| | | One of industry's main objectives is to produce certain items in various shapes and sizes which when combined together, following precise rules and criteria, allow objects to be produced which are necessary in all sectors (home, chemical, industrial etc.). For this purpose systems are necessary which are able to manipulate material such as marble, glass, wood, metals, plastics, synthetics etc. and assemble more items to produced semi-finished or finished goods. These systems must be equipped with testing or process control cycles. "Precision" or error is a fundamental parameter in every system: it must be considerated for all evaluations and operations to be carried out. A system which transforms materials needs a "Measuring System" able to check every instant, that the work underway conforms to the product desired constantly satisfying the requirements imposed by the designer. In systems where are measurement sensor the most delicate and extremely important role is played by "Position sensor" or Encoders. Encoders supply the value measurement taken from the unit of control which in its turn uses it to check, for example, the size of the object being worked at a certain temperature and also that everything is being carried out correctly. It is therefore obvious how important the role of encoders are in a system: the accuracy of the finished product depends largely on the reading it supplies. The designer must exercise a great deal of caution when designing a system. Care must be taken in choosing the right encoder and must take into account the following factors: - mechanical error (transmission, gears, play, etc); - accuracy required; - type of measurement: relative or absolute; - environmental, electrical and mechanical stress; - type of electrical/electronic connection and mechanical coupling; - speed of mechanical rotation and counting rate. Error of measurement and precision working To obtain the accuracy desired all the errors which are intrinsic in a system and the connection with the measuring instrument must be carefully considered; this allows us to choose the correct resolution of the encoder and obtain the degree of accuracy most suitable for the application. Theoretically the degree of accuracy varies from 2 to 10, which consists of the number of samples of the segment to be measured, supplied by the measuring instrument (the greater the number of samples the greater the precision). Attention must be paid so as not to measure play and mechanical or elastic movements (a feature common to all moving system). The correct choice of the | | |
| | | Relative and Absolute measurement readings. An Encoder is a sensor which is able to convert proportionally the movement made the shaft into an electrical signal: the electrical signal, which is either incremental or coded, is used by the data acquisition system to quantify and control the movement of the moving components. There are two types of reading: Relative and Absolute. A relative reading is one in which the encoder gives an indication of a previous position plus an increment: this is a simple indication to menage in that it is sufficient to associate to the unit of movement or degree of accuracy the unit of resolution of the encoder. The incremental information consists of a two bit Gray code (phase A and B) in quadrature (duty cycle of 50%). The different between the two bits is 90 electrical degrees and it is necessary to check the direction of rotation. Honer Automazione srl assumes for the sake convention that the increment is positive when phase B precedes the phase A(the leading edge of signal B arrives 90 electrical degrees before signal A) and the shaft of the encoder turns clockwise, when viewed from the shaft itself. There is also a reference point (Zero or Marker) which indicates the competion of the revolution underway: this is an impulse whose size can be supplied, on request from 90 electrical degrees up to a few periods (whith reference to the duration of the impulses). | | |