3-D Profile Scanner
Aaron Trask
Mariusz
Zaczek
ADSL
Spring 1999
1.0 Introduction
Recreating physical objects into three dimensional computer graphics can be an expensive and time-consuming task. Today, many methods for scanning physical objects and recording dimensional information exist. One type is to probe the surface of an object and record the probe position. This method is usually done with expensive and bulky machines, but the 3-D Profile Scanner (3DPS) presented here is a cost effective alternative to the hobbyist. The 3DPS has a 2mm probe giving a resolution of 2mm. It can scan a 150mm cube and record the profile as a set of points, which can be meshed to display the surface of the scanned object.
2.0 Mechanics
The 3-D Profile Scanner (3DPS) uses a basic 3-axis table [Fig. 2.1]. All three axes slide on rails with linear bearings and are pushed by lead screws. DC stepper motors, giving a resolution of 1/12mm per step in the x-axis and y-axis and a 1/77mm per step resolution in the z-axis, power the lead screws.
Figure 2.1: 3-Axis Table
The probe consists of a
limit switch with a return spring to push the probe to rest [Fig 2.2]. As the
probe is lowered and comes into contact with the object being scanned, the
limit switch is pressed. This sends an interrupt to the program and the
positions of the three stepper motors are recorded.
Figure 2.2: Probe Design
3.0 Electronics
3.1 X and Y direction motors
The motors used for the x and y
positioning of the probe are 24 VDC, 1.26 A Werner Electric stepper motors.
Each motor has four color wires (black, red, yellow, brown) that are used to
initiate the stepping action. For each step pulse the motor is able to turn 15
degrees. This motor is an a 3-phase motor that uses the common wire (black) as
the LO (ground) or HI (24 VDC) wire while the HI or a LO signal is rotated
through the other three wires in a red-yellow-brown sequence (the order is
reversed to generate opposite rotation). For this 3D scanner, the black wire
was connected to HI using a 12 VDC source rather than the rated 24 VDC which
was not necessary [Fig. 3.2]. The other three wires were connected to the
signal obtained from the computer. A TIP142
NPN Darlington transistor. assigned to each of the three signal wires, was
used to rotate a HI signal to control the action of the stepper motor [Fig. 3.1 and Appendix]. The original signal, that is obtained
from code via the parallel port, is first inverted prior to reaching the TIP142
transistor primarily to avoid a back
wash of current which may hurt the parallel port. A SN54/74LS04 Hex Inverter (see appendix for data sheets) is used for
the signal inversion. The circuit design was taken from motor control circuit
obtained from the ECE 110 Laboratory Manual by Prof. Ricardo B. Uribe (p 43).
Figure 3.1: TIP 142 Darlington Transistor (Uribe,
1997)
Figure 3.2: X & Y Stepper Motor Circuit
3.2 Z Direction Motors
The x-direction motor (Astrosyn Size
17) uses the typical application circuit found in the data sheet for the
LMD18245 [Fig. 3.3 and Appendix]. M1 through M4 pins are set to the 5V supply,
the Direction pins are the signals from the parallel port, and the Brake pins
are set to ground.
Figure 3.3: Z Stepper Motor Circuit (National
Semiconductor, 1998)
3.3 Computer Interface
The
parallel port of a PC is used to control the entire scanner assembly which
includes all motors and interrupt switches. A guide on the use of the printer
port for control and data acquisition is attached in the appendix. Pins 2
through 7 (DATA) generate control signals for the three motors which correspond
to an eight bit word where bits 0, 1, 2 control the y-direction motor, bits 3,
4, 5 control the x-direction motor and bits 6 & 7 control the z-diection
motor - the most significant bit (MSB) being 7. Figure 3.4 below shows the pin assignments for a parallel port of
a PC.
Figure 3.4: Parallel Port Pin Assignments
As mentioned before, the signal
received from the parallel port is inverted prior to reaching the motors. As a
result, while a logic 1 (HI) is shifted through the wires of the motors, to
control the stepping, and logic 0 (LO) must be generated by the code and sent
through the parallel port. A typical code output for the DATA word would look
as follows: (bit 7) 11 110 110 (bit 0).
The zero value in bits 0,1,2 and 3,4,5 is shifted rotated through to generate a
step. Bits 6,7 use a sequence specified by the LMD18245 chip to control the
z-direction stepper motor [Appendix].
In order to output data via the
parallel port pins, the address of the parallel port must first be determined
for the particular PC being used. This can be done by using the DOS debug
program to display the memory locations 0040:0008 by typing at a DOS prompt
(Anderson):
DOS
PROMPT> debug
debug
mode: -d 0040:0008 L8
example
result: 0040:0008 78 03 78 02 00 00 00 00
For the above
example result, the LPT1 (1st parallel port) address is 0x0378
(hexidecimal) while LPT2 is 0x0278 and LPT3 and LPT4 are not assigned. For the
computer used by this project the DATA word was assigned to the 0x0378 address
while the corresponding STATUS and CONTROL words were assigned to 0x0379
(DATA+1) and 0x037a (DATA+2), respectively. The STATUS word is used by the
mechanical interrupt which is generated by the limit switch when the probe
makes contact with the part surface. The schematic of the connection necessary
for the interrupt are shown in the figure below. Pin 11 is an input pin of the
parallel port that is scanned continously during the vertical (z-direction)
movement of the probe. When the limit switch (or push button) is engaged the
input of pin 11 is connected to ground (pin 18) which alerts the code to stop
the probe movement and record the current position.
Figure 3.5: Schematic of Mechanical Interrupt
Circuit (Anderson)
4.0 Code Description
The control code used to automate
the action of all motors on the 3D scanner is written in the C programming
language - C++ can be used with slight modifications. A copy of this code - scanner.c
- is attached in appendix A of this report. This code was compiled using a
Borland C compiler for DOS and Windows – the run file generated is called scanner.exe. Output of each scan is sent
to a file called scan_out.txt for
later retrieval
4.1 Program Walk-Through
In order to run
the scanner program with minimal problems it is advised that the computer be
restarted in DOS mode. To run the program, enter the directory containing the
executable file as well as a file called: EGAVGA.BGI
which is the video driver required to display graphics in DOS.
Upon exectution of the program the
first prompt instructs the user to initialize the position of the probe by
moving the tip to the lowest plane possible – this will set the zero (0)
position of the z axis. An “Interrupt” message will be generated on the screen
to instruct the user to position the probe at the starting scan location for
the part on hand. This starting location corresponds to the upper most (z-axis)
position of the probe in the lower left portion of the scan area [Fig. 4.1].
Figure 4.1: Starting Probe Position
Once the upper most position is locked in
place by hitting ENTER, the program will prompt the user to input the maximum x
and y dimensions of the part in milimeters. This is followed by a request for
the maximum x and y step distances in milimeters – i.e. how far should the
probe head move in the x and y direction during the scan. Upon completion of
this initialization the screen will clear and a main opening screen will appear
displayin the text: “3D Scanner ... Zaczek, Mariusz & Trask, Aaron”. Scanning
will start when the user hits the ENTER key.
The scanning of the part is a raster type scan shown in the figure below. The probe starts at the maximum height in the lower left portion of the part [Fig. 4.2]. It descends the probe until contact is made and then returns. Next the probe proceeds along the +y axis stopping after one after the specified distance entered during the initialization process. Once again, the probe lowers until contact is made and finally retracts. This process repeats until the maximum y position is reached, during which time the probe will move over one specified step to the right and repeat the previous process but in the reverse direction.
Figure 4.2: Scanning Motion of Probe (Top View)
During the scanning of the part, points
are graphically displayed to the screen representing the probe contact
locations. Upon completion of the scan the entire image on the screen will
begin to rotate in 3D space and thus the contours of the part can be seen with
more detail. Rotation and 3D
perspective code examples are attached in the Appendix. This code was used to
complete the scanner program.
(Note: Due to the limitations of the motors, scan times even for small parts can be long. For example a part of max height of 2cm having 200 points scanned can take approximately 1hr)
5.0 Conclusion
The
3-D Profile Scanner has been a successful project but there are still many
possible additions and improvements that can be made to it. In its current
state this scanner can only scan the contour of very simple shapes. Curved
surfaces pose difficulty because the probe tip tends to slip off and follow the
surface face. An improvement for this case may be to use a rubber tip on the
probe as well as to stiffen the probe itself so that it does not bend and flex.
In addition, the vertical screw used, used to move the probe up and down, has a
bend in it and should be corrected to get more precise movement while avoiding
the wobble induced due to this imperfection. The current 3-D display only joins
each point scanned by a single line and a true wire-frame representation woud
be the next step to improve this . A final improvement would be to use shading
to display the part a a fully three dimensional image.
References
Anderson, Peter
H., "Use of a PC Printer Port for Control and Data Acquisition",
http://et.nmsu.edu/~etti/fall96/computer/printer/printer.html,
1999
Bikker, Jacco, “Building a 3D portal Engine”,
http://www.flipcode.com/portal/issue5.htm,
1999
Uribe, Ricardo
B., "Introduction to Electrical and Computer Engineering - ECE 110
Laboratory
Notes and Manual" , Spring 1997
