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Even with hundreds, thousands, or even millions of products sold, many companies will individually stamp their logo on every product they sell. The marking of logos and the etching of logos are done by laser marking machines , a process that requires very high precision. As technology advances, in order to perform more delicate markings, the designers of these systems are under pressure to make laser marking machines more accurate.
Laser markers ( Figure 1 ) use high-intensity, low-power lasers to etch very precise designs on anything from cell phones to hand tools to printed circuit boards (PCBs). To achieve the desired output, the laser needs to be very carefully guided with the aid of a precise digital-to-analog converter (DAC).
Figure 1: Laser cutting machine
So how does the DAC control the laser? The DAC is responsible for providing a very precise output voltage that will be used as an analog input to the motor. Each specific analog input code to the DAC is associated with a specific motor position. This motor is responsible for moving a mirror that can be repositioned in the x, y, or z plane to direct and reflect the laser and position it on the end device to change the material surface of the end device and etch a logo, text, or barcode. See Figure 2 .
Figure 2: TI’s DAC11001A provides the output for an analog motor control loop
As products that need to be etched become smaller (such as PCBs and some consumer products), the accuracy of laser marking systems must increase. TI's DAC11001A and DAC91001 have 20-bit and 18-bit resolutions, respectively. Resolution , an important parameter, translates into the number of voltage steps available at the DAC output. For example, 18-bit resolution has 262,144 unique codes ( see Figure 3 ), allowing the laser to be controlled from multiple motor positions. A 20-bit DAC has 1,048,576 unique codes, providing finer granularity and higher accuracy.
Figure 3: DAC resolution to code number calculation (16 to 20 bits)
What are the other benefits of a laser marking system that takes advantage of 20-bit DAC performance? If a full revolution of the motor is equal to 1 radian, what step size do you need? Existing systems have a resolution of 10 microradians , compared to a full-scale range of about 1 radian . Ideally, this would equate to 18 bits of resolution, but many designers want 20 bits of resolution due to system-level nonlinearities . At this point, the DAC11001A can provide nearly four times the number of output codes and, by extension, finer control of the motor.
Another issue to consider is motor vibration. Any fault in the laser marking system will have an adverse effect on the final etch. Such systems are very sensitive to residual motor vibration due to the multi-stage transfer function of the control loop . From selecting low-vibration motors to using complex control system logic, designers use a variety of complex techniques to optimize performance. A key contributor to motor vibration is the spike pulse generated by the DAC due to input code-to-code changes . The DAC11001A and DAC91001 have very low 1nV-s code-to-code, code-independent spike pulse output. This is achieved by integrating a track-and-hold circuit that isolates the output of the DAC from the spike pulse caused by the code change inherent in the internal ladder resistor network.
As we can see, laser marking machines must deal with many variables to achieve higher accuracy. DAC plays a key role in solving this problem and can make the designer's job easier. Innovative solutions with higher resolution can improve accuracy and achieve better error prevention, which is very important in laser marking system design.
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