ISO 25178-601: Design and Characteristics of Contact (Stylus) Instruments

1. Design and Characteristics of Contact Stylus Instruments

ISO 25178-601:2025 (second edition) specifies the design, metrological characteristics, and nominal characteristics of contact stylus instruments for areal surface topography measurement. This standard replaces both ISO 25178-601:2010 and ISO 3274:1996, consolidating profile and areal stylus instrument requirements into a single document. The instrument must comprise a probing system, an areal reference guide, a lateral scanning system with drive units for both x and y axes, and a lateral position sensor. For profiling-only instruments, the y drive unit is optional and the areal reference guide can be replaced by a linear reference guide.

The main components of a contact stylus instrument include the lateral scanning system with its reference guide, the probing system, and the software for mathematical processing. The probing system houses the stylus and generates electrical signals proportional to surface height variations. Three basic probing system designs are described: inductive probing systems using a pivot and measuring force generation, interferometric probing systems using laser interferometry for position detection, and scale-based probing systems with integrated linear encoders. Each design has distinct characteristics regarding linearity, hysteresis, and critical dynamic behavior.

2. Probing System Design and Measurement Process

Three basic probing system designs serve different measurement requirements. Inductive probing systems use a pivot mechanism with a spring-loaded measuring force generation system, where the angular displacement of the stylus arm is detected by an inductive sensor. These are the most common type due to their robustness and good linearity over moderate measurement ranges. Interferometric probing systems use a laser interferometer integrated into the probe to measure the vertical displacement of the stylus with higher resolution and better linearity, making them suitable for ultra-precision applications. Scale-based probing systems incorporate miniature linear encoders directly in the probe assembly, offering good linearity and insensitivity to environmental factors.

The measurement process follows a sequential pattern: profile acquisition along the x-axis, return to starting position, step in the y-direction, and repetition. The extracted surface contains n profiles separated by the y-sampling distance. The arcuate motion of the stylus introduces a systematic distortion that is negligible for small deflections but must be considered for contour-capable systems. The critical dynamic of the probing system (vdyn,c) defines the maximum scanning speed before output signal distortion occurs, which depends on the mechanical inertia of moving parts and the surface characteristics being measured. Engineers should optimize scan speeds to operate below this critical threshold for accurate measurements.

3. Influence Quantities and Engineering Best Practices

Key influence quantities include: stylus tip geometry (radius and cone angle determine lateral resolution and maximum measurable slope), probing system transfer function (affects amplification and linearity), areal reference guide flatness (superimposed on measurements as systematic error), and lateral position sensor accuracy (determines x-y mapping fidelity). The Hertzian contact pressure between the stylus tip and the surface is another critical consideration, especially for soft materials where permanent deformation can occur. The standard specifies that the nominal static measuring force is 0.00075 N at the mean position of the stylus, with a nominal rate of change of 0 N/m.

The standard replaces traditional ISO 3274 concepts with the comprehensive metrological characteristics framework from ISO 25178-600. Terms like “probe linearity deviation” and “total deviation of the stylus instrument” are superseded by generalized concepts including amplification coefficient, linearity deviation, and topography fidelity. This unification ensures that stylus instrument performance can be directly compared with optical instruments on the same metrological basis. For practical engineering work, diamond probe tips with a 60 degree cone angle and 2 micrometer radius are the default configuration. Larger cone angles and tip radii improve durability and reduce contact pressure at the cost of reduced sensitivity to fine surface structures. When measuring soft materials such as polymers or thin films, the contact pressure must be carefully evaluated to avoid surface damage.

4. Frequently Asked Questions