Geo Tech

Mechanical

ME 85O I

Design Special

THE GEORGE W. WOODRUFF SCHOOL OF

MECHANICAL ENGINEERING

Georgia Institute of Technology

Atlanta, Georgia 30332-0405

Topics

Nasa/University

Advanced Design Program

Proof of Principle

Model

for a

Lunar Dumptruck

June 1989

Jill Ann Harvey

An EqualEducationand Emplo_ent OpportunityInstitution AUnitof theUniversitySystemof Georgia
Problem Statement

Background

It was desired to design a proof-of-principle model of a two

wheeled lunar dump truck, to display at the 1989 Summer

Conference in Huntsville, AI. The dump truck, which was designed

fall quarter, consists of a cylindrical bucket suspended between two

wheels with the center of gravity of the bucket lower than the axis of

the wheels (ref. fig I). This gives the truck great stability in rough

terrain. One motor on each wheel controls the movement of the

truck and limits the moving parts to only two.

Require ments

It is desired in the future, to use the curvilinear

motors,currently under design at the V.A. hospital in Atlanta, Ga., to

power the lunar version of the truck. However, for the proof-ofprinciple model, the motors and drive system of an Everest and

Jennings wheel chair were modified to meet the minimum

requirements. The truck needed to be operated from a D.C. source,

and be remotely controlled. Also, to facilitate dumping, all parts

must be contained within the wheel base.
Constraints

As previously mentioned, a wheelchair was to be modified for

the truck. Therefore, the motors that were used were 24 Volt D.C.

motors. Also, in order for the truck to be able to invert, the driver

pulley had to be relocated within the wheelbase. The belt length had

to remain constant as the poly-rib belt used on the chair was not

available in a variety of lengths. Finally, the truck needed to remain

as safe as possible; the batteries had to bolted in the bowl to

minimize the chance acid spill during dumping.
Mechanical Drive Design

The mechanical drive consists of three separate parts for each

side of the dump truck. The driver pulley is powered by the motor.

The driven pulley is attached to the wheel and pulled be a k section

poly-rib belt. Because of the necessity that the wheelchair must be

reassembled, none of the parts could be altered. There were basically

three possible methods to construct the system: shortening the span,

adding a double pulley, or using a mule drive. Shortening the span

would be the easiest method of attachment. However, the

tolerancing would be very tight, and it is difficult to find poly-rib

belting in variable lengths. Using a double pulley with a slightly

smaller diameter than the driver pulley would help the tolerancing

problem but, adding and extra pulley and another belt became too

expensive. The mule drive, while not desirable, satisfied the

necessary requirements for short term operation.

A mule drive consists of four pulleys arranged so the belt

makes a 90 degree turn (see fig 2). It was decided that pulleys 2

and 4 would be .5 in. diameter McGill bearings. These bearings will

also be used to tension the belt after the driver pulley is mounted in

place. The motors will be mounted below the axis of the wheels to

help keep the center of gravity low. The relatively short length of

the belt predetermined the placement of the pu)Tevs to attain the
most wrap in all directions. It is necessary to attain at least 50%

wrap on both the driver and driven pulleys. By placing the idler

bearings relatively close to the wheel pulley, about 270 degrees of

wrap can be obtained. After compensating for the belt length, this

leaves approximate spans of around 6 inches to the driver pulley.

After adjusting this pulley for proper tensioning, the values for the

angular placement of the idlers can be determined.
Electrical Syste m

An electrical system to connect the motors to the

microcontroller was needed. The circuit needed to be isolated from

the motor to guard against current spikes. Also, the circuit needed to

be responsive to pulse width modulated input signals.

Generally, circuit design at this level of electronics consists of

simple relays, resistors, and transistors. The circuit outlined in Fig. 3

will drive a motor with relatively low current needs. All inputs are

active low, and are configured for TTL compatible signals. The logic

source would be connected to the same 5 Volt source connected to

the processor. The transistors provide the current gain needed to

energize one of the relays. The wire across the base of the motors

allows for the pulse width modulated signal. The diodes are simply

protection from current backflow after the circuit is de-energized.

The circuit is fully designed for a 12 or 6 Volt motor which would

pull less than I0 amps. of stall current.

Unfortunately, the motors on the wheelchair had no

specifications. A few simple measurements were made to determine

the current requirements of the motor. The starting current was

around 40 amps. After gathering data from similar motors, the stall

current was estimated to be somewhere between 60 and 100 amps.
This is far too much current for the simple electrical system shown

above. In order to obtain a relay capable of handling just the startup current, a double pole relay would have to be wired across the

connection plates.

Included with the electrical system are two ten turn 10k ohm

potentiometers. These provide the input signal to the microprocessor

for speed and direction control. They will also be powered to 5 VDC

and wired to the A/D converter located on the processor (see fig

below). This will provide the control system with 255 different

levels of direction and speed control.
Control System

The objective of the control system is to coordinate the control

of the two motors via a remote control station (i.e. self contained). It

was decided that digital control would provide the simplest method

of integrating the desired input with the output signals. After

careful investigation, the Motorola MC68HCII was chosen as the

microcontroller. The HCII provided a 7 channel A/D controller

onboard in addition to 5 output compare registers for pulse width

modulated output. Also located on the chip, are 512 bytes of

EEPROM memory for permanent memory storage so the controller

can be disconnected from the computer.

The input signals are provided by the two potentiometers.

After A/D conversion, the variable voltage of the potentiometer is

converted to an 8 byte hexadecimel number between 00 and FF. The

median value of input is HEX 7F. This corresponds to the division

between forward and reverse on the speed control or left and right

on the direction control. The machine code program reads the

values from the A/D registers then determines which control needs

to be taken.

The output signals are generated through the output compare

registers. These registers are loaded with a HEX number between

0000 and FFFF. These numbers correspond to the free running clock

on the chip. When the clock reaches the number stored in the
register, a specified action is taken (the line either goes high or low).

This generates a waveform which in turn drives the motor. The

waveform actually turns the motor off and on but, it happens so

quickly that it is impossible to notice. Since the electric circuit in the

previous section was designed for active low signals, the register

drives the output high when a match occurs. This stops the ontime

of the motor.

HC68HC11 / ,

Pot ent iomet er s Cont ro 1 ler Driver Circuit

Electric

Circuit

Truck

The correlation between the direction control and speed control

is linear. Once the desired speed is determined, a direction offset is

calculated and that amount is added to motors ontime and subtracted

from the others. The input signals are given a "gain" of HEX I01 to

convert from the FF scale to FFFF. This required some creative

programming as the chip will not perform 16 bit multiplications. To

compress the code further, the conversion process could be placed in

a subroutine rather than repeated so often.
The state space equations of control are:

[SI=K[-I I ] [U]

[ I I]

where the [S] matrix corresponds to the speeds of the motors;

K is the gain HEX I01; and [U] is the input signals from the pots.

[u] _11-_1 [s] _"--

The program, as written, uses 284 cycles of processing time to

calculate the output signals for the motors. This corresponds to a

sampling time of 14.2 milliseconds. Relative to the slowness of

response of the inertia of the mechanical system, this is extremely

fast control, If a closed loop system were implemented, a time delay

would have to be added to maintain system stability. The system

would easily over calculate while waiting for the motor shaft to

attain the desired speed. In the open loop system, the operator

closes the loop by controlling the rate of change for input

parameters.
The simple graph shown below demonstrates the input signals

for desired speed control. The non-linear curve demonstrates the

input necessary for the dumping action. Dy quickly advancing the

pot then quickly decreasing it, the dumping action will occur. The

other two are linear and correspond to slowing down or speeding up.

L

0

_=_

'!1

5

Time

Decrease

Increase

Dump
Conclusions and Recommendations

The mechanical drive is questionable. The poly rib belt is not

designed to carry a load on the back side. Therefore, twisting the

belt is against the desired operating conditions. However, for the

relatively short duration of the demonstration, this would suffice.

The electrical circuit which connects the motors to the board

and provides current protection for the board is designed properly

for low power circuits.

electric interface circuit.

work.

The 24 Volt motors require a heavy duty

Therefore, the circuit as shown will not

The MC68HC11 provides excellent control for the lunar dump

truck proof-of-principle model. This chip has all the functions

necessary to implement the control on board. Also, the E clock rate

of 500 nsec per cycle provides fast response time to input signals.

It is recommended that 12 Volt, 10 amp motors be used for the

design or, an electrical engineer work on the interface circuit. For

the short duration of the demonstration, the mule drive will give

sufficient torque. However, if a more permanent model were to be

built, a direct drive system would perform much better. Finally, to

improve the reaction of the truck to obstacles, closed loop feedback

control should be implemented.
Bibliography

Heiseman, David. I_/_i_J_gn and Build Your Own Custom Robot.

Blue Ridge Summit, Pa. :TAB Books, 1981.

Motorola. Advance Information : MC68HC11A8 HCMOS Single-Chii_

Microcontroller. Phoenix, Az. :Motorola, 1988.

Motorola. M68HC11 HCMOS Single-Chip Microcontroller ;

Programmer's Reference Manual. Phoenix, Az. :Motorola.

Oliver, L.R., Johnson, C.O., and Breig, W.F.. Agricultural V-Belt Drive

Design. Dayton, Oh.: Dayco Corp. 1977.

Peatman, John B.. Design with Microcontrollers. New York: McGrawHill Book Company, 1988.

Staugaard, Andrew. 6801, 68701, and 680__

Programming and Interfacing, Indianaplois, In. :Howard W. Sams

& Co., Inc, 1980.

Waugh, Dale L., David G. Fisher, and Melvin D. Gayer, eds.

Automotive Belt Drive Design Handbook. Springfield, Missouri,

1980
Appendix A

Electric Circuit Diagram

and

Controls Program

for

MC68HCI I
L.

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