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How to Measure Position and Speed

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In many fields, industrial or otherwise, motion measurement (position or speed) is essential information to ensure the proper operation of systems and machines.

There are multiple devices available to perform these measurements. These tools, often called position sensors, present characteristics that vary from one another. It is therefore necessary to select the sensor which presents the characteristics best matching the measurement to be carried out.

This article aims to explore the different solutions available to the user by presenting the most suitable for each type of application. For this we will separate the analysis of the applications in two: the type of motion and the type of measurement.

TYPE OF MOTION

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We will first look at the type of movement to be measured. Motion is normally divided into linear and rotational components (vectors). These two types of components are generally analyzed (i.e. measured) independently.

ROTATION

Rotation is when a system rotates around an axis. Examples include mechanical arms and motor valves. The measurement will be given in angle units.

Singleturn

A rotation is called "single turn" when it does not exceed 360°. In this context, the orientation of the system with respect to a reference axis is simply measured. Two different position sensors can then be used to make such a measurement:

The main advantage of inclinometers is that they can be mounted virtually anywhere and in any position on the system: they do not need to be attached to the rotation axis to measure an inclination. Considering in addition their very compact size, they will be therefore naturally recommended when space is a major constraint.

In comparison, single-turn encoders (absolute single-turn rotary encoders) must be mounted on the shaft. However, their dynamic performance and accuracy are often better. They also have very good shock and vibration resistance and present the huge advantage of being available with all interfaces on the market. Finally, the magnetic version of these encoders is both more economical, more compact and less sensitive to environmental aggressions (humidity, dust, shocks, temperature).

For more detailed information, you can consult our analysis Comparison of Inclinometers and Singleturn Rotary Encoders .

Multiturn

Multiturn rotations concern movements logically involving several turns. For these movements, not only the position and/or speed of a revolution but also the number of turns will be measured. In these cases, only absolute multiturn rotary encoders allow motion tracking.

You will find more information in a detailed article on absolute encoders .

We have also written an explanation on the difference between single-turn and multiturn encoders .

TRANSLATIONAL MOVEMENT

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A translational movement is a movement that occurs along an axis, parallelly to it. This is often referred to as linear motion. The measurement is processed in length units.

Draw Wire

Linear motion sensors consist of a rotary encoder coupled to an adapter whose purpose is to convert a translational motion into rotary motion. A cable is wound and unwound around an axis, allowing measurement via a rotary encoder. One end of the cable is attached to the system whose displacement is to be measured while the encoder part remains fixed with respect to the reference frame.

Draw wires can include any encoder and therefore can have all of their characteristics. The user consequently has all existing output interfaces at his disposal. However, these have the disadvantage of their relatively important housing volume and the need for wire placement. The resolution will depend on the linear measuring range .

Measuring Wheel

A measuring wheel is a component that is fixed to the shaft of a rotary motion sensor. This is driven in rotation by adhesion to its outer diameter. This is usually done by attaching it to a conveyor belt. It is the same principle as the pinion/rack and pinion torque (the gear wheels being replaced by the grip).

This system is interesting when you have a small space dedicated to the linear motion sensor and/or an "endless" movement, such as a conveyor belt.

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Type of measurement

The section will define the type of device to be used, at least in part. Indeed, it is also necessary to define the type of measurement that one wishes to carry out. Depending on this, one will be directed towards different characteristics.

Absolute vs Incremental

RELATIVE (INCREMENTAL)

In some applications, it is only necessary to measure a movement in comparison with an initial situation (no fixed reference point) i.e. a displacement from the previous moment or a speed. This kind of relative measurements can be perfectly realized thanks to incremental encoders. This applies not only to rotational movements but also to translational movements where the motion sensor consists of 50% of a rotary encoder.

Incremental encoders have the significant advantages of being very economical and compact in size. When only speed and/or relative position are required, this solution is preferred.

ABSOLUTE

However, some control systems need to know the exact position in relation to a reference frame that does not evolve as the work steps progress. Absolute encoders and inclinometers have been designed exactly for this purpose. It is obviously possible to add an offset and perform a reset at a given time. Being able to measure speed too, the main difference will therefore lie in this ability to follow the movement throughout the cycle, even after a power cut!

But this additional function is not without counterpart. The technology required for turn counting comes at a cost. Absolute encoders will therefore be significantly more expensive than an incremental encoder.

It is interesting to note that POSITAL turn counting technologies are battery-free (less maintenance!). In particular, the Wiegand effect technology represents a technological leap in the sensor field.

Conclusion : which one to choose ?

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Eventually, when asking yourself the question “which position sensor should I choose for my application?”, try to first analyze what type of movement you intend on measuring and what sort of measure is necessary for your control system to work. POSITAL tries to provide you with the best products and solutions while assisting you during your design phase.

WHY POSITAL?
POSITAL is a manufacturer of sensors for motion control and safety assurance systems. The company’s products, which include rotary encoders, inclinometers and linear position sensors, are used in a wide range of settings, from manufacturing to mining, agriculture to energy. POSITAL is a member of the international FRABA Group. FRABA Group is a market-leading enterprise that makes use of advanced product design and manufacturing process to ensure that its customers enjoy the benefits of technology leadership, choice, product quality and competitive prices. FRABA group is also an innovator in product design and manufacturing processes and a pioneer of Industry 4.0.
POSITAL has a global reach with subsidiaries in Europe, North America and Asia – and sales and distribution partners around the world. Products are manufactured in advanced production facilities. The computer-guided semi-automated production system tracks each device from order, through assembly and testing, to final delivery.POSITAL developed an advanced manufacturing and order fulfilment system that embraces 21st Century technologies. With this system in place, POSITAL has been able to achieve “mass customization”.
This is how POSITAL Combines the Cost Efficiency and Variety of Scaled Manufacturing with the Flexibility and Accuracy of Specialized Projects.
POSITAL is a market-leading enterprise that makes use of advanced product design and manufacturing process to ensure that its customers enjoy the benefits of technology leadership, choice, product quality and competitive prices.