Thickness Measurements Using Laser Triangulation | Sensors ... - IIPE

Thickness Measurements Using Laser Triangulation | Sensors Magazine 

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Motion/Velocity/Displacement 

Thickness Measurements Using Laser 

Triangulation 

April 1, 2010 

By: Don Welch, MTI Instruments 

Sensors 

Understand how best to use laser displacement sensors to measure 

the thickness of products at high speeds and from long standoff 

distances. 

Many processes require continuous monitoring of the thickness of a product to provide feedback 

to control product characteristics. Product thickness may change due to production line wear or to 

changes in the consistency and integrity of the feedstock. Typically you're measuring either piece 

parts (Figure 1A), or a sheet type of product as shown in Figure 1B. As production line speeds 

increase so does the cost of scrap product—you need to catch any thickness change quickly or 

provide active feedback to the production line. 

http://www.sensorsmag.com/sensors/motion-velocity-displ...ess-measurements-using-laser-triangulation-7050?print=1 (1 of 12)23/04/2010 10:56:30 AM


Figure 1. Laser sensors can measure the thickness of piece parts (A) or a continuous feed (B) 

Why Use Laser Sensors? 

As optoelectronics have improved, so have laser displacement sensors, with increased 

resolution, accuracy, linearity, and frequency response. The introduction of CMOS optical 

displacement sensors has resulted in lower-cost laser displacement sensors that are starting to 

rival other traditional displacement sensors such as eddy current and capacitive sensors. Laser 

displacement sensors (see sidebar "The MTI Microtrak II") are also suited to make difficult 

thickness measurements. For example, they are not bothered by humidity or water vapor that can 

affect capacitive sensors; they are relatively immune to temperature and acoustic variations that 

can affect ultrasonic sensors; and they are not sensitive to target conductivity or magnetization 

that affects eddy current sensors. Laser displacement sensors can measure equally well to just 



about any target material and typically have operating ranges of several inches, with standoff 

distances ranging from several inches to several feet; capacitive and eddy current probes 

typically have operating ranges of tenths of inches. Large standoffs help keep the sensors out of 

harm's way when something goes wrong with the process line as well as allowing for relatively 

large changes in thickness without the need to reset the sensor for different production runs. 

Recently, laser displacement sensors have begun to shed their bulky controllers, integrating their 

control electronics within the sensing head. 

Computing the Thickness 

The standard thickness equation is shown 

in Equation 1: 

where: 

G = total gap 

T = target thickness 

gap from sensor 1 to 

A = 

target 

gap from sensor 2 to 

B = 

target 

(1) 

To solve for target thickness (Figure 2) we 

can rewrite the equation, solving for T. This 

isn't a serious issue when using absolute 

sensors such as capacitance probes; you 

can run the voltage outputs into a summing 

amplifier that inverts the output. When 

using sensors that have a relative 

calibration with bipolar outputs, as shown in 

Figure 3, it is slightly more difficult to Figure 2. Measuring thickness 

compute the thickness. For example MTI's 

Microtrak II controller outputs a bipolar ± 5 V voltage centered about the specified standoff 

distance. You can't run the analog outputs into an inverting summing amplifier because the 

Microtrak's output voltage polarity changes, resulting in errors when you pass through the center 

of the range. Mathematically the thickness equation looks like Equation 2: 

(2) 



Figure 3. Using sensors that have a relative calibration and bipolar outputs 

Practically speaking, most users ignore the standoffs and treat them as one big DC offset and 

only deal with the active range of the heads once they're set up. For example, we could sum 5 

VDC with each of the heads to make the controller output always positive (0–10V unipolar) and 

then sum the two outputs, invert the output, and run it into a voltmeter. After setup, place a target 

of known thickness—the golden target—between the lasers and then zero the meter. All readings 



are now relative to the target with known thickness. Multiply the voltmeter's voltage by the 

sensitivity factor of the laser heads to get the change in thickness relative to the golden target. 

Displaying the Thickness Data 

There are many ways to display the thickness data depending on your budget and how elaborate 

your measurement setup is. One simple method that can be used for one to three slow-speed 

thickness channels (with two heads per thickness channel) is to connect stand-alone measuring 

heads to a laptop computer through an RS-485 to USB converter. One converter can handle from 

one to six heads to provide up to three thickness points. The heads all tie in parallel to the same 

USB converter and the heads are addressed individually by the display software (Figure 4). 

Figure 4. The display shows a real-time plot of the individual head's displacement and the resulting 

thickness measurement 

For those who do not like programming or don't have the time, another solution is to use a 



precision analog summing amplifier with two laser heads, a digital panel meter (Figure 5), and a 

switch. This allows you to select between individual heads during setup and then to select the 

thickness module's output when running. Typically, once the heads are set up you insert a target 

of known thickness and adjust the thickness module to 0 V. Many panel meters also have one or 

more built-in alarm contacts that allow triggering an alarm event based on voltage set points. 



Figure 5. Using a precision summing amplifier, two laser heads, and a panel 

meter, you can display thickness data 

High-speed systems for industrial control and alarming require higher speed communications. 

Ethernet connections allow communication speeds of up to 100 Mbps and there are a number of 

off-the-shelf analog to digital converter (ADC) components available to build such a system. 

In the system shown in Figure 6, a precision MTI thickness module is used to combine the 

analog outputs from the laser heads and to provide a high-speed, real-time analog thickness 

signal. Advantech ADAM 6017 DA modules convert the thickness and individual displacement 

readings into high-speed Ethernet packets that are read by an off-the-shelf HMI. Advantech 

ADAMView software ties the whole system together. An extra ADAM module used in conjunction 

with a proximity sensor alerts the system that a target is present if discreet sheets are being 

measured. A standard multiport Ethernet switch ties all the Ethernet cables together. 



Figure 6. Coupling laser sensors with DA modules and an HMI provides high-speed analog 

thickness data 



Why Use Synchronized Lasers? 

When measuring thickness, the lasers must sample the 

thickness at the exact same instant or else an error will be 

introduced. The size of the error is dependent on several 

external parameters such as target speed, vibration, and the 

nature of the surface of the target material. Even though slow 

process speeds may be involved, high-frequency vibrations can 

contribute fairly significant errors if the heads sample the target 

even tenths of milliseconds apart. Let's say that you have a 

sheet of aluminum with 60 Hz vibration present from a conveyor 

motor. If the vibration was 0.002 in. and the laser heads 

sampling was delayed by 0.008 s (125 Hz sampling) then you 

could see a thickness variation of up to .004 inch. MTI's lasers 

share control signals that ensure synchronous sampling occurs 

within 0.00001 s. In the same application the thickness variation 

due to vibration would be


Figure 8. Proper design concept where both sides of the sensor supports are fixed to prevent 

unwanted vibration. One side can be made removable so the structure is easily disassembled for 

access or service 

Laser Adjustment 

One of the simplest ways to adjust the lasers is first to adjust their standoff to the target for 

nominal center-of-the-range standoff distance. Remove the target and replace it with a flat sheet 

of translucent white paper so the two laser spots may be observed simultaneously. The sheet of 

paper should be at the same location as the target material would be. Next, adjust the heads such 

that the spots remain superimposed upon each other as the paper is moved back and forth 

between the range of the lasers in a perpendicular direction. As you move the paper back and 

forth, perpendicular to the heads, you'll see one spot gets larger while the other gets smaller. This 

is because the laser beams are focused at the nominal standoff distance. As you move away 

from the nominal standoff, the beams expand. Once you have the laser adjusted for continuous 

concentric coaxial alignment throughout the range (Figure 9) be sure to lock the adjustment 

screws down or cap them to prevent unauthorized users from tampering with them. 



Figure 9. A set up to adjust the lasers 

System or Thickness Calibration 

Although the individual laser heads are already calibrated, you'll have to calibrate the assembled 

system for thickness. Place a target of known thickness in between the lasers and tare the 

system to zero or set the readout device to read the actual thickness of the known target. The 

system function for thickness is Thickness = Total gap – (laser gap 1 + laser gap 2). 

Overall System Accuracy 

Assuming you haven't introduced any additional error with poor fixturing, the overall measurement 

uncertainty will be 2*max linearity error + 0.5 the peak-to-peak (pk-pk) noise at the bandwidth 

selected because you're using two heads. For a head linearity of ± 0.05% F.S. where F.S. = 10 

mm, if the dynamic noise for the same heads is 3 µm pk-pk then the total measurement error 

could be as high as 10 µm + 1.5 µm = 11.5 µm. 

Fixture vibration and surface finish effects will also add to this uncertainty, and could easily 

increase the uncertainty to 23 µm. Most laser head dynamic noise or uncertainty is given at a 

specific bandwidth and when measuring to white paper. Actual targets can easily cause larger pkpk 

noise values. Averaging techniques such as low-pass filtering or a digital moving average can 

help reduce uncertainty due to surface finish effects and head noise and can sometimes make an 

impossibly noisy measurement both possible and statistically useful. 

Closing Thoughts 

The above are just a few ideas to help make your thickness setup as foolproof as possible. 

Following these guidelines will help ensure the highest accuracy and repeatability of your 

thickness data. 

ABOUT THE AUTHOR 

Don Welch is Director for New Business Development at MTI Instruments, Albany, NY. He can 

be reached at 518-218-2503, dwelch@mtiinstruments.com. 



The MTI Microtrak II 

MTI's Microtrak II stand-alone laser sensing devices have an integrated CMOS sensor within 

the head and a built-in processor for data averaging and filtering. Both sensors communicate 

with each other to synchronize their data sampling, which ensures the correct thickness reading 

and eliminates the need for a separate controller. The system provides both analog (0–10V or 

4–20 mA) and digital outputs (RS-485 binary format). Either can be used to determine 

thickness, but analog is preferred for very high frequency (>100 Hz) response. 

The sensors provide high-speed resolution within a combined accuracy of ±0.1% of the 

measurement range, a data-sampling rate of 40 kHz, a bandwidth of 20 kHz, and standoff 

distances from 25–300 mm. A dynamic laser feedback loop allows accurate measurement 

without regard to color, ambient light changes, variations in object reflectivity, speed, 

temperature of material, or general plant environmental conditions. The feedback loop allows 

the sensor to make virtually error-free measurements with high reliability and repeatability up to 

40,000 times per second within a defined spot, while operating from a single 12–30VDC supply. 

The thickness system has measurement ranges of 2–200 mm, depending on the head 

selection, and can operate at temperatures from 0°C–40°C. The sensor is enclosed in a sealed 

aluminum housing to protect it from moisture and dust. An external air purge provision can be 

provided to keep the optics dirt and dust free. 

About the Author: Don Welch 

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About Don Welch 

Articles by Don Welch 

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