Basics for measuring force, displacement, flow, speed
The technical features of the force transducers are substantially fixed by VDI/VDE guideline 2637. The most important terms are described below:
The load range, for which the guaranteed error limits will not be exceeded.
The nominal load is the upper limit of the measuring range. Depending on the sensor, the nominal load can be a tension or compression load.
The working load is the load that can be applied to the sensor, as well as the nominal load, without affecting the specified characteristics. The working load range should only be used in exceptional cases.
The load limit is the maximum permissible load that can be applied to the measuring cell without expecting a destruction of the measuring system. At this load the specific error limits are no longer applicable.
The breaking load is the load where a permanent change or destruction occurs.
Maximum dynamic load:
Rated force related oscillation amplitude of a sinusoidally changing force in direction of the measuring axis of the sensor. At a load of 107 cycles the sensor, when being repeatedly used up to the rated force, is not subject to significant changes regarding the metrology characteristics.
The drift error is the maximum permissible change of the output signal of the sensor over the specified time at constant load and stable environmental conditions.
ALMEMO® Force Measurement
ALMEMO® force transducers allow to adjust the constant load (tare) to zero and to enter the final value as nominal value. The correction value will be automatically calculated from this by the measuring instrument. An ALMEMO® connector that switches on this resistor for the adjustment is available for force transducers with integrated reference resistor.
Measurement of fast changes in force and force peaks with digital ALMEMO® D7 sensors
The new ALMEMO® V7 measuring system makes it possible to measure fast changes in force and force peaks with a temporal resolution of up to 1ms.
The (passive) measuring bridge of the force transducer measures the change in force without (electrical) delay.
The digital ALMEMO® D7 measuring plug ZKD7 00-FS works with an A/D converter integrated in the plug and with a measuring rate of up to 1000 measuring operations per second (1ms per measuring operation).
In combination with the ALMEMO® V7 measuring instrument, e.g. ALMEMO® 710, it is therefore possible to record fast changes in force and force peaks.
The measured values are evaluated in the WinControl software as table or line diagram (see chapter “Software”).
The overall accuracy of the measurement is determined only by the force transducer with the connected ALMEMO® D7 measuring plug, and is unaffected by the ALMEMO® display device / data logger and extension cables used. The complete measuring chain, consisting of force transducer and connected ALMEMO® D7 measuring plug, can be calibrated.
High resolution measurement with digital ALMEMO® D7 sensors
The digital ALMEMO® D7 measuring plug not only enables fast measurements but also high resolution measurements. Thereby, the measuring plug works with reduced conversion rate. Thus, stable measured values with high resolution can be achieved by using high-precision sensors. The user can easily configure the ALMEMO® plug on the ALMEMO® V7 measuring instrument.
Depending on the boundary and environmental conditions of the measuring task, different measuring methods can be used:
Linear inductive displacement transducer and displacement sensor:
Advantages: high accuracy, high resolution, robust, acceleration resistant, inexpensive, resistant to interference, very long-term stable, environmentally stable (dirt, humidity), spot measurement combined with basically non-contact measurement, easy assembly and handling
Conductive plastic potentiometers:
Advantages: high resolution, good linearity, inexpensive, good temperature and humidity coefficients, extensive application temperature range.
ALMEMO® Displacement Measurement
Our Potentiometric displacement sensors have been pre-aligned in the factory by storing the correction values in the ALMEMO® connector before delivery. The precise adjustment can be locally performed by the user with final measures after the installation
Measurement of fast changes in displacement with digital ALMEMO® D7 sensors
The new ALMEMO® V7 measuring system makes it possible to measure fast changes in displacement with a temporal resolution of up to 10ms. The potentiometric displacement transducer measures changes in displacement without (electrical) delay. The digital ALMEMO® D7 measuring plug ZKD7 00-FS works with an A/D converter integrated in the plug and with a measuring rate of up to 100 measuring operations per second (10ms per measuring operation). In combination with the ALMEMO® V7 measuring instrument, e.g. ALMEMO® 710, it is therefore possible to record fast changes in displacement.
The measured values are evaluated in the WinControl software as table or line diagram.
The overall accuracy of the measurement is determined only by the displacement transducer with the connected ALMEMO® D7 measuring plug, and is unaffected by the ALMEMO® display device / data logger and the extension cables used. The complete measuring chain, consisting of displacement transducer and connected ALMEMO® D7 measuring plug, can be adjusted.
The sensor contains a vane or paddle that starts rotating when a flow is present. Unlike the optical method, this method also allows for measurements in cloudy and non-transparent liquids. The rotational speed is proportional to the corresponding quantity of flow. The electrical output signal can be generated by two different methods:
Inductive Proximity Switch:
The rotor blades are provided with special steel caps, therefore, the rotor blades approaching the transducer cause a change of the inductance and the generation of a pulse type output signal.
The rotor is provided with permanent magnets that affect a Hall sensor, which is located on the transducer. The transducer electronics transforms the Hall signal into a pulse type electronical output signal.
For measuring the volume flow rate or for dosing tasks, the ALMEMO® sensor range includes turbine flowmeters for different measuring ranges and operating conditions:
Radial turbine flowmeters for large flow quantities.
Axial turbine flowmeters with rotating vane for small flow quantities
Optical Rotational Speed Meters
The optical reflection method has become the most accepted method for the measurement of revolutions of shafts, wheels, fans etc.
With single unit retroreflective photoelectric sensors the transmitters and receivers form one single unit. The light sent by the transmitter is, by an opposite located object, reflected to the receiver. The sensor performs a switch when the reflected amount of light exceeds a specific, adjustable limit value at the receiver. This quantity of light depends on the size and the reflection properties of the object. Special reflective tapes are used to increase the sensing range and to improve the signal-to-noise ratio.
ALMEMO® rotational speed sensors can be used in two measurement setups:
Retroreflective photoelectric sensor (DIN EN 60947: Type D)
Detects only opaque objects. The sensing range depends on the reflectivity of the object, i.e. on the surface quality and colour. Sensitive with regard to contamination and against changes of the reflective properties of the object
These influences can (within limits) be compensated by means of a sensitivity adjustment control Only small mounting efforts are required as the sensor is a single unit device and a rough alignment is sufficient in most cases.
Retroreflective light barrier (DIN EN 60947: Type R)
Retroreflectors allow for long sensing ranges and an improved signal-to-noise ratio. Low susceptance to interferences, therefore, highly suitable for use under harsh conditions, e.g. outdoor applications or dirty environments