Measurement System Types and Characteristics


What is an Arm Type Three-Dimensional Measuring Machine?

An arm-type three-dimensional measuring machine, or coordinate measuring machine (CMM)is a type of CMM that measures using an arm with multiple joints.
Compared to a gantry type CMM, the accuracy is generally inferior, but the degree of freedom in installation is relatively high, and this device is often used for three-dimensional measurements at manufacturing sites. Originally, these CMMs would measures using a contact probe, but recently there have been an increasing number of types that can measure both with contact and non-contact.

Measuring Principle of Arm Type Three-Dimensional Measuring Machine

Each joint is equipped with a rotary encoder from which the angular information of the joint movement is obtained. The distance between each joint is defined by the arm length between the encoders.
By combining this angle information with the distance (arm length) information between each joint, the coordinates of the measuring unit at the tip are calculated.
The number of rotation axes of joints varies depending on the model, but 6 axes and 7 axes are generally used.


Arm-type three-dimensional measuring machine configuration

Arm part
Control PC

The measurement unit consists of a base, arm, joint, and a probe with a measuring trigger and a stylus. The stylus can be replaced with different types depending on the purpose.
In addition, the software-based measurements are generated on a dedicated PC, and it is used by connecting to the measuring unit.

Features and disadvantages of the arm-type three-dimensional measuring machine

Features of the arm-type three-dimensional measuring machine

The advantage of the arm-type CMM is the flexibility of the measurement location. Because it is relatively light and portable, it can be installed in a place where measurement is desired for measurement.
Generally, it is used by fixing the base part with a magnet, etc. as shown in the figure below.

It is also used to measure objects that are relatively large compared to the gantry type CMMs. Compared with a large-scale gantry-type CMM, an arm-type CMM can achieve measurement without taking up a large space.

Disadvantages of the arm-type three-dimensional measuring machine

On the other hand, the arm-type CMM also has some disadvantages.

①Concerns about measurement accuracy due to mechanical structure

The accuracy of the arm-type CMM is guaranteed on the assumption that the physical distance between the encoders is a constant. Therefore, there is a concern that the accuracy will deteriorate if the arm or joint, which is a component, is deflected or twisted, or if the joint is worn due to repeated use. Therefore, it is common to perform maintenance and recalibration regularly.

②Limit of the measuring range

Of course, the reach of the arm is determined by the length of the arm.
The measuring range of a typical arm-type CMM is from about 3m to 4m, even if it is large. The longer the arm, the greater the effect of deflection as described above.

③Measurement Routing and Ease of Handling

The base of the arm-type CMM and the measurement target point must always be physically connected. When measuring a deep part or measuring a narrow place, it may be difficult to make measurement because of the physical arm part. Also, when attempting to measure the edges of a large object, the arm must be moved greatly each time. Since the arm is quite heavy, the person taking measurements can easily become fatigued when measuring for a long time.

Keyence WM Series is recommended for large three-dimensional measurements

The WM series is a three-dimensional measuring machine for large products that measures by placing a handheld wireless probe against an object. Coordinate measurement is possible in the range up to 15m using the probe and camera head unit.

①Continuous measurement with stable accuracy

Because of the optical measurement, there is no fear of wear on the mechanical parts, and repeated use will not adversely affect the accuracy of the measurements.
It is equipped with a self-diagnosis function, so it is possible to check whether the measurement is performed with the correct accuracy before measurement. It is also equipped with a function to self-compensate if the accuracy is affected by some cause.

②Wide measuring range

A large measuring range of up to 15m allows coverage for parts of all sizes.
The WM Series can automatically follow and measure anywhere in the range with probes.
In addition, even when the probe’s line-of-sight is blocked, the camera can be moved so that it does not become a bottleneck of measurement.

*Maximum measuring range

③Ease of use for perfect wireless probes without arms or cables

The WM series probe is lightweight and only need one hand for use. You can take measurements simply, by holding it in a place where you want to measure it. In addition, since the main measurement elements can be selected from the touch panel on the probe, there is no need to move back and forth between the control PC and the object to be measured.

Please be sure to check the information on the WM series, which is more user-friendly than the arm-type CMM.