, brake and regen with the ESC. The ESC will drive the hub motor, using hall sensors for commutation feedback.
The SoC will be connected to a stepper driver to move the steering column, and a potentiometer to measure the absolute (although not precise) steering angle.
Depending on the complexity of the control protocol with the
Controller Electronics
The controller is designed similarly to the drive wheel and steering control system. The peripherals connected to the SoC are the LCD, and the controls used to control the mobile.
The LCD shows information such as speed, charge status, connection status, drive mode, instantaneous torque, and other important information.
The X-axis of the thumb stick will be used for steering. The Y-axis may be used for throttle / brake, although those will likely be another control.
Buttons will be used to navigate the menus, control the mobile (change drive modes, etc.) and act as a power switch for the controller and the drive wheel and steering control system.
Wired Communications Electronics
Wired communications will use the SPI protocol for maximum performance. A mclk of 5 MHz will be used, allowing for 5 Mb/s link performance. To support hot-plug, when a wired link is not established, the steering control system will pull up the MISO pin. The controller will connect the MISO pin to GND (low). When the controller is plugged into the steering control system, the MISO pin will be driven low which should wake up the steering control system. The steering control system shall now set MOSI to LOW, which should be pulled HIGH by the controller. This should wake the controller up to initiate reconnection.
To allow for hot-unplug, if two or more packets are lost (20 ms), the link will be considered broken and a wireless command will be established.
If the controller is unplugged while the mobile is driving at a speed higher than 1 MPH, the mobile will E-stop.
Wireless Communications Electronics
The wireless communications will use the ESP32’s built in support for WiFi. No extra electronics are necessary, except possibly a high-gain 2.4 GHz antenna with an IPEX connector.
Communications Protocol
Communications will use packets, individual pieces of information that are separate from each other. Wired packets will be slightly different from wireless packets. Wireless packets will be encrypted using AES-CTR 128 bit (explained later). The encrypted payload of the wireless packets will be similar to the wired packets. All packets can be represented by a packed structure.
The diagram above shows what a wired packet looks like. It includes a packet type, a payload length, a payload, and a hash (may be a different hash) to verify integrity.
The payload of above wireless packets is a wired packet encrypted with AES-CTR 128-bit.
The subpacket above (payload in the packet) updates the steering angle (signed, maps between -90 deg to 90 deg) and the throttle (signed, maps between 100% brake and 100% throttle). This subpacket has a type, 1. This packet is sent from the controller to the steering control system.
The subpacket above, with type 2, negotiates an AES 128 bit key for secure, encrypted wireless communications with the AES-CTR 128-bit cipher.
As this project is developed, more subpackets will emerge. They will be explained in future articles.
Curveballs
This wouldn’t be a project by Yash without some curveballs!
The steering column needs to be entirely 3D printed using a small delta 3D printer. This requires much more care as I need to manage the stress on the plastics in all situations. I might even need to 3D print low-speed disk/drum brakes to engage when the drive motor is too slow to brake electromagnetically.
Technically encryption is not necessary for this project because who will are about hacking this?! I’m using encryption because it is forcing me to learn how to properly use AES, choose the correct cipher for the job and implement it securely.
I need to minimize latency over WiFi.
3D Printing the Entire Thing
The rotor is designed to be as small as possible to fit the required parts, but as large enough to fit in my printer by diameter to maximize the volume supporting the wheel mount.
The stator will be either printed in multiple pieces then glued together with super glue, or printed on a school printer (yeah, I hate the lead time) which can print it in one go.
Lower-Level Communications Protocol and Security
To skip the bulk of having a heavy IP stack, I will be transmitting and receiving raw packets via the 802.11 baseband. This will hopefully reduce the connection latency.
Of course, we have the issue that security is no longer implemented for us via WPA2. We gotta secure ourselves!!!
For this project, the wired connection is being considered as a secure channel. Through this secure channel, a pre-shared key (PSK), 128 bits of length is shared to allow for secure communication via AES-CTR 128-bit.
All packets received with the correct MAC header will be “decrypted” using the key, nonce and CTR (counter). The end of each message is a hash. If the message does not match the hash, the message will be discarded.
To prevent replay attacks, the highest-received CTR will be tracked. If a packet is received with a CTR below the highest recorded CTR, it will be discarded. The CTR, while sent unencrypted, cannot be modified, because modifying the nonce or CTR would cause decryption to fail, and the packet to be discarded.
If replay attacks become a huge issue ever, a timestamp can be sent with each packet. The clock would be synced every time the controller is connected via a wired connection.
Encryption
The communications protocol I am using is lossy. Most AES ciphers work on streams of continuous data. Missing pieces of data must be retransmitted to decrypt later bits of data.
We are sending a ton of extremely short packets. If we use the same AES key over-and-over again with most ciphers, we slowly expose our key more and more. Stream ciphers allow the same AES key to encrypt multiple blocks by mixing in data from the previous block, fudging the relationship between the plaintext and ciphertext for the block being encrypted. Thus, we need the previous block to decrypt the next block.
We can “change” the AES key used for every encryption/decryption by specifying parameters for an “initialization vector,” which can be viewed as a salt for the encryption key.
Instead of using the bare AES key to perform the encryption and decryption, we derive a new working AES key by applying a different initialization vector every time we encrypt. This is how we accomplish this:
Our initialization vector is the same length as the AES PSK. For us, this is 128 bits. We initialize this IV on the sender with a random number. This is called the nonce (number used once). Generally, with AES-CTR, we only fill the bottom 64 bits with the random number, and the upper 64 bits with the counter (or some other split). Instead, this project will implement the nonce using a 128 bit random number for initialization.
To allow the sender and receiver to use the same AES PSK, the controller will increment the nonce for every encrypted block. The steering column will decrement the nonce every time a block is encrypted. Note that each side will track its own nonce, and this won’t get updated when data is received.
I hope you enjoyed this!
This is a fun project I’m working on with a friend of mine, Orkut. Most complexities laid out in this article have the sole purpose of creating a practical environment through which I can learn a bunch of new concepts. Hopefully, I can share what I learn with the world!
A Beginner’s Guide to Binary Heaps
Introduction to Binary Heaps
Today, we will discuss a binary heaps, a common data structure in programming. Heaps have multiple uses in programming including:
- Priority Queues: a queue-like data structure which has a priority field as a key. In a priority queue implemented with a min-heap, the element with the lowest value for priority will the next available element. In a priority queue implemented with a max-heap, the element with the highest value for priority will be the next available element.
- Finding the k’th [small/larg]est element: Heaps are really good at solving this problem, because after creating a heap from the target elements, the lowest (min-heap implementation) or the highest (max-heap implementation) values can be extracted with ease.
Heaps are very useful when you need to retrieve the max (max-heap) or min (min-heap) quickly (\(O(\log n)\)), need to add an element with a certain value quickly (\(O(\log n)\)), and need to be able to build the structure quickly (\(O(\log n)\)).
Artificial Finger
Introduction
I was thinking, how do we determine what we are touching when we aren’t looking at our fingers? I realized that our nerve endings are very sensitive and can feel some of the lightest vibration. It also dawned on me that perhaps the texture on our fingers – our fingerprints – vibrate differently when feeling a different surface.
Why Does This Matter?
Physiology
Understanding how we are able to sense our environment is important for many reasons. Perhaps, if we determine the sequence of action potentials that take place when we feel a material, we may be able to develop an electro-mechanical transducer for prosthetics.
More importantly, we get to better understand our senses.
Robotics
A large portion of robotics surrounds the field of “localization,” the process through which a device captures, processes and understands the environment it is in. In the future, a robot could be built, which learns its surroundings by touching materials and taking pictures of them.
The Sensor
Our fingerprints composed of many ridges in a random, but organized pattern. If you place your finger carefully on a surface, without rubbing it at all, while not looking, you cannot determine what surface you are touching. But as soon as you rub the surface, you can feel vibrations that determine physical properties such as smoothness of the surface.
You can also determine the flexibility of a surface by applying a constant pressure to it, but determining how far in your finger moved.
The actual sensor consists of a motion sensor that can determine vibrations, as well as position, and an air pressure sensor to determine contact pressure. These sensors are placed within an inflated, air-tight chamber such that any external pressure on this chamber will cause a rise in the internal air pressure. This can be measured by the air pressure sensor.
A picture of how this sensor could look is below.
Front:
Side:
Bottom (External):
Build Guide
At the bottom of the finger, to the IMU, there is a mechanical link. If the sensor is just suspended inside the air chamber, very little vibrations will be transmitted into the IMU.
The PCB should be made of a flexible material like DuPont Pyralux. This will allow the sensors to move in the finger, constrained by the mechanical link.
The airtight bag should be heat sealed around the PCB. If this bag loses air, the sensor may not work properly. An alternative would be to use a different type of a pressure transducer. Air pressure sensors are very cheap, but there are many types of flexion sensors.
My Prototype (Under Development)
The prototype will currently only have an IMU. For the IMU, I will be using a six axis IMU from ST Microelectronics, the LSM6DSM. It has a 16 bit resolution for its accelerometer and gyroscope axes. Its accelerometer is capable of about 5k samples per second per axis. Its gyroscope can handle 1.5k samples per second per axis.
The main processor for this project will be an ESP32 by Espressif Systems. It’s a dual core, 240 MHz CPU with WiFi and Bluetooth. I am using a prebuilt module, the esp32-wrover. I chose the esp32-wrover module because it has an extra 32 Mbit of external SRAM, allowing for a significantly larger capture buffer. The ESP32 can capture and send its data over WiFi to a computer running data acquisition software, or directly onto a server.
The main processor board has a 32 GB SD card for headless data acquisition and a Lithium polymer PMIC for power.
See ya next time!
KiCAD Introduction
Introduction
I am currently trying to understand how to use KiCAD. KiCAD is a complete open source schematic capture and PCB design toolkit, supported by CERN. To begin, one of the pros of KiCAD is that it runs on most common operating systems – Linux, Windows and Macintosh.
With the purchase of Eagle, the industry-standard PCB design software, by Autodesk, and the shift to a subscription-based license, there has been significant development in both the software and the community.
Since KiCAD is new to me too, I will use this blog as a beginners guide.
Installation
I am running KiCAD on Windows 10 currently. The installation was rather straightforward. I downloaded and ran the KiCAD installer from KiCAD’s homepage. Within no time, it was installed. I also installed Wings3D, the optional 3D toolkit recommended for installation. I’m assuming it is required for KiCAD to render 3D models. However, for 3D CAD, my go-to software is Autodesk Inventor Professional, or Fusion 360.
First Run
I ran KiCAD for the first time and created a project. To create a project, use the file menu, select “New Project” and then select “New Project.” Now, you need to select the directory to save all of the KiCAD project files in, and name the KiCAD project file (*.pro). The name of this file is the name of the project. Below is a screenshot of what it should look like:
In orange, is circled the menu. The buttons in green allow you to create or open a project. In purple, is a navigator, which shows the hierarchy of the project. In red is a set of buttons which allow you to open different tools.
First Impressions
My first impressions of KiCAD is that it is a rather complex tool to use. Instead of being built as a single, well-connected applications, it is a set of individual applications, or components, which have been selected and chosen to interact with each other. This is a little unnatural to me.
Schematic Capture
KiCAD comes with a schematic capture tool, Eeschema. It seems to have all the major features required to draft a circuit’s schematic.
On the right, is a set of tools we can use to create our schematic. At the top is a cursor button, the main tool which allows us to select components. Upon hovering over many buttons and options, and sometimes even written on the button, are keyboard shortcuts which allow faster development.
The arrow keys move the cursor around, with the grid – one point at a time. It seems as though KiCAD is designed to allow minimal mouse usage. That’s always a plus!
We have two maroon buttons on the right side – “Place component” and “Place power port.” We cal also place wires, buses, junctions, not-connected, net names, global labels, hierarchical labels, hierarchical sheets, hierarchical pins, lines/polygons, text and bitmaps. I do not know what most of these buttons do, but we will likely find out while using KiCAD.
Placing Components
To place components, we can use the “Place component” dialog, click on the schematic to select a target location, select a component and then use the cursor to finally place it.
The “Filter” field seems to allow me to search for a component I want. The size of the bundled library is very small. It will be necessary to create schematic symbols and footprints for most components! All the common components and passives seem to be in the “device” library.
I placed a battery and a few passives on my schematic sheet. To place a component, align your cursor to where you want to drop the component, and left-click or press enter.
Here’s how my schematic looks:
Keyboard Shortcuts in Eeschema
Upon scouring through the menus in KiCAD’s Eeschema, I found quite a few useful keyboard shortcuts:
- Move component: M
- Drag component: G
- Move literally moves a component, disconnecting everything connected to it. Drag moves the connections with the component.
- Rotate clockwise: R
- Delete component: [delete]
- Copy: C
- Add component: A
- Add power port: P
- Place wire: W
- Junction: J
- Label: L
While most keyboard shortcuts are capitalized (use shift when pressing these keys), KiCAD seems to perform the correct action when pressing the lowercase key. Perhaps this is a bug, or a feature.
Random Tidbits
I really like how in Eeschema moves your cursor back to where it was before trying a command, when a command finishes. If you are placing components next to each other, for example, you could use the arrow keys to move the cursor, say, two grid spaces, and issue another command. It’s also really efficient being able to control the cursor with arrow keys. As a programmer, I am much more efficient and fast at typing than using a mouse.
In Conclusion
I’ve now covered basics on the KiCAD schematic tool, Eeschema. In the next blog post, we can try to dive deeper and design something cool with it. So far, I’m impressed with KiCAD. At a first sight, it seems like an under-developed tool, but when you try using it, it’s filled with features that make working with schematics so much easier. The best part is that KiCAD is open source, and may be used for commercial projects – something that most other free electronics CAD software lack.
See ya next time!
I’m Designing an EEG!
I’m designing an EEG from scratch as a prototype for a startup! It’s a very ambitious project, as I am very rusty at electronics. My goal is to design a very simple, inexpensive, versatile, easy-to-use and self-contained device, which confirms to medical standards (though FDA approval isn’t a goal for this project).
I cannot disclose the feature set of this device, or the problem this aims to solve. However, I will try my best to use this blog as a journal for the project.
Since I can be classified as a “noob” in electronics/electrical engineering, this blog will be a great resource for those who are getting their feet wet in electronics. This blog is a tool for me to communicate what I learn, and what traps I run into while working on this project, and how to avoid them.
Welcome!
This is my new blog. It will allow me to have conversations on what I am currently working on, create tutorials on cool stuff and share ideas!