I believe the best way to learn STEM and engineering is through hands on experience. Hacking a four legged spider robot with my 5 year old son is my way of learning as well as teaching various engineering disciplines. With that said, let's dive into the details.
Currently, I'm using the Spider robot chassis designed by regishsu on Thingiverse. If you own a 3D printer, you can print your own with virtually no cost. Otherwise, you can have it printed for you by an online service such as CraftCloud. Last time I checked, they quoted me $27 USD to have someone print it for me in PLA, delivered in 10 - 18 business days.
The goal of this project is affordability. Here is a list of parts and cost:
- ESP32 MCU - $14.99 ( 2 pcs )
- SG90 Servos - $20.99 ( 12 pcs )
- 4.8V Battery - $11.99 ( 1 pc )
- Perfboard - $6.39
- 2.54mm Pin Headers (Male & Female) - $10.98
The total cost of the robot would be $65.34.
If you don't have a 3D printer and opt to use a 3D printing service. You will need to add about $27, and the total comes out to $92.34.
If you opt-in to order the custom PCB from JLCPCB (which is optional if you know how to solder some pins to a perfboard), you will need to add about $20, and you don't need to buy the perfboard and 2.54mm pin headers.
The brain of the spider robot is a ESP32 Micro Controller Board I bought on Amazon. You get 5 of them for $32 USD. If you want less, you can also buy this one, which will give you 2 of them for $15 USD.
This MCU has Bluetooth and WIFI support. I use Bluetooth to control the robot via a game pad such as PS4 / PS5 controller. WIFI is used to host a small web app where you can tweak various settings for the robot. The web app is not yet implemented. The planned use is to expose a UI for servo calibrations, but other ideas are welcome. If you have some cool ideas, please suggest it by filing an issue, and I'd be more than happy to read and discuss with you.
Since this is a 4 legged robot with 3 degrees of freedom on each leg, there are 12 servo motors in total. The SG-90 model servos are used in this project. I picked this model because it is very popular among hobby builders and is very very cheap. Again, I bought some cloned ones called Miuzei SG90 9G Servos from Amazon. They work pretty good for me. They come in 10 pieces, but since we need at least 12 of them, I bought two orders, giving me 20 of them so if some of them break, I can always replace them. There is also this one on Amazon comes with 12 of them for $21 USD. However, I have not tried those so I do not know if they are of good quality.
You can probably hand solder a perfboard and hook up the ESP32 board, servos, and power together, but I made a custom PCB to make things a bit easier for me. This PCB allows you to hook up a 4.8V NIMH battery to power the MCU and the servos connected via the pins on the 4 sides of the board.
I then order 5 pieces of this custom PCB via JLCPCB. It costed me $35 USD. You can probably get it cheaper with new customer coupons. It can get even cheaper if you opt in to solder the components yourself. But $35 USD for custom made PCB is already dirty cheap in my opinion.
I personally bought a 7.4V LiPo Battery as the power source. This means the servos are using 7.4V - 8.4V instead of the manufacturer recommended d 4.8V - 6V. I don't think this is a big deal because the servos are so cheap and they seem to be working alright under those voltages, but your milage may vary. I only chose to take the dangerous route because I don't want to spend the money to buy another battery. But if you want it up to spec, then you should probably get a 4.8V NIHM Battery like this one instead.
I used the official ESP-IDF as the base library for ESP32 boards. This is because it offers all the flexibility such as ledc fade functionality to provide efficient control over leg movements as well as bluetooth connectivity which allows me to use a PS4 controller to control the robot. I also used the following 3rd party libraries for various purposes:
- Bluepad32 - Bluetooth gamepad connectivity.
- btstack - Needed by
Bluepad32. Low level bluetooth stack. - ESP32Servo - Arduino compatible servo interface with ESP32 ledc. This will be replaced with custom implementation soon.
- esp32-arduino - Arduino component for ESP32. Provides Arduino interfaces. Needed by
Bluepad32andESP32Servocomponents.
You need to install the Espressif IoT Development Framework in order to build this project.
For windows, please follow the official documentation to install.
For Linux & MacOS, please follow this official documentation.
git clone https://github.com/aq1018/spidy.git --recursive
In a terminal, initialize the ESP-IDF environment by using the command below:
. ~/esp/esp-idf/export.sh
Note, this is assuming you have installed esp-idf under ~/esp/esp-idf.
Next, go to the btstack directory.
# assuming you are in the project root, navigate to btstack by:
cd vendor/bluepad32/external/btstack
# now you need to go to the esp32 port:
cd port/esp32
# now run the following to integrate `btstack` to `esp-idf`:
./integrate_btstack.py
# check your esp-idf components dir, and you should see files there.
ls ~/esp/esp-idf/components/btstack
If you haven't done so, in a terminal, initialize the ESP-IDF environment by using the command below:
. ~/esp/esp-idf/export.sh
Now under your project root run the following terminal command:
idf.py build
This should build the project.
Alternatively, if you use VSCode, you can install the Espressif IDF plugin, and then you can build, flash and monitor with a single button.
Run the following to flash your MCU:
idf.py flash
Or use VSCode Espressif IDF as described in previous section.