A simple speed measurement is easily done with two switching information. 

The distance s divided by the time difference t between the two switching events is the speed of the moving object assuming no acceleration or deceleration between the two measuring points.

Since it is a fairly easy task to do this calculation, there are possibilities to get the speed of an object without the usage of a PLC or computer.

Possibility 1: SIG100

The calculation of the speed could be done by SIG100. Switching events are created by two standard sensors. In this case two WTB4.
In case of rounded objects (or objects with rounded edges) or objects with different/ changing colour/ glossiness we strongly recommend the usage of reflective sensors WL... or through beam sensors WSE... because at these detection principles the outer shape of the object covers the complete lighbeam which leads to more stable/ accurate detection distance/ colour/ glossiness independent results. 
Please note that a rotation of the object can have an influence on the detected object length and thus on the measured speed. Therefore a good object guiding is recommended.

A small logic is calculating the speed and transfers it via IO-Link process data:

The distance s is stored in the constant CON.

Possibility 2: Smart sensor A72

There are smart sensors in our portfolio which are designed to do exactly this task. Measure the speed of an object.
Currently (2022) available sensors are:

1098509 WL4SLC-3P2232A72
1098510 WSE12C-3P2430A72

A smart sensor with A72 functionality is connected to a second standard sensor in fixed distance s. This distance has to be configured via IO-Link / SOPAS.
The speed value will be transferred via IO-Link process data.

FAQ for A72 sensors:

- what is the interest to activate/deactivate "Inverter Ext. Input" ?

This check box is not "activating / deactivating" the input. It is giving you the possibility to invert the input signal.
The smart task is always triggered by a positiv edge. So if you have a trigger sensor which is switching the other way round (light switching) you just have to tick this box to make the smart sensor work.

For easy cabling consider using the Smart Sensor Y-connector SYL-8204-G0M11-X2 (6055012) or SYL-1204-G0M11-X1 (6055011).

- measurement threshold 1 and 2 => it is the distance between the sensors and the object ?

No - these thresholds are just needed if you like to generate a switching signal out of your speed measurement. In the example above the sensor would give a switching output (rising edge) on QL1 if the measured speed is between 50 and 500 mm/s.

- what is the difference between the switching modes : within and out of window ?

Using the switching mode you can invert the behavior of the above mentioned switching output QL1. It decides if the QL1 switches if the measured speed value is inside or outside of the given range.

- what is the meaning of impulse width and impulse shift ?

These two settings device how the switching result of the speed measurement is looking like.
Impulse width: The width of the digital output signal. E.g. digital output signal QL1 is active for 1sec. (1000ms).
Impulse shift: Possibility of shifting the complete digital output signal. 0ms means the digital output will switch immediately after speed is calculated.

Principle is shown here:

- I checked all settings, the cabling and my trigger sensor is switching correctly. But the resulting speed value is always 0?

Most probably the reason for this behavior of the device is that your distance between the two sensors is bigger than the object itself.
The physical distance between the measuring points (smart sensor and trigger sensor) has always to be shorter than the shortest measurement object.
In case the object is shorter than the distance between the measuring points, no measurement value is being calculated.