Dimitar Stefanov

1
Lecture 19

• Dimitar Stefanov

2
Powered Wheelchairs

• 1940s first powered wheelchairs, standard
  manual wheelchairs adapted with automobile
  starter motors and automobile battery

Prescription of the type of the wheelchair
depends on the users mobility. In some cases
manual wheelchair for indoor and sort-distances
and a powered wheelchair for long distances.
3
Powered wheelchair with combination of other
devices communication devices, computer access
devices, respirators, and reclining seating
systems.
54 commands
HMI by voice, switch, joystick, wheelchair
controls.
NEMO, Madentec Limited, http//www.madenta.com/
4

• Depending on their construction
• Power bases
• Power wheelchairs
• Scooters.

Four classes wheelchairs (depending on their
power) Class 1 carry up to 100 kg, maximal
speed of 1.1 to 1.8 ms-1 , capable to carry up to
6 slope, travel over than 10 km on a single
charge, climb a 25 mm obstacle, for indoor use
Class 2 carry up to 100 kg, maximal speed of
1.8 to 2.7 ms-1 , capable to carry up to 9
slope, travel over than 16 km on a single charge,
climb a 50 mm obstacle, designed for indoor and
moderate outdoor use. Class 3 carry up to 113
kg, maximal speed of 1.8 to 2.7 ms-1 , capable to
carry up to 9 slope, travel over than 16 km on a
single charge, climb a 50 mm obstacle, designed
for active indoor and outdoor use. Class 4
carry up to 130 kg, maximal speed of 10 to 12
km/h , capable to carry up to 15 slope, travel
over than 35 km on a single charge, climb a 150
mm obstacle, designed for active outdoor use in
heavy terrains.
5
Scooter an example
Front-wheel driven (for indoor use)
Rear-wheel driven (for indoor and outdoor use)
Scooter can be broken down to fit into the trunk
of an automobile.
6
Powered wheelchair system schematic
Electromagnetic brakes immobilize the wheelchair
in case of inclined ground surfaces
7

• Motor selection
• Armature controlled permanent magnet direct
  current motors high torque, high start torque,
  simplest control

• In some prototypes
• Brushless motors
• AC motors special controllers, where the motor
  power is changed by varying the frequency of the
  motor supply.

Power rate of the DC motor describes the
ability of the motor to transfer electrical power
to mechanical power.
Motor control of powered wheelchair
Dynamic stiffness how closely a moving system
follows a desired motion profile in the presence
of disturbance torques.
Closed-loop systems are used to increase the
dynamic stiffness.
8
Bandwidth the limiting frequency of motion
commands to which a system can respond Speed
contour of the controller Typically 150 -250 Hz
Servo controllers
Voltage control is used to control the motor speed
Current contour (inner) Speed contour
(outer) Switching (or chopper) drive
9

• Switching drive
• Unidirectional (half-bridge) used in scooters
• Bi-directional (full-bridge, H-bridge)

Speed reverse by relay

• Switching drive
• With dynamic braking
• Without dynamic braking

Pulse width modulation (PWM)
Frequency 5 -30 KHz
10
Motor current in the switch mode

1. Imax should be limited to 5 times the rated
   current of the motor to insure that
   demagnetization does not occur
2. The ripple (Imax Imin)/Iaverage should be
   minimized to improve the form factor, and reduce
   the conduction loss in the switching devices.

11

• Switching elements
• Silicon controlled rectifiers (SCR) old
  constructions
• Powder metal oxide semiconductor field effect
  transistors (MOSFETs)

• Powered wheelchairs based on SCR use typical
  switching frequencies of 500-1000 Hz.
• Powered wheelchairs based on MOSFETs use typical
  switching frequencies of 5 -30 KHz. Efficiency
  90.- excellent to battery application.

12
Basic Full-bridge circuit

• High forward voltage (up to 50 V) on the MOSFETs
  transistors during the turn-off period due to the
  motor inductivity.
• Multiple MOSFETs transistor in parallel in order
  to achieve big output currents.

13

• Typically - MOSFETs transistors rated at 100V and
  30-50 A continuous operation.
• In some constructions up to 8 transistors,
  connected in parallel to increase the reliability
  of the wheelchair
• Dynamic braking special fast recovery diodes
  are switched in parallel of the MOSFETs and the
  motor During the braking process the current is
  returned back to the supply, i.e. the battery is
  charged during braking.