Differences between Discrete and analogic modules

[Juliano]

What is the main difference between a discrete output and an analogic output module.
May I use a FP-DO-xx to control a proportional valve.

[Aaron_G]

A discrete output is an output with two states; ON and OFF. For most FieldPoint discrete modules, the ON state can be between 5-30 VDC (depending upon external power supply) and up to 1 Amp per channel.

An analog output channel has a range of values that it can assume based upon the resolution of the channel. An 8 bit analog output channel can assume up to 255 (2^8) different output values. For example, the FP-AO-210 is a 12 bit analog output module with a range of 0 - 10.2 Volts. Thus an output value of 0 bits is 0 volts, and an output of 100 bits is 0.249 V ((10.2 V - 0 V)/4096 * 100 bits)). Generally, an analog output channel can drive less current than a discrete channel. The FP-AO-210 can provide a maximum output of 10 mA.

Can you use a FP-DO-4
0X to control a proportional valve? It depends upon the input of the control circuitry of the valve. As a general answer, I would probably not recommend it since it provides for an ON/OFF control of the valve and does not provide an easy in-between value. However, by turning the channel ON and OFF quickly varying the amount of time that the signal is ON verses OFF, you can vary the duty cycle and provide for in-between positions of the valve. The common implementation of this is called Pulse Width Modulation (PWM). This is normally done with circuitry that has been optimized to control the PWM timing rather than a discrete channel (as you generally will not get the same level of timing control through the communications link). In FieldPoint, the [c]FP-PWM-520 module exists for these types of applications. With the [c]FP-PWM-520, you control the duty cycle (ratio of ON time to OFF time) and period.

Regards,
Aaron

[Juliano]

Very interesting solution, I´ve never saw PWM beeing used for that kind of application. But I still have some doubts. Is it necessary to have a demodulation circuitry between the FP-PWMxx module and the proportional valve?

[Aaron_G]

It really depends upon the particular valve and drive implementation of the valve as to which type of circuitry you use to drive it. Some projects that I have worked on used 0 - 10 V signals for valves, others used 0 - 20 mA, some used PWM with a specified period.

Regards,
Aaron