That is the idea behind the optical motion sensors from INTACTON, a business unit of the Cologne-based FRABA Group. Based around specialized digital camera technology, these sensors are designed to be mounted a fixed distance from the surface that they monitor. By measuring changes to the optical image that they ‘see,’ these devices are able to determine exactly how much and in what direction the surface in their field of view is moving.

INTACTON optical motion sensors are a drop-in alternative to code wheels in manufacturing facilities producing roll products such as paper, fabrics, foil, plastic film or wire. 

Thanks to their no-touch feature, there are no problems measuring the movement of hot, wet, delicate, sticky or even gooey surfaces, and no chance of surface contamination. Moreover, because they monitor the surface directly, there is no loss of accuracy due to slippage, such as can occur with roller or wheel-based measurement systems.

These devices have also proven themselves to be useful in applications such as control of automated guided vehicles (AGVs). Here, a key advantage of INTACTON optical sensors is that they can deal with a wide variety of floor surfaces, even recognizing special navigational symbols painted on the floor.

In this application, optical motion sensors can be significantly more reliable than wheel-mounted rotary encoders since they aren’t affected by wheel slippage.

INTACTON produces two main types of optical motion sensors. The OPTIPACT product line uses an image correlation technique to measure displacements of the observed surface. Each device contains a light source and an optical image sensor. As the object moves, an image of the object moves across the face of the optical sensor. ‘Snapshots’ of these images are recorded at regular short intervals (Δt). 

The shift in the image position is determined by an image correlation algorithm. Knowing the shift in the image position and the properties of the optical system (focal length of the lens and distance between the sensor and the monitored surface) it is straightforward to calculate the displacement of the target object (Δx and Δy) and the object’s velocity (Δx/Δt and Δy/Δt).

Most OPTIPACT sensors are equipped with a red LED light source that has the advantage of requiring no special eye protection for staff working near the devices. However, models with a laser light source are available for use with materials such as foil or plastic films that have extremely smooth surfaces. 

OPTIPACT and COVIDIS optical motion sensors.

These devices are compact and relatively inexpensive. They measure motion in two dimensions, which makes them a great choice for many manufacturing processes and for special applications such as AGVs. Accuracy is typically better than 1% of measured displacement while velocities as high as 4 m/s can be reliably monitored.

COVIDIS sensors, which are larger, make use of a sophisticated spatial frequency filtering technology that yields outstanding accuracy – typically within 0.05% of measured results. These devices also have extremely good dynamic response that means that they can detect standstill, changes of direction and accurately track accelerations or decelerations of the object being monitored. They have proven to be extremely useful in production facilities where accurate length measurements contribute to reduce material wastage. 

Both OPTIPACT and COVIDIS motion sensors are designed to stand up to the challenge of industrial environments. In both cases, the housings are protected from dust and moisture to IP65 level, although of course the optical paths need to be kept reasonably dust and moisture-free to enable accurate measurements. 

Both types of instruments feature RS232 interfaces for connections to control systems along with the standard quadrature pulse interface.

Christian Fell is with FRABA Inc. in Hamilton, NJ.
www.fraba.com