Transcript No Slide Title - Electrical & Computer Engineering

Lecture 19
Dimitar Stefanov
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
user’s mobility.
In some cases – manual wheelchair for indoor and sortdistances and a powered wheelchair for long distances.
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/
Depending on their construction:
1. Power bases
2. Power wheelchairs
3. 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.
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.
Powered wheelchair system
schematic
Electromagnetic brakes immobilize the wheelchair in case of inclined
ground surfaces
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.
Bandwidth – the limiting frequency of motion commands to which a
system can respond
Speed contour of the controller
Typically – 150 -250 Hz
Servo controllers
Velocity
reference
+
+
Switching
drive
M
Current
sensor
Velocity
sensor
Voltage control is used to control the motor speed
Current contour (inner)
Speed contour (outer)
Switching (or chopper) drive
Switching drive:
1. Unidirectional (half-bridge) – used in
scooters
2. Bi-directional (full-bridge, H-bridge)
Speed reverse by relay
Switching drive:
1. With dynamic braking
2. Without dynamic braking
Pulse – width modulation (PWM)
Frequency – 5 -30 KHz
R
Pulse
Generator
L
DC
motor
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.
Switching elements:
1. Silicon controlled rectifiers (SCR) – old
constructions
2. 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.
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.
•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.