A frequency converter triggers an electric motor. It has an analog input of 0 to 10 V, whereby 0 V corresponds to a speed of 200 rpm and 10 V a speed of 3000 rpm. To store the speed in rpm in the process image, you have to select the parameters for the output range 0 … 10 V as follows:
Multiplier = 100, Divisor = 28, Offset = -714
The slew rate settings have the following meaning:
Let’s assume you have selected an output range 0 … 10 V. In the process image there is an output value of 3 V for channel 1. This value is now to change to 8 V.
Therefore you write the number 8000 instead of the 3000 mV to the corresponding offset in the process image. In the next PiBridge cycle, the figure is then consigned to the DAC (Digital to Analog Converter) which then generates the analogue voltage for the output.
This takes place with a certain delay time of 10 … 40 ms depending on the load on the PiBridge (which depends on the configuration).
After this delay, the output voltage would jump from 3 V to 8 V very quickly. The slew rate of the voltage is only dependent on the total capacity at the output (which amongst other things is dependent on the connected devices). As a rule, it should amount to a few µs.
The slew rate is often not desired because it has an extremely high frequency ratio that can lead to interference radiation and other problems. You can therefore artificially reduce the slew rate.
To do so, set Output1EnableSlew bzw. Output2EnableSlew to On.
Now the DAC will, as soon as it has received the new data (8000), increase the output voltage in small steps until 8 V is reached.
You should set the amplitude by yourself by going to Output1SlewStepSize or Output2SlewStepSize. This step size is indicated as a LSB (Last Significant Bit).
As our converter works with a 16 bit resolution in every output range, the step size of the lowest valued bit (LSB) measures, in the 0 … 10 V range, 10 V/2^16 = 0.15 mV (or respectively 0.3 µA in the 4 … 20 mA range).
The number of minute steps that the DAC needs depends therefore only on the voltage jump: In our example, it is, for a step size of 1, therefore 5000 mV / 0.15 mV = 33,333 steps.
The time in which these steps are performed is determined by the clock speed by which the steps are integrated into the signal. They can be set by yourselves by means of the Output1SlewClock or Output2SlewClock parameter.
When the clock speed is set at 3.3 kHz, the edge needs from 5 V, therefore 33,333* 1/3300 s = 10 s. When set at 258 kHz, it is therefore 130 ms. The slew rate is, for a 3.3 kHz clock speed, therefore 0.5 V/s or 38 V/s respectively when the clock speed is 258 kHz.
You can drastically increase the slew rate with larger step sizes. However, the signal will also skip so that it will receive significantly greater share of high frequency signals.