The calibration of a transducer with static forces
This section only discusses the calibration of a transducer with static forces. The dynamic aspect is not covered.
When the relationship between the force applied to a dynamometer and the measurement of its output signal cannot be accurately determined during manufacturing or by means of a calculation, it is necessary to calibrate the dynamometer. This operation comprises establishing the exact relationship between the force applied to a dynamometer – input – and the electrical signal it releases – output.
In essence, the operation involves applying forces that can be accurately measured to a dynamometer and registering the values provided by the electronic equipment connected to the transducer.
The results obtained are displayed in a value table or mathematical polynomial enabling the person using the dynamometer to know the value of the force from the indication provided by the connected electronic equipment.
Determination of the related uncertainty is also part of the calibration. This is established according to the “Guide to the Expression of Uncertainty in Measurement” published by the International Bureau of Weights and Measures (BIPM).
Calibration is usually performed by applying the protocol established by ISO 376. Standard ASTM E74 is also a protocol issued in the USA. Other calibration protocols can be defined according to the use of the dynamometer and the level of uncertainty sought.
Calibration of a dynamometer must be performed with forces referenced to the force unit – the newton – as defined by the International System of Units (SI). Traceability of this system is only formally guaranteed by calibration laboratories applying ISO 17025 and accredited by one of the national accreditation bodies (SMB, COFRAC, DKD, UKAS, etc.).
Application of effort to the transducer
The calibration bench must be able to apply unidirectional forces to the force transducer. For this purpose, the axis according to which forces are applied by the bench must be materialised and the transducer must be centred on this axis. This is performed by means of a rigorous mechanical assembly of the transducer on the calibration bench.
Regardless of metrological quality, a calibration bench will never generate perfectly unidirectional efforts. Due to imperfections in the bench, interfering effort and torque components are also applied to the transducer.
Indications provided by the dynamometer are affected according to the sensitivity of the transducer to these efforts. The electrical signal increases or decreases according to the direction of these interfering components. However, the direction of these are generally constant for a given force of the calibration bench. The importance of the interaction between the calibrated dynamometer and interfering efforts generated by the calibration bench is highlighted by a particular procedure.
Several series of efforts are applied, and between each series the transducer turns around its effort axis in an angle which represents a whole divided by the number of measurement series, resulting in a:
- 180° angle for two series,
- 120° for three series, or
- 90° for 4 series.
Average results obtained at a given load level largely removes a lot of the influence of interfering efforts and their dispersion, or reproducibility, and is a good indicator of this influence.
In fact, a transducer's response to these solicitations depends largely on the way the applied strains are distributed inside the transducer. For this purpose, geometric and functional characteristics of mechanical pieces mounted on the calibration bench are essential and condition the calibration results. Thus, they must be accurately defined and described.
Electronic device connected to the transducer
Two cases must be considered, depending on whether or not the transducer is delivered with its associated electrical device.
Force sensor and measuring electronics
If this measurement chain is fitted with a calibration device, this device must be operated, its results registered and eventually adjusted according to the user's needs. If a calibration adjustment is performed, this will entail a loss of traceability of measurements taken with the dynamometer, so calibration values obtained before and after the adjustment must be registered as any modification after calibration renders this operation invalid.
Single force sensor
When only the force sensor is being calibrated, the calibration laboratory connects it to a measurement device supplying power to the force sensor and allowing the output signal to be measured. The characteristics and adjustments of this electrical supply and measurement devices must be accurately defined in agreement with the user of the force sensor. Electrical connections must be specified. Traceability of electrical measurements to the International System of Units (SI) must be guaranteed.
Calibration results will only be applicable if the user then connects the transducer to a measurement chain with similar characteristics and settings as those obtained during calibration, and with a similar guarantee of electrical traceability.
In all cases, when calculating the uncertainty of the forces measured by means of this dynamometer, the user must include components that take into account the replacement of the electrical device associated with the transducer.
NOTE : Spanish version /en/tecnologia/sensor-estandar/calibracion-de-un-dinamometro