LUMBER 

DEFECT 

DETECTION 

BY 

ULTRASONICS 

FOREST PRODUCTS LABORATORY 

FOREST SERVICE 

U.S. DEPARTMENT OF AGRICULTURE 

MADISON, WIS. 

RESEARCH PAPER 

FPL 311 

1978 
ABSTRACT 

Ultrasonics, the technology of highfrequency sound, has been developed as a 

viable means for locating most defects In 

lumber for use in digital form in decisionmaking computers. Ultrasonics has the potential for locating surface and internal defects in 

lumber of all species, green or dry, and rough 

sawn or surfaced. 
LUMBER DEFECT DETECTION 

BY ULTRASONICS 

By 

KENT A. McDONALD, Forest Products Technologist 

Forest Products Laboratory,1/ Forest Service 

U.S. Department of Agriculture 

INTRODUCTION 

More efficient use of our Nation's timber 

resource can be realized by increasing 

product yield through improved processing 

techniques and equipment. At present, nearly 

all processing decisions in the wood Industry 

are made and executed by people rather than 

by computers. A poor decision usually results 

in less product from the raw material and 

means that more timber must be removed 

from our forests to meet the wood product demand. With the objective of relieving the demand on our forests, the Forest Service has Initiated research programs to increase utilization efficiencies through improved processing 

and decision-making techniques. The 

technology exists to make the difficult 

processing decisions correctly with a computer programed with carefully designed 

decision-making algorithms (2,3,5).2/ The 

computer can, however, only make these 

decisions when supplied with a complete 

description of the raw material quality in digital 

form accurately obtained at production 

speeds. 

Any attempts to devise a lumber scanner 

for obtaining an adequate description of individual board quality for the computer must 

take into account the wide range of conditions 

germane to the wood material referred to as 

lumber. Such conditions as moisture content 

(green or dry), surface quality (rough sawn, 

planed. or sanded), species (hardwoods or 

softwoods), and point-of-manufacture (edger, 

cut-off saw. rip saw, etc.) have an effect on accurate recognition of lumber quality. Any 

lumber scanning technology developed, unless for a very specialized application, will have 

to be applicable to these various conditions of 

the lumber being scanned. 

Generally, the known flaw detection 

techniques fall into two categories: those sensitive only to changes in surface qualities of the 

material, and those sensitive to internal, or 

subsurface, qualities as well. 

For some lumber scanning applications, 

surface recognition alone will provide adequate information for making processing 

decisions. For most applications, significant 

improvements in product yield can be made if 

a description of internal quality can be obtained. 

1/ Maintained at Madison, Wis., in cooperation with the 

University of Wisconsin. 

2/ Numbers In parentheses refer to literature cited at 

the end of this report. 
BACKGROUND 

Surface Scanning 

One approach to sensing surface quality 

requires that the defects be visually identified 

and tagged with a light-reflective or magnetic 

mark. The locations of these marks are then 

sensed by photoelectric sensing devices. A 

variation on this idea is to index the location of 

lumber defect areas into a computer using a 

manually positioned pointer system. Both of 

these approaches rely heavily on human ability 

(or inability) to recognize and mark defects at 

production speeds. 

A popular approach to surface scanning 

without human involvement is the use of optical scanners to locate defects. Optical 

systems. electronic imitations of the human 

eye, create images dependent upon the 

amount of light from the wood surface that 

reflects back to a lens. Of particular Importance In considering optical scanning techniques for lumber Is that “What it sees is what 

you get.” Optical systems can record 

superfluous data that have to be identified and 

removed from the meaningful data. For example, dark heartwood areas, dirt, and sticker 

and scuff marks usually have enough contrast 

in light reflection to be picked up as defects. 

Conversely, defects which have little or no 

contrast In color or light reflection, due to 

species characteristics or a rough-sawn surface, can be missed with this scanning system 

(fig. 1). The optical scanning techniques are 

proficient at locating defects that have a distinct dark appearance in contrast to adjacent 

clear wood, such as knots in pine lumber. 

One reported optical technique is able to 

detect cracks and checks in lumber. The 

technique uses a laser as a light source and is 

being developed and tested by industry. 

Internal (Subsurface) Scanning 

The location of internal defects in wood 

products requires a sensing technique that 

penetrates the surface of wood. Microwaves, 

X-ray, and ultrasonics are “throughtransmission” flaw-detection techniques that 

are used in various applications to locate Internal flaws and are thought to be applicable for 

scanning lumber. 

Microwave and X-ray.–Generally, 

microwave and X-ray flaw-detection techniques function by measuring the amount of 

electromagnetic radiation transferred through 

a material. The amount actually transferred is 

Figure 1.–Rough sawn hard maple board (top) and planed ponderosa pine board (bottom). 

M 144 528 

2 
a function of material density; thus a flaw 

associated with an excessive change in density 

can be detected. In applying these densitysensitive flaw-detection techniques to wood, 

some difficult problems are encountered. 

Flitches, fresh from the log, have a high, nonuniformly distributed moisture content. This 

moisture has a relatively high density that 

overshadows any smaller density variations 

that may be attributable to wood defects, thus 

limiting the techniques to scanning lumber that 

has been dried to a uniform moisture content. 

Furthermore, normal density differences 

between species would limit application to 

scanning one species per machine setting, or 

at best, species groups that have similar densities. 

Ultrasonics.– Ultrasonics is a science that 

deals with the effects of vibrations of ultrasound and with the apparatus used to 

produce, measure, and record these waves 

(9). Ultrasound (“ultra“ = beyond) is highfrequency sound beyond the audible range of 

human hearing. 

Positive results from early experiments in 

measuring ultrasound velocity in wood showed 

significant differences in velocity in areas of 

localized steep grain. Wood is anisotropic with 

respect to sound, resulting in sound traveling 

faster along the grain than across the grain. 

Further research has led to the design and 

testing of an ultrasonic technique for sensing 

and locating defects in wood products (6,7,8). 

This research is based on the time it takes 

sound to travel through wood of varying 

moisture content and density. 

ULTRASONIC 

THROUGH-TRANSMISSION 

TECHNIQUE FOR WOOD 

The ultrasonic through-transmission 

technique for wood is aided by wood’s 

anisotropy. Wood has three perpendicular 

axes: The longitudinal axis is parallel to the 

fiber (grain); the radial axis is normal to the 

growth rings (perpendicular to the grain in the 

radial direction); and the tangential axis is perpendicular to the grain but tangential to the 

growth rings (fig. 2). The velocity of sound in 

the wood is a property closely associated with 

this anisotropy (1,4). 

Because the grain of clear wood is normally straight (parallel to the longitudinal axis 

of a piece of lumber) and defective wood is 

often associated with locallzed areas of distorted grain, the rate of sound travel across the 

grain of clear wood remains fairly constant for 

any given species, and the rate of sound travel 

through defective wood is significantly 

different. Continual designing, modifying, and 

testing of the ultrasonic technique has revealed that sufficient sound energy can be 

transmitted through wood to detect the grain 

changes associated with defects. A threedimensional model of actual ultrasonic velocity 

measurements taken every 1⁄2-inch along and 

across a piece of softwood lumber shows (fig. 

3) the correlation between grain direction and 

velocity of sound. Note the uniformity in the 

velocity data shown in relation to the clear, 

straight-grain areas of the board. Obtaining 

this “image” of the board from the sound 

velocity data provided the impetus to pursue 

an automated lumber scanning system that incorporates the ultrasonic technique. 

Method 

Sound velocities are calculated from transit time data obtained by a basic throughtransmission technique transmitting sound 

through wood and measuring the elapsed time 

(fig. 4). An ultrasonic wave is created by shock 

exciting the transmitting transducer with a 

single pulse from the pulse generator. In this 

application, a 400-volt pulse of 1 μsec duration 

is used. When this pulse is initiated, a time interval counter for obtaining transit time starts 

running. The transmitted sound wave 

propagates through wood Immersed In a water 

TANGENTIAL 

LONGITUDINAL 

Figure 2.–Three perpendicular axes in wood. 

M 140 728 

3 
bath, and Is picked up by a receiving 

transducer. When the receiver senses the 

sound wave, the time interval counter is 

stopped to get the transit time. The velocity of 

sound in wood can be obtained after measuring the distance between the transducers, 

measuring the thickness of the wood, and then 

subtracting the time to transmit sound through 

the water. 

Transducers are devices that transform 

energy from one form to another. in this 

application, piezoelectric transducers were 

used to transform high-frequency electrical 

energy into high-frequency mechanical energy 

and back again. Frequencies usually 

associated with ultrasonic throughtransmission techniques for sending sound 

through wood range from 150 kHz to 1000 kHz. 

At the lower frequencies (150-200 kHz), the 

size of the piezoelectric element in the 

transducer Is 1.5 to 2 inches in diameter. With 

this size, and with the large beam angle of the 

sound created (fig. 5), flaws cannot be located 

with much precision. 

Figure 4.–Schematic of through-transmission 

ultrasonic technique used to make 

ultrasonic transit time measurements 

through wood. See figure 12. 

M 144 788 

Figure 3.–Three-dimensional model of sound velocities and the respective board area showing clear 

wood and a defect. 

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4 
At the higher frequencies (500-1000 kHz), 

it is possible to get piezoelectric elements in 

the transducer as small as 0.5 to 1 inch in 

diameter. This smaller diameter and higher 

frequency means a smaller beam angle (fig. 5) 

and thus permits greater precision and better 

definition of defects in wood. 

The water between the immersed board 

and the transducers is used as the ultrasonic 

couplant necessary to propagate sufficient 

sound energy into and out of the wood. This is 

based on the similarity of the specific acoustic 

impedances of water and wood, which results 

in very little reflection of sound at the 

wood/water boundaries. If the impedance 

values of two media are quite different, such as 

for air and wood, considerable sound energy is 

reflected at the wood/air interface (fig. 6). The 

water, as a couplant, makes good contact with 

a rough sawn surface of wood, as well as a 

sanded or planed surface–a phenomenon 

that broadens the potential field of application. 

Preliminary Results 

Sound velocities were calculated from 

transit time data taken on several species of 

clear, straight-grained wood samples (table 1). 

These samples were carefully prepared to 

provide the three perpendicular axes, and did 

not contain any known defects such as knots, 

crossgrain, decay, checks, or splits. 

In these tests, sound velocities were 2 to 3 

times faster longitudinally along the grain than 

radially or tangentially across the grain. Less 

difference was found between the tangential 

and radial directions, although radial velocities 

were consistently faster than tangential 

velocities. The sound velocities recorded in 

Figure 5.–Upper: The wide beam of sound is a result of the low transmitted frequency (150 to 250 

kHz) and means poor resolution of a defect area. 

Lower: The narrow beam of sound is a result of the high transmitted frequency (500 to 1000 kHz) 

and means good resolution of a defect area. 

M 146 231 

5 
Figure 6.–Schematic and received waveforms showing sound reflection using air couplant (top) and 

water couplant (bottom). 

Table 1.–Sound velocities for several wood species 

Softwood Hardwood 

Douglas- Western Eastern Ponderosa 

Sound fir red cedar white pine pine Beech Hickory Red oak 

propagation (dry) (dry) (dry) (green) (dry) (green) (green) 

Longitudinal 4350 5850 4350 4390 6610 5270 4110 

Radial 1980 2160 2470 1620 1980 2250 2040 

Tangential 1770 1980 1550 1460 1770 1680 1790 

6 
rotation from a longitudinal to a radial direction continually decrease (fig. 7). The same is 

true in moving from a longitudinal to a tangential direction. However, the velocities obtained 

in moving from a radial to a tangential direction do not change consistently but slow considerably about halfway through the rotation 

(fig. 8). These results are attributed to the cell 

structure of the wood with respect to the alinement of the sound path between the transmitting and receiving transducers. 

When transmitting sound through water 

alone. through water and 3⁄4-inch clear wood, 

and through water, wood, and a knot, the 

relative differences in the received ultrasonic 

signal wave form and its arrival time can be 

seen (fig. 9). The faster sound velocity through 

the wood than through water alone verifies that 

sound propagates through the cell wall structure. Similarly, the knot and the associated 

steep grain causes an increase in sound 

velocity as compared to clear wood. 

To illustrate the principle, transit times 

were recorded from a ponderosa pine board 

along three selected scan lines (fig. 10) with 

data taken along each scan. One scan along 

line C to D is clear, straight-grained wood, one 

along line A to B is through a knot with 

associated localized steep grain, and another, 

Figure 7.–The change in sound velocity from 

longitudinal to radial axis in wood. 

M 145 387 

line A to C, is across the width of the board to 

show the relationship between tangential and 

radial propagation of sound. The board was 

sawn after scanning to expose the grain 

pattern along each scan. The pronounced 

changes in the transit times associated with 

the localized steep grain around the knot can 

be clearly seen, along with the uniform transit 

time data recorded from the clear, straightgrained wood (fig. 10). 

Other Considerations 

The use of rolling transducers to contact 

the wood has been suggested to eliminate the 

water couplant and still have adequate coupling of the sound to the wood. Experience and 

investigation indicate that the rolling transducer covering needs to be wear resistant, 

and that the internal ultrasonic elements must 

be small enough for wood defect identification. 

The rolling transducer also requires good 

contact between the transducer’s flexible cover 

and the material being scanned. If the material 

is rough-sawn lumber, then poor contact is 

unavoidable, thus limiting the usefulness of 

the rolling transducer. 

Focusing of the ultrasonic beam produced by the transducers is possible and has been 

Figure 8.–The change in sound velocity from 

tangential to radial axis in wood. 

M 145 385 

7 
Figure 9.–Velocity and received waveforms of sound propagated through water alone, through clear

 wood, and through a knot. 

8
Figure 10.–Upper: Scanning paths on a ponderosa pine board through a defect (A to B) and through 

clear wood (C to D). 

Lower: Edge view of wood and corresponding ultrasonic transit time data showing the relative 

changes that occur. 

M 142 640 M 143 101 

9 
considered as having potential for locating 

lumber defects, but has had limited investigation. It is understood that maintaining the correct distance between the transducer and the 

material (wood) is critical to the focal point of a 

focused beam of sound. Varying wood 

thicknesses make such a system unfeasible. 

The piezoelectric crystal element in the 

ultrasonic transducers is usually barium 

titanite, lead metaniobate, or lead zirconate. In 

our experience, commercially available 

transducers made from lead metaniobate or 

lead zirconate have been adequate. When a 

specific application is developed, the potential 

exists for further refinement of the ultrasonic 

signal levels through improved design of the 

ultrasonic transducers. 

EXPERIMENTATION 

Experimental Lumber Scanning 

System 

A computer-controlled lumber scanner, 

called a defectoscope (fig. 11). was built to test 

and demonstrate the principle of the ultrasonic 

lumber-defect detection technique. 

This system was designed as a prototype 

to model the critical parameters associated 

with a commercial lumber scanning application, as well as to provide for positioning and 

control of the transducers. 

Under direct computer control, X- and Ystepping motors accurately move a pair of ultrasonic transducers along a board immersed 

in a water tank at speeds up to 50 lineal feet 

per minute. In this manner, transit time data, 

are taken at regular intervals along and across 

a board, normally at 1⁄2-inch increments for 

testing purposes. The X-stepping motor controls the movement of the transducers along 

the length of a board with an accuracy of 1/200 

inch. The Y-stepping motor controls the position of the transducers across the width of a 

board with an accuracy of 1/1600 inch. The 

carriage, driven by the X- and Y-stepping 

motors, firmly holds the transmitting and 

receiving transducers at a fixed distance apart. 

Various arrangements of multiple transducer 

assemblies can be mounted on this carriage 

Figure 11.–The computer-controlled lumber scanning system for laboratory evaluation of ultrasonic 

lumber defect detection. 

M 140 796 

10 
for testing under computer control. 

The computer, given scanning instructions by an operator via teletype, has complete 

control of the scanner, positioning the 

transducers, and collecting the transit time 

data (fig. 12). Data are stored on magnetic tape 

for analysis after scanning is complete. 

A computer-controlled line plotter is connected to the system for output of the results 

obtained from the lumber scanner. The plotter 

will make X, Y pen movements in 0.01-inch increments at a speed of 400 increments/second. 

Data Analysis for Defect Location 

Transit time data from each board 

scanned are recorded to the nearest 0.01 sec 

and stored in the computer for analysis. A 

contour plot (fig. 13, upper) of transit time data, 

where each contour line represents equal 

values. shows the strong correlation between 

the visible grain pattern (fig. 13, center) of the 

board scanned, and the data. Knowing that this 

correlation did exist enabled the investigation 

of various methods of analyzing the data to 

obtain the location of the defects on the 

board (6.7). 

The data analysis algorithm selected was 

designed to classify the board area scanned as 

being either defective or nondefective. Each 

area classified was a square centered on the 

scan line, and located between the data points 

(fig. 13, lower. and fig. 14). This mathematical 

description is a binary classification of each 

area made by comparing every two adjacent 

transit times along each scan Iine. If adjacent 

transit times differ by less than a preset 

difference, Δt, the area is designated as 

"clear." if the difference is equal to or greater 

than Δt, the area is classified as “defective.” 

Formula Classification 

[ti - ti+1] < Δt "Clear" 

[ti - ti +1] ≥ At "Defective" 

The preset difference, At, is a variable 

dependent upon the defect resolution desired 

for a particular configuration of the ultrasonic 

apparatus at the time of scanning. Results of 

the analysis using different Δt’s from 0.1 μsec 

to 0.9 μsec at increments of 0.2 μsec reveal the 

changes in classification of defect areas that 

occur over the range of Δt's (fig. 15). The Δt 

selected to get the best resolution is also 

affected by the distance between data points 

and/or transducer diameter. 

This approach to processing transit time 

data separates those areas of the board that 

exhibit sharp changes in sound velocities from 

those areas that have gradual changes 

associated with normal. clear wood. Often, 

sharp changes in sound velocities do not ocFigure 12.–Schematic of computer-controlled lumber scanning system showing lines of communication for transducer control and data collection. See figure 4. 

M 146 233 

11 
cur in the center of defects, giving the false impression these areas are clear. 

A normal procedure for locating defects 

in a 3⁄4-inch pine board is to use 1⁄2-inchdiameter, 1,000-kHz transducers positioned 4 

inches apart, with data recorded every 1⁄2 inch 

along each scan and across the board. With 

this setup, a difference level (Δt) of 0.5 μsec is 

used to classify the board areas. 

Using Defect Location Information 

To demonstrate the application potential 

of having a two-dimensional model of defect 

locations in a computer, a sawing algorithm 

was designed which maximizes the percent 

clear area sawn from a board scanned by the 

defectoscope system using the ultrasonic 

defect sensing technique. 

The sawing algorithm selected for 

demonstration simulates a finger-jointing 

operation producing wood moulding. It considers total board size, defect location, saw 

kerf, minimum length of usable clear cutting. 

and desired widths of clear cuttings. In this 

process, random-length clear cuttings are 

produced by first ripping the lumber into 

predetermined widths, and then crosscutting 

out the defects. All permutations of rip widths 

that occur from one edge of a board are considered. For each permutation, a “percent 

clear yield” solution is calculated, and compared to the previous best solution. 

The best solution is plotted under computer control showing the board outline, the 

defect areas, and the rip and crosscut 

solutions (fig. 16). The undesirable localized 

Figure 13.–Upper: Contour plot of ultrasonic transit time data, showing the close correlation to the 

grain pattern of this pine board. 

Lower: Plot of the results of defect analysis designed to provide a mathematical description of 

this board. 

M 147 971 M 147 972 

12 
Figure 14.–Upper: Locations of transit time readings on scan lines form squares. 

Center: Shaded squares represent six defective areas. 

Lower: Defective areas are outlined to include unusable material. 

M 137 107 

13 
Figure 15.–Defect analysis results for various difference levels, Δt’s, ranging from 0.1 μsec to 0.9 sec. 

14 
Figure 16.–A section of a board ultrasonically scanned showingthe ripping and crosscutting solution 

to remove defectsand obtain clearcuttings. 

M 145 842 

steep grain associated with most defects is not 

included in the ends of cuttings determined to 

be clear. After a board had been scanned for 

defects, it was ripped and then crosscut according to the computer’s decision. An overlay of 

the actual detect locations was used to 

evaluate the results and make comparisons 

with the area removed by sawing (fig. 17). 

Given the existing numerical control 

technology and the demonstrated capability of 

the ultrasonic lumber-defect detection system, 

it is assumed that saws could be numerically 

controlled by this system to do the actual sawing instead of the plotter drawing the board, Its 

defects, and the sawing pattern. 

15 
Figure 17.–Closeup of defects removed after making a sawing decision with the computer after 

defects were located. Board from figure 13. 

M 145 750 M 145 751 

16 
COMMERCIAL APPLICATION 

POTENTIAL 

The ultrasonic lumber scanning system 

has profoundly demonstrated that a mathematical description of lumber quality can 

be electronically obtained in digital form 

and that processing decisions can be made by 

the computer with this information. However, 

this system is a laboratory device, not commercially applicable to an industrial situation 

that requires handling and scanning boards at 

production speeds. 

Multiple Transducer Concept 

Early in the ultrasonic defect sensing 

research program, it was determined that 

commercial application would require the 

ability to move the board past a bank of 

transducers, without the calculating. handling, 

and analyzing of large volumes of transit time 

data. For example, a 12-inch-wide. 16-footlong board has 4,096 transit-time datacollection positions that amount to a rate of 

25,600 data points per minute in scanning at 

100 lineal feet per minute, or 153,600 data 

points per minute for scanning at 600 lineal 

feet per minute. 

To avoid the problems associated with 

significant numbers of transit time data and 

making defect decisions in the computer, consideration was given to making a defect decision immediately upon receiving the sound 

pulse through the board. This entailed 

transmitting a sound pulse through the board 

and comparing Its arrival times at two adjacent 

receiving transducers (fig. 18). The difference 

In arrival times was computed as clear (“go“) 

or as defective (“no-go”). Efficiency is improved by transferring only one data bit indicating a “go” or “no-go” condition at every 

decision point on a board rather than 

transferring the 48 bits necessary in determining transit time data. Handling the single 

data bit is simpler and results In less electronic 

noise interference. 

Electronic crosstalk between adjacent 

receiving transducers, a potential problem, 

was found to be manageable when the 1⁄2-Inchdiameter receiving transducers were mounted 

on 3⁄4-inch center. Tests comparing this 

method of obtaining defect location with the 

original comparison of transit time data show 

similar results. 

Figure 16.–Two transducers are used to receive the transmitted sound pulse for comparison of the 

sound velocity through wood at two points. 

M 146 232 

17 
Longitudinal Scanning 

The Laboratory scanning system described here holds the board stationary in a 

water tank, while a single pair of transducers 

is moved back and forth along its length until 

all data are obtained. At this rate, a 12-inchwide, 6-foot-long board is scanned in 4 

minutes (about 120 bd. ft./hr.). 

In designing a commercial ultrasonic 

lumber scanner, the initial approach was to 

have a bank of transducers covering the full 

width of the board, with the board moved 

lengthwise between the transducers. A tank 

long enough to accommodate lumber 16 feet 

long was considered to be impractical, and an 

alternative was sought. The alternative required a “water dam” arrangement to accommodate the necessary water couplant between 

the transmitting and receiving transducers. 

The last design for longitudinal scanning included a single water gate on the input end 

with the board moving upward at an angle (fig. 

19). Water lost around the board at the input 

gate would be pumped back into the scanning 

tank. The scanning speed of this design was 

calculated to be less than 150 lineal feet per 

minute, or about 9,000 board feet per hour. 

One of the difficult problems in the design 

of the longitudinal scanner was scanning unedged flitches (fig. 20). It was found that too 

much water would be lost due to the irregular 

profile and tapered edges. Another problem 

was the need to maintain longitudinal movement of the board past the transducers in a 

Figure 19.–Proposed method of scanning 

lumber moving longitudinally past a bank 

of ultrasonic transducers that spans the 

full width of a board. 

M 146 234 

straight line. This is similar to the difficult 

problem of moving a board in a straight line 

through a rip or edger saw. 

A number of disadvantages to this system 

are evident: 

1. Scanning speed of 150 lineal feet per 

minute was only a fraction of the rate of 600 

lineal feet per minute that would be required 

commercially. 

2. There appeared to be no way to design 

around the problem of excessive water loss in 

scanning unedged flitches. 

3. Excessive space would be required In 

a mill for the longitudinal movement of the 

lumber. 

4. The system could not adequately 

resolve defect location In the width direction 

where it is most important. 

5. Alinement of lumber passing through 

the scanner could not be adequately controlled. 

When a new concept-transverse scanning-was conceived without the above 

limitations, the longitudinal scanning system 

was abandoned. 

Transverse Scanning 

The new concept, designed to overcome 

previous difficulties, involves moving the 

board transversely past ultrasonic transducers 

that span the full board length. This approach 

permits a slow scanning speed, allows the 

scanning of unedged boards, provides adequate defect resolution, and adapts well to 

present product flow rates. 

The board transporting mechanism consists of opposing sets of chains that carry the 

boards into the water couplant and past a bank 

of transducer modules. The chain sets are 

staggered so the chain does not obstruct the 

transducers (fig. 21). The chain sets maintain 

equal pressure and constant speeds when 

transporting unedged boards and varying 

board thicknesses. 

The transducer modules are designed to 

hold 17 half-inch-diameter receiving 

transducers spaced 3⁄4 inch on center. Each 

module has a scanning path of 12 inches. By 

incorporating the comparative technique for 

locating defects, each module will provide 16 

bits of “go” or “no-go” information. Alternate 

modules are staggered, with the end 

18 
Figure 20.–Unedged lumber flitches as they are sawn from a log 

M 144 495 

transducer of each module scanning the same 

path as the end transducer of the next module 

(fig. 21). The transmitting transducers will be 

directly opposed to the receiving transducer 

banks. 

A scanning rate of 50 feet per minute will 

mean an actual product flow rate of 600 to 650 

lineal feet per minute. Data, handled by a 

microprocessor. can be taken every 1/10-inch 

across the board width, giving excellent 

resolution to defect width. One concern with 

his approach Is the large number of ultrasonic 

transducers necessary to span the full board 

length. Simpler design, increased flexibility, 

and smaller space requirements are expected 

to offset this concern. 

A board profile scanner using an optical 

technique and a board identification marking 

system are included in this scanner design. 

These functions, obtained before the board 

enters the water couplant, are necessary in addition to defect location to make and execute 

the sawing decisions. 

Figure 21.–Upper: Proposed 1-foot transducer modules arranged for 100 percent 

scan of a board and enabling handling 

with chain conveyors. 

Lower: Side view of chain conveyors 

proposed for moving lumber sideways 

(transversely) into a tank of water and 

past the ultrasonic transducers. 

M 146 235 

19 
SUMMARY 

The research reported in this paper has 

been aimed at eventual development and use 

of a production-type lumber scanner that will 

scan at production speeds. An ultrasonic 

system is now capable of describing lumber 

quality in digital form so a computer can make 

and carry out complex processing decisions to 

maximize conversion efficiency. The ultrasonic 

lumber-defect-detection system described in 

this report has been demonstrated to be a 

viable means for locating defects In lumber of 

all species, green or dry, and rough sawn or 

surfaced. 

With any flaw-detection system 

developed for detecting defects in lumber, 

there will be detection errors; some from identifying too much area as defective, and some 

from missing defective areas. It is known that 

the ultrasonic technique, as developed, will not 

reliably locate all of the defects found in 

lumber. Defects such as cracks, splits, and 

grub and worm holes can be oriented in such a 

way that the sound waves are undisturbed by 

their existence, and therefore escape detection. Future refinements and improvements to 

the technology are expected to keep these 

errors to a minimum. Optical techniques for 

locating visible surface defects would seem to 

complement the ultrasonic technique quite 

successfully, as well as to provide board 

profile data to the computer. Through experience in this research, it is expected any 

lumber scanning system will incorporate more 

than one flaw-detection technique. 

Research efforts have been intentionally 

aimed at distinguishing between clear areas 

and defect areas in wood. However, many 

decisions made in processing lumber involve 

permitting certain defects or blemishes in the 

product where they might enhance final 

product appearance or where they will not 

show in the final product. Being able to locate 

the defects will allow the computer to place 

them in an acceptable position at the time the 

processing decision is made. Additional 

research is necessary to determine if defects 

can be type classified so that certain ones, 

such as sound knots, can be excluded as 

defects in the processing decision. 

Successful application of these reported 

research results will require a difficult but 

presumably possible conglomeration of disciplines to eventually realize a commercial 

lumber scanning system that will operate at 

production speeds. These talents include 

mechanical and electrical engineers along with 

interested personnel from the wood industry, 

machinery manufacturers, the computer industry, and the electronics industry. 

Finally, it has been determined that sound 

can be successfully propagated through wood 

for detecting defects. In the future, ultrasonics 

could possibly lead to determining internal 

quality in logs, and making computerized, 

quality-based, log-sawing decisions. 

20 5.0-21-5-78 
LITERATURE CITED 

1. Burmester, Arno. 

1967. Nachweis von Asten im Kiefernholz 

durch Ultraschall (Detection of Knots 

in Pinewood by Ultrasonics). Holz RohWerkst. 25(4):157-163. 

2. Erickson, B., and D. C. Markstrom. 

1972. Predicting softwood cutting yield 

by computer. USDA For. Serv. Res. 

Pap. RM-98. Rocky Mountain For. 

Range Exp. Stn., Fort Collins, CO. 

3. Hallock, H., and L. Gallger. 

1971. Grading hardwood lumber by 

computer. USDA For. Serv. Res. Pap. 

FPL 157. For. Prod. Lab., Madison. WI. 

4. Lee, I.D.G. 

1958. A non-destructive method for 

measuring the elastic anisotropy of 

wood using an ultrasonic pulse technique. J. Inst. Wood Sci. 1:43-57. 

5. Lucas, Edwin L., Jr. 

1973. Long length cuttings from No. 2 

Common hardwood lumber. USDA 

For. Serv. Res. Pap. NE-248. Northeastern For. Exp. Stn., Broomall, PA. 

6. McDonald, K. A., Roger C. Cox, and E. H. 

Bulgrin. 

1969. Locating lumber defects by ultrasonics. USDA For. Serv. Res. Pap. FPL 

120. For. Prod. Lab., Madison, WI. 

7. McDonald, K. A. 

1971. Locating defects in softwood 

lumber. M.S. Thesis. Department of 

Forestry, University of Wisconsin, 

Madison, WI. 

8. McDonald, K. A., and Roger G. Cox. 

1972. Ultrasonic detection of lumber 

defects. United States Patent No. 

3,664,180. 

9. McMaster, R. C. 

1959. Nondestructive Testing Handbook. 

Vols. 1 and 2. Ronald Press, New York.