MOTOR RESEARCH AND SELECTION PROCESS
the initial literature search and a trip to Motoman in Dayton, Ohio, several motor options were
defined. The overriding decision was
whether to proceed with AC or DC motors.
Previous design teams have utilized DC motors, since the wheelchair
already had this power source available.
AC motors, however, presented some enticing options specific to the
project that merited further investigation.
One such advantage is the wide spread use of AC motors in industry. This is a two-fold benefit; the cost of these
motors should be lower, for a given set of characteristics, over a similar DC
motor due to their widespread use, second any control specific questions might
more easily be answered by those in industry since they are more likely to have
faced similar questions in their own designs.
Another advantage AC motors have over DC motors relates to the power
consumption of the motor at its different states. As pointed out by Michael Ondrasek, an
engineer from Motoman, AC motors require the least amount of power in a holding
position and the most power while running at full speed, in the case of DC
motors the most power is required at stall and the least power is required
while running at full speed. The AC
motor characteristic seems to better suit the needs of the Gateway design,
since the arm will spend a good deal of time in a rest or stationary holding
The AC motors are not without their drawbacks however. Two main concerns are the need for an
inverter and the speed-torque characteristics of AC motors. First, the need for an AC inverter, to
convert the available 24 VDC to 120 VAC was investigated. Since DC drill motors were being considered,
as a cost saving measure, the same thinking applied to AC motors. In the current arm design there are five main
motors, to get an idea of the size of inverter required, Statpower, a company
that specializes in power conversion, maintains a list of typical appliances
that can utilize their power conversion equipment. This list is shown below in Table 8.1.
Table 8.1 –Power Consumption of Typical Appliances
Est. Power Requirements
Cell Phone Charger
100 W Work Light
According to Statpower and several other
inverter manufacturers, small motors like those found in power tools require
between 400 and 500 Watts of power. This
being said, the Gateway arm fitted with five AC drill would require between
2000 and 2500 Watts of power from the AC inverter. Because of the price of this size inverter,
it was determined that the cost outweighed the benefits. MajorPower, a large supplier of power
inverters, carries several models in the 2500W range, that retail for between
$1,800 and $2,000. Other drawbacks to
the power inverter option are the dimensions and weight of the inverter. The 2500W inverter from MajorPower weighs 32
pounds and has overall dimensions of 20” x 15” x 5.5”, which would make it
difficult to position the inverter in a suitable place on the wheelchair for
accessibility to the occupant or for the protection of the device. The inverter also adds unnecessary complexity
to the design. For example, operating
the controls for the inverter would be difficult for the disabled user, and any
malfunctions or blown fuses would disable the arm. Although AC drill motors are less expensive
for similar capacities, the large cost of the inverter all but negates this
advantage. The second disadvantage to AC
motors is the need to run the motor at high speeds to obtain the rated torque
for a given motor. AC motors produce
most of their torque at higher speeds, and this torque drops off rather quickly
as the motor speed is reduced. The
Gateway design does not require high speeds in any of the main motors, so
additional gearing would be required to obtain the necessary torque at lower
speeds. This gear reduction adds weight
and additional cost to the design.
Therefore these drawbacks outweigh the gains AC motors have over DC
DC motors were determined to be the choice for the Gateway arm, the next
decision relied upon, which type of DC motor to use. In previous Gateway designs, several
different types of DC motors were used because the motor’s specific advantage
provided the best result for a specific arm function. This allowed the arm to be optimized with
these different motors, but made controls a more tedious job trying to balance
the various inputs and outputs required by the assortment of motors. The decision was made to settle upon a single
type of DC motor, simplifying the controls, and the different interfaces needed
by each type of motor. The main
advantage to any of the different types of DC motors is the ability to produce
many different speed-torque relationships that can be easily tailored to the
needs of the project. Most DC motors
when properly configured can also produce as much as three to five times their
rated torque for short bursts. Wound DC motors, permanent magnet DC motors, and
brushless DC motors were three types considered for the Gateway arm. Stepper motors were not considered, since the
power requirements for the arm were above those provided by reasonably priced
stepper motors. Brushless DC motors were considered briefly since they react
more like AC motors than DC motors. They
also exhibit more durability than conventional wound DC motors since they
contain no commutator or brushes, which wear over time. This advantage is also a disadvantage since
the added circuitry increases the overall cost of the motors. This leaves permanent magnet motors and wound
DC motors. Permanent magnet DC motors provide many of the same
characteristics of wound DC motors, but variability in the magnetic material
that goes into construction can lead to inconsistencies from motor to
motor. The PM motors are also
susceptible to shock, vibration, and temperature variations that are not
concerns for wound DC motors. The final
DC motor option considered, are wound DC motors. These motors come in several configurations
that lead to varying control options and motor characteristics. These configurations include: series wound,
shunt wound, and compound wound. The series
wound motor is useful for low speed-high torque applications, but has high
starting torque and poor speed regulation.
The shunt wound motor is available in both long and short varieties and
provides good speed regulation and a flat torque-speed curve. The compound motor is a combination of a
series wound motor and a shunt wound motor.
It provides a compromise by offering better starting torque than the
shunt motor, but less accurate speed regulation. The DC motor obtained from a cordless drill
is most likely a series wound DC motor.
previous arm designs utilized expensive specialized actuators and gearboxes to
provide movement. A main goal for this year’s
design was to reduce cost. Consumer products afford the cost savings of mass
production. After researching the
available cordless products, the cordless drill had the best combination of a
motor torque rating and gearbox. The
drills researched ranged in price from $100 to $200+ and the advertised torque
ranged from 210 to 500 in-lb. In the
interest of time saving a baseline inexpensive drill would be tested with
future purchases based on how well the test drill faired.
test drill purchased was a Black & Decker 18.0 Volt Firestorm, Model
HP932K-2. The drill was disassembled and
the key parts are shown in Figure 8.1.
The main components are the motor, speed control, gearbox, and clutch
pack. The gearbox consists of three sets
of planetary gears. The two sets closest
to the motor that are under the lowest torque have plastic idler gears. The speed rating on the drill is 0-400 rpm
for low range and 0-1400 rpm on high.
For our application low range would be tested. The low range has a gear ratio of
21.4:1. The clutch packs of the drill
enable a maximum torque limit. As a
safety factor the packs could be set to reduce the maximum force provided to
the arm. For all testing, the packs were
set to the drill mode, which delivers the maximum torque available. The motor was rewired at its terminals to
provide current and voltage readings for testing.
Magtrol HD-710 model dynamometer was used for testing. This dynamometer uses a strain gage to
calculate torque input to a brake unit.
This strain gage has a voltage output that is converted back into a
torque output. An encoder produces a
pulse frequency and an oscilloscope was used to convert to rpm. A stand had to be designed and built so the
drill could be effectively coupled to the brake unit. The stand was designed to be adjustable in
the horizontal and vertical axes to accommodate different drills for future
Figure 8.1 - Firestorm 18V drill motor
See Figure 8.2. Since the drill had a 3/8-inch chuck a shaft
was designed to transmit torque to the brake.
Three milled flat lands were placed on the chuck end of the shaft to
eliminate the possibility of slippage of the chuck. A keyway was placed on the brake end to drive
the dynamometer. See the shaft in Figure
8.3. The shaft completed the mechanical
changes to the stand. See the final
setup in Figure 8.4.
The Magtrol required both 22 volt and 5
volt power supplies to operate the data outputs. An automotive 12-volt battery was wired in
series with an adjustable voltage supply to deliver the 22 volts. This was done because the voltage supply had
a maximum voltage of 17 volts. A
separate 5-volt supply was used for the 5 V circuit. The connector shown in Figure 8.5 was wired
to provide the input power to the data acquisition of the Magtrol. The testing procedure, which was followed to
carry out the testing, can be found in Appendix E.
Testing of the setup proved very
brief. Unfortunately, there was a short
between the 22 V and the 5 V circuits.
Two loud pops were heard and electrical smoke emanated from the stand. We think the short occurred in the orange end
of the connector. The Magtrol case was
opened and a 15 V voltage regulator was destroyed along with a capacitor. See Figure 8.6 for a photo of the board
components. These parts were inexpensive, $1.45 for the voltage regulator, and
hopefully they will fix the board.
Magtrol was contacted and a replacement board costs $175. The team installed a new control board, and
the dynamometer was recalibrated. At
this time it was determined that the tachometer encoder was also damaged,
preventing accurate speed dynamometer speed measurements. This component was order and replaced at a
cost of $55 dollars. The only data
obtained in the brief test was that the motor drew a maximum current of 35 Amps
at the 18.0 V battery voltage. At this
high torque load testing the 1 amp-hr drill battery was drained in less than
three minutes. After all the part
replacements, and recalibration, the dynamometer did not produce the required
data for motor torque speed curves, since the dynamometer only produced usable
data up to 3.4 N-m (30.19 in-lb).
Figure 8.7 shows the torque speed curves
for the motor alone provided by the motor manufacturer, Johnson Electric. Because the maximum torque provided by the
motor and 57.6:1 drill gearbox combination was approximately 250 in-lb, this
motor could be used in the shoulder joint with gear reduction to increase
torque and decrease rotational speed. Gear ratio calculations and gearbox
efficiency calculations are shown in Appendix E. A further 80:1 reduction would be required to
reduce speed from 400 to 5 rpm as required by the arm design, the cost of this
gear reduction would negate any savings gained by the use of motors obtained
from mass produced consumer goods.
The cost-prohibitive nature of the drill
motor adaptation, led to a search for alternative motor and gear drive
combinations. Upon further
investigation, a supply of surplus gear motors was located. These motors operated on 24 VDC providing 325
in-lb of torque at 35 rpm. These motors
utilize a worm drive mated to the electric motor to provide this reduction.