lecture 19 dimitar stefanov powered wheelchairs 1940s – first powered wheelchairs, standard manual...
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Lecture 19Lecture 19
Dimitar Stefanov
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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 sort-distances and a powered wheelchair for long distances.
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Powered wheelchair with combination of other devices – communication devices, computer access devices, respirators, and reclining seating systems.
NEMO, Madentec Limited, http://www.madenta.com/
54 commands
HMI by voice, switch, joystick, wheelchair controls.
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Depending on their construction:1. Power bases2. Power wheelchairs3. 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.
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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.
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Powered wheelchair system Powered wheelchair system schematicschematic
Electromagnetic brakes immobilize the wheelchair in case of inclined ground surfaces
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Motor selection:
•Armature controlled permanent magnet direct current motors – high torque, high start torque, simplest controlIn 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 wheelchairDynamic 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.
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Bandwidth – the limiting frequency of motion commands to which a system can respond
Speed contour of the controller
Typically – 150 -250 Hz
Servo controllers
M
Currentsensor
Switchingdrive
-+
-+
Velocityreference
Velocitysensor
Voltage control is used to control the motor speedCurrent contour (inner)Speed contour (outer)Switching (or chopper) drive
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Switching drive:1. Unidirectional (half-bridge) – used in
scooters2. Bi-directional (full-bridge, H-bridge)
Switching drive:1. With dynamic braking2. Without dynamic braking
Speed reverse by relay
Pulse – width modulation (PWM)
Frequency – 5 -30 KHz
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Motor current in the switch mode:
PulseGenerator
R
L DC
m
otor
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.
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Switching elements:1. Silicon controlled rectifiers (SCR) – old
constructions2. 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.
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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.
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•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.