1. Introduction Currently, diamond stylus profilometers using contact measurement methods still dominate the measurement of surface roughness in engineering. These instruments have advantages such as reliable measurement, convenient operation, moderate price, and compliance with international standards for surface roughness evaluation. However, diamond stylus profilometers have the following disadvantages: (1) the sharp diamond stylus may damage the surface of the workpiece under certain measurement pressure, and affect the authenticity of the measurement results; (2) the stylus may be damaged during the measurement process; (3) the range of the stylus profilometer is small (approximately ±100μm), making it difficult to measure the surface roughness of curved surfaces with large curvature and deep grooves. Therefore, stylus profilometers are generally not suitable for measuring soft metal surfaces, biochemical material surfaces, rubber surfaces, information-containing surfaces, and ultra-precision machined surfaces. In order to overcome the disadvantages of contact measurement, optical needle scanning profilometers using non-contact measurement methods have been rapidly developed in recent years. For the measurement of engineering surface morphology, the focused optical needle scanning profilometer is a more ideal measuring instrument and has been applied in industrial production. However, this type of profilometer also has certain drawbacks: (1) it requires a high degree of cleanliness of the surface being measured; (2) the inclination of the surface being measured has a significant impact on the measurement results; (3) the reflectivity of the workpiece material and the height of the surface peaks and valleys have a significant impact on the measurement results. Therefore, it is best to have both of the above instruments in production to meet the needs of engineering surface measurement. At present, diamond stylus profilometer technology is mature, the price is moderate, and it is widely used in production; optical needle scanning profilometer has products on the market, but the price is high and the performance is still to be improved, and it is the key research and development object in the field of surface measurement. Therefore, it is of great significance to research and develop a surface measurement instrument with a large range, low price, and both contact and non-contact measurement methods, and the non-contact measurement technology is relatively mature. 2. Measurement System Structure and Principle The structure and principle of the surface roughness dual-purpose measurement system is shown in Figure 1. [align=center]Figure 1 Block diagram of the measurement system structure and principle[/align] The instrument is designed with two measuring heads, the left one is a non-contact measuring head, and the right one is a contact measuring head. When using a contact measuring head, the non-contact measuring head should be removed; when using non-contact measurement, the non-contact measuring head should be moved in so that the probe of the measuring head contacts the moving part of the voice coil motor. In this case, the contact measuring head becomes the second-stage measurement system of the non-contact measuring head. The optical probe sensor and the diamond probe sensor of the measurement system share a set of position detection circuits, data acquisition and control circuits, scanning system, worktable, computer system, and evaluation software, thus combining contact and non-contact surface roughness measurements and integrating the advantages of both methods. Different measurement methods can be selected on the same instrument according to the actual situation, thereby expanding the range of measurement objects. 3. Design of the Non-Contact Measuring Head The non-contact measuring head of the measurement system adopts an optical probe sensor based on the focusing detection method. The focusing detection method uses a focused beam as a probe and a photodetector to measure the minute defocusing amount of the microscopic undulations of the measured surface from the focal point of the microscope objective. The linear measurement value of the defocusing amount reflects the morphology of the measured surface. Methods for surface topography measurement using the focusing detection principle mainly include the Foucault knife-edge method, critical angle method, astigmatism method, and eccentric beam method. The focusing detection method has a vertical resolution of up to 1 nm and advantages such as simple optical path and ease of use; its disadvantages include a narrow linear range (approximately 30 μm) and the photodetector's sensitivity to changes in the reflectivity and microscopic slope of the measured surface. The dynamic focus tracking measurement method used in this system effectively overcomes these shortcomings, achieving a vertical measurement range of up to 500 μm and a measurement resolution of 10 nm. Its working principle is shown in Figure 1. The beam emitted by the semiconductor laser diode is expanded and collimated into parallel light. After passing through the polarizing beam splitter BS, a quarter-wave plate, and the microscope objective L1, it converges onto the surface of the workpiece being measured. The reflected light returns to the polarizing beam splitter BS along the original optical path, but at this point, the linear polarization direction of the reflected light has rotated by 90° compared to the original incident light. This beam then passes through lens L2 and is split into two beams by beam splitter L3, which are projected onto two sets of two-quadrant photodetectors, generating focusing error signals. When the beam focal point is located on the surface of the workpiece, the two reflected spots are located in the middle of the two sets of photodetectors, and the focusing error signal is zero. When measuring the surface morphology of the workpiece, the optical probe scans along the surface at a constant speed. The microscopic undulations of the surface morphology cause the photodetectors to continuously generate focusing error signals. These signals are processed and compensated to control the voice coil motor, which drives the microscope objective to make corresponding adjustments to ensure that the relative position between the microscope objective L1 and the measured point on the workpiece surface remains unchanged, so that the light spot is always focused on the measured surface. The continuous change in the position of the voice coil motor reflects the continuous change in the height of the measured point (i.e., the surface topography information). This change is acquired by an inductive measurement system independent of the focusing control system, thus ensuring good linearity of the measurement signal within the measurement range. Because a focusing error feedback control device is introduced into the measurement system, its vertical measurement range remains within the linear interval. Therefore, this system features a large measurement range, high resolution, and fast scanning speed. The microscope objective L1 is a key optical element in the system's optical path. Its function is to converge the laser beam output to form a high-quality diffraction-limited spot on the surface of the workpiece and collect the reflected beam for focusing error detection. The lateral resolution and sensitivity of the sensor are related to the optical characteristics and imaging quality of the objective. The main parameters of the microscope objective include the numerical aperture NA, the diameter of the diffraction spot d, and the depth of focus ΔZ. According to optical diffraction theory, the diffraction spot diameter d = Kλ/NA. If the spot is an Airy circle, when the intensity decreases radially to half of its maximum value, the corresponding K = 0.52, i.e., d = 0.52λNA, and the focal depth ΔZ = λ/2NA². It can be seen that as the NA value increases, the spot diameter d decreases, which is beneficial for improving the lateral and vertical resolution of the measuring head. However, with a fixed objective lens diameter, the lens is closer to the surface of the workpiece, which is not conducive to adjusting the measuring head, and the system wavefront aberration increases with the NA value. Therefore, the NA value needs to be optimized. The microscope objective lens used in this system has an NA value of 0.45, resulting in a focused spot diameter d = 0.76 μm and a focal depth ΔZ = 0.16 μm. The focusing servo system is an important subsystem of the non-contact measuring head, and its positioning speed and accuracy directly affect the speed and accuracy of surface roughness measurement. The task of the focusing servo system is to move the microscope objective as quickly as possible so that the focus falls back on the surface of the workpiece. To achieve this goal, a focusing actuator with good dynamic performance and a reasonably designed control circuit are required. The focusing servo mechanism uses a position loop, and its design needs to address issues such as the system's tracking accuracy and control system stability. After the actuator is well-designed, a well-designed focusing phase compensation network can improve system performance. 4. Design of Contact Measuring Head The stylus measurement method is the most widely used surface profile measurement method and is currently the internationally recognized standard method for two-dimensional surface roughness measurement. The stylus used in the stylus measurement method is generally made of diamond, with a tip diameter of approximately 2μm. During the measurement process, the stylus slides along the workpiece surface. The microscopic peaks and valleys of the surface cause the stylus to move up and down vertically along the measured surface. The sensor converts this vertical displacement of the stylus into an electrical signal, which is amplified and sent to subsequent circuits for processing, thus obtaining the microscopic profile information of the measured surface. Such instruments can generally be classified according to the type of sensor used, including inductive, piezoelectric, interferometric, and grating types. This measurement system uses an inductive sensor, which converts the vertical displacement of the stylus into a change in inductance. This change is then modulated, amplified, and detected by a sine wave to obtain a voltage signal proportional to the vertical displacement of the stylus. The AD698 is the core of this measurement system for inductance signal measurement. The AD698 is a monolithically integrated linear variable displacement sensor signal conditioning subsystem. Compared with traditional inductance measurement circuits composed of discrete components, it has advantages such as small size, low gain drift, low zero-point drift, and high signal-to-noise ratio. Therefore, it is easy to miniaturize the instrument and improve the measurement accuracy of the system. The peripheral interface circuit of the AD698 is shown in Figure 2. [align=center]Figure 2 AD698 peripheral interface circuit[/align] 5. Test Results The multi-line sample (Ra = 0.344 μm) calibrated by the China National Institute of Metrology was tested using both a non-contact measuring head and a contact measuring head. The measured surface profile curves are shown in Figures 3 and 4. From the profile curves, it can be seen that the measurement results obtained by the two methods are basically consistent. This measurement system does not use a guide head for mechanical filtering on the measuring sensor, thus more accurately reflecting the surface morphology of the workpiece. Since factors such as table motion error, workpiece surface shape, and surface waviness affect surface roughness assessment, the system's assessment software uses a least-squares orthogonal polynomial as the best-fit mathematical model for surface roughness assessment, and separates the surface roughness profile through digital filtering. The filtered template surface roughness profile curve is shown in Figure 5. [align=center]Figure 3 Template surface profile curve measured by non-contact method[/align][align=center]Figure 4 Template surface profile curve measured by contact method[/align]Figure 5 Filtered template surface roughness profile curve Edited by: He Shiping