Food & Drink Process & Packaging Issue 9 2016 - Page 11

Level Triggering Level triggering, sometimes referred to as gating, causes the sensor to output measurements until the trigger condition is satisfied (see Fig 2). As the processing occurs sequentially In this example, only measurements in time and parallel in space, the for a particular range of positions are next measurement is available at the to be examined more closely. The output after a further 400µs. trigger signal (green) initiates the The sensor time pattern therefore requires a chronological interval between two consecutive trigger signals of at least four cycles with Trigger pulse values The trigger pulse duration ti must be at least one cycle period (= 1/ measuring rate). With a slower measurement in the sensor. The time offset of the trigger signal and output signal of the sensor (blue) can be seen clearly on the oscilloscope. edge triggering, or five cycles for level The sensor needs this time for the triggering. Thus a measurement is not internal processing and output of the output that was valid four cycles ago, measurement results. Encoders are but instead the measurement object also suitable as trigger signal sources. position is acquired exactly at the time Rotary transducers can be found in of the trigger. all numerically controlled industrial systems. measuring rate, the trigger pulse As a result, a maximum trigger duration must therefore also be frequency of 625Hz occurs for a The rotational speed or the closeness extended (e.g. from ti = 400µs at a measuring rate of 2.5 kHz. There of the pulses from the encoder is a measuring rate of 2.5 kHz, to ti = remains a time uncertainty of a direct measure of a linear change, 3.2ms at a measuring rate of 312.5 maximum of one cycle because the e.g. of a conveyor belt. If an encoder, Hz). The necessary level adaptation sensor continues operating in its cycle for example, supplies 1,000 pulses to the LVDS specification (see Fig 3) sequence and the trigger edge can be per revolution and if the internal of the sensor occurs via a controller located within the first cycle, i.e. from processing time of 1.6ms in the that permits trigger levels from 2.4V the start to the end of the first cycle. sensor is taken into account, then the to 24V. The trigger input is interrogated within this period. The leads to a minimum length of the trigger pulse of at least one cycle. encoder may revolve at a maximum of 0.625 revolutions per second (or 37.5rpm), so that the maximum trigger frequency for the measuring rate of 2.5kHz is not exceeded. Application Summary A typical measurement application Edge triggering supplies single Time offset of trigger and output with triggering is the bridging of gaps signal with single parts on a convey ܈[H[܈X]Y\H\[‚YHY JKYX\\[Y[ۈH]]وB[܈[\X\[\[›\][ۋ[H\X[Y[[ܞB[]]وHYX\\[Y[‚ݙ\\X]Y]۝[[\‚[X\ˈ\[Z[HX^[][BYX\\[Y[\[][Y YX\\[]Hو Z 0\\‚X[\\[^X[YۛY[Z[\]X\܈^\K]Y[HYX\\[Y[[BXY[[\]][ۈ[[YKYH[܈\X[Y]۝[ˈH[\[و\ H۝\Z[[X[BK\\\YܙHH\X^[][HY\\]Y[Y\[YX\\[Y[\]Z[XH]BXۛZX[HXX[YX\\[Y[]] []\XY܈X[ۈ[\]]X][ۈ\X][ۜ˂ԐTHHZXܛQ\[ۂHԒUSHZXܛQ\[ۂ H˙ ˝Z LB