Electric Armchair V2

Improved version of the previous electric armchair, featuring electric motors and suspension.

Improved version of the previous electric armchair, featuring electric motors and suspension.

What’s new in V2

This is a complete rebuild of the first armchair.
The main enhancements in this version are:

  • Lithium-ion battery - No more trouble with limited charge cycle durability of lead-acid batteries.
  • Suspension - Way better traction and smoother ride.
  • Electric chair adjustment - Seating position can be adjusted effortlessly.
  • Overall larger - More comfort with a larger armchair.
  • Easy detachment - Armchair can be removed from the frame quickly for further upgrades

image New armchair in flat position

Specification

  • Top-speed: 9 km/h
  • Range: Approx 1 hour
  • Turning radius: 0.4 m
  • Weight: 99.5 kg

  • Current Features:
    • Individual tire control
    • Joystick operation
    • Remote control via WiFi
    • Electric seat adjustment
    • Massage function
    • USB charging port
    • OLED display
  • Future Features (Work in Progress):
    • Additional sensors: e.g. Accelerometer, Lidar sensor, GPS receiver
    • Anti-slip regulation
    • Self-driving algorithm
    • Integrated lighting


image Parked armchair

Components

Mechanical

  • Armchair: Second-hand
  • Drivetrain:
    • Chain: 06B-1(3/8’ x 7/32’)
    • Sprockets:
      • Motor: Z10
      • Axle: Z30
    • Axle diameter: 20 mm
  • Shock absorbers:
    • Length: 125 mm
    • Spring: 750 lbs
    • Type: Spring-only (no dampener)
  • Tires:
    • Rear:
      • Type: 4.10-4 for Mini-Quad
      • Diameter: 250 mm
      • Width: 76 mm
      • Max load: 80 kg (each)
    • Front:
      • Type: Swivel caster with pneumatic tire
      • Diameter: 210 mm
      • Height: 250 mm
      • Max load: 100 kg (each)

image Rear view

image Front tires and rear axle assemblies

Motors

  • Power: 350 W each
  • Voltage: 24 V
  • Type: Brushed DC motor
  • Rotating speed: 2500 rpm (without gearbox)
  • Attachment: Gearbox 9.8:1

image Motor with open gearbox attachment

Battery

  • Type: Self-made Li-ion
  • Battery cell:
    • Name: Samsung INR18650-35E 3500mAh 3,7V
    • Count: 105
    • Price: 2.45 EUR per Cell (Ebay)
  • Voltage: 25 V (21-29.4 V)
  • Capacity: 1.28 kWh (25 V 49.5 Ah)
  • Cell connection: 7 series, 15 parallel (7s15p)
  • Battery management system (BMS):
    • Max current:
      • Discharge: 150 A
      • Charge: 75 A
    • Type: Li-ion 7S
    • Features:
      • Over charge protection (4.25 V)
      • Over discharge protection (2.7 V)
      • Over current protection (260 ±50 A)
      • Balancing (30 mA)
    • Price: 70 EUR (Aliexpress)

image All battery components included

image Self made battery pack

Control System

  • Microcontroller: Custom pcb with ESP-32
  • Motor driver:
    • Brand: Sabertooth
    • Voltage range: 6-30 V
    • Current:
      • Continuous: 2x60 A
      • Peak: 2x120A
    • PWM frequency: 32 kHz
    • Interface: Serial (TTL RS232)
  • Sensors:
    • Speed measurement: 2x photoelectric sensor + custom encoder disk
    • Current measurement: 2x analog hall-sensor
    • Temperature sensors: ds18b20 (motors and driver)
  • User interface:
    • Analog joystick
    • 64x128 OLED display
    • Rotary encoder
    • Button
    • Wireless control: WiFi + React web-app


image Control board

image Box with motor driver and current-sensors

Electrical overview

All electrical aspects were planned in the following document:

Complete Document

Firmware

The firmware for the ESP32 microcontroller was created using the following components:

  • C++ (Programming language)
  • ESP-IDF (Development framework)
  • diagrams.net (Flowcharts)
  • Git (Version control)

The repository with the source code will be made public in the future.

image Joystick mapping

3D-Model

The entire build was planned in detail using CAD software. The model below showcases all the constructed and 3D-printed components:

Technical Drawings

All components were designed in FreeCAD, and manufactured according to the created technical drawings.

You can explore the detailed plans of all metal parts in this document: technical-drawings.pdf

Difficulties

Not enough pins

Problem:
The control interface of the previously used motor drivers already required 6 GPIO pins in total (A, B, PWM). With the introduction of new components like the display, rotary encoder, speed sensors and planned future sensors, there weren’t enough available GPIO pins on the ESP32.

Solution:
Two-board approach: One board controls the motors (driver, current limit, fading, etc.). The other handles the user interface and generates motor commands. Those boards communicate via UART.

image The two boards newly populated according to plan: V2.1 (page 4-6)

Driver bug

Problem:
The cheap motor drivers caused a dead time for several seconds when reducing duty cycle while the motors were still generating voltage. This was due to an error in hardware design of the driver. The control voltage being the difference between input and output voltage, without a separate regulator.

Debugging Driver: Does not respond to signal, when generator voltage exceeds 14V at low duty cycle.


Solution:
As workaround two relays and power resistors were added to briefly toggle the motors on and off. This helped in the recovery of the control voltage and also could be used to dissipate power during braking, protecting the motor drivers.
Schematic: V2.1 Driver box overview (page 7)

image

image Aluminium plate with brake resistors and Relays added below the motor driver box

Unreliable Drivers

Problem:
Initially, the inexpensive motor drivers from the previous armchair design were used: HC240A-PWM rated for 8-30 V and 30 A. However, even after the previously mentioned optimizations these drivers continued to burn out. This was primarily due to a slightly higher battery voltage and increased tire grip, resulting in higher current demand that slightly exceeded the specified limits.

Solution:
A switch to a more powerful dual-channel motor driver (2 x 60 A) was made. It is controlled via a single-pin serial interface (RS232). This switch made the second board and braking resistors obsolete. Overview of the current setup: V2.2 (page 1-3)

image Previously used cheap motor drivers

Videos

Tests

Suspension test

Test drive frame

Rotate

Drive

Fails

Crash into fireplace

Crash into bench

Gallery