lecture 19 dimitar stefanov powered wheelchairs 1940s – first powered wheelchairs, standard manual...

Lecture 19Lecture 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 sort-distances and a powered wheelchair for long distances.

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.

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.

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 Powered wheelchair system schematicschematic

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 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.

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

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

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.

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.

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.