In many fields, industrial or otherwise, the measurement of movement (position or speed) is essential information for the proper functioning of systems and machines.
There are multiple devices for performing these measurements. These tools, often referred to as position sensors, have characteristics that vary from one to another. It is therefore necessary to select the sensor which has the characteristics most in accordance with 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. To do this, we will separate the analysis of the application into two: the type of movement and the type of measurement.
TYPE OF MOVEMENT
We will first look at the type of movement to be measured. A motion is normally split into linear and rotational components (vectors). These two types of components are generally analyzed (i.e. measured) independently.
We talk about rotation when a system rotates around an axis. Examples will be mechanical arms, motors or valve flaps. The measurement will then be given in angle unit.
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:
- Single-turn rotary encoders
Each with its own advantages and disadvantages:
The major 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 axis of rotation to measure a tilt. If we add to this their compact size, they are therefore naturally suitable when space is a major constraint.
Single-turn encoders (single-turn absolute rotary encoders) must in comparison be mounted on the shaft. However, their dynamic performance and precision are often better. They also have very good resistance to shock and vibration while having the enormous 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 attacks (humidity, dust, shocks, temperature).
Multiturn rotations relate to rotational movements logically involving several turns. For these movements, we will not only count the position and / or the speed over a turn but also the number of turns.
In these particular cases, only absolute multiturn rotary encoders allow motion tracking.
A translational movement is a movement that takes place along an axis, parallel to it. We speak colloquially of linear motion. The measurement will then be taken in units of length.
This device consists of a rotary encoder coupled to an adapter whose mission is to convert a translational movement into a rotary movement. A cable coils and unwinds around an axis, allowing measurement via a rotary encoder. One end of the cable is hooked to the system whose displacement is to be measured while the encoder part remains fixed with respect to the reference frame.
The reels that can include any encoder therefore have all the characteristics. The user therefore has all the existing output interfaces at his disposal. However, these have the disadvantage of being bulky due to the volume of the housing and the need for the wire. This being relatively thin and fragile, nothing should disturb it. The resolution will depend on the linear measuring range.
A measuring wheel is a component that is attached to the shaft of a rotary motion sensor. This is rotated by adhesion to its outer diameter. This is usually done by attaching it to a treadmill. This is the same principle as the pinion / rack pair (the toothed wheels being replaced by the grip).
This system is interesting when there is a small space dedicated to the linear motion sensor and / or an “endless” movement, such as a treadmill.
TYPE OF MEASUREMENT
The type of application will define the type of device to use, at least in part. Indeed, it is also necessary to define the type of measurement that one wishes to carry out. Depending on this we will move towards different characteristics.
In some applications, it is only necessary to measure a movement in comparison with an initial situation (no fixed reference point) i.e. a movement with respect to the previous moment or a speed. This kind of relative measurements can be perfectly realized thanks to the incremental encoders. This obviously concerns rotational movements but also translational movements, of which the movement sensor is made up of 50% of a rotary encoder.
Incremental encoders have the significant advantages of being very economical and compact in size. Since only the speed and / or the relative position are necessary, this solution will be preferred.
However, some control systems need to know the exact position in relation to a frame of reference that does not change as the work steps go. 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 movement throughout the cycle, even after a power failure!
But this additional function is not without its price. The technology required for lap counting comes at a cost. Absolute encoders will therefore be significantly more expensive than an incremental encoder.
It is by the way interesting to note that POSITAL’s lap counting technologies are without batteries or batteries (less maintenance!). In particular, the Wiegand effect technology constitutes a technological leap in the field of sensors.
CONCLUSION: WHICH TO CHOOSE?
Ultimately, when you ask yourself the question “which position sensor to choose for my application”, first try to analyze what type of movement you want to measure and what type of measurement is necessary for your control system to work. POSITAL strives to provide you with the best products and solutions while supporting you in the design phase.