There are numerous ways to control and monitor equipment in the bulk handling industry.
In this chapter we address the very basics on Digital, Analog and AS-i controls as for each corresponding the pros and cons.
Some example equipment that will be used in this comparison are;
- Level transmitter
- Valve control
Each of the components above requires a power supply which comes in the following variations;
The controlling options are;
NO: Normally open (for control)
NC: Normally closed (for alarms)
4-20 mA (for sensors)
For bus communication (ASi, profibus)
Digital and/or Analog control
Central controllers and most terminal unit controllers are programmable, meaning the direct digital control program code may be customized for the intended use. The program features include time schedules, setpoints, controllers, logic, timers, trend logs, and alarms.
The unit controllers typically have analog and digital inputs, that allow measurement of the variable (temperature, humidity, or pressure) and analog and digital outputs for control of the medium. Digital inputs are typically (dry) contacts from a control device, and analog inputs are typically a voltage or current measurement from a variable (temperature, humidity or pressure) sensing device. Digital outputs are typically relay contacts used to start and stop equipment, and analog outputs are typically voltage or current signals to control the movement of the medium control devices. Usually abbreviated as “DDC”.
Figure 1: a) Conventional digital system
Pros and cons of Digital and/or Analog control
- Pros Analog control: robust (doesn’t crash), easy repair of wires in case of damage, controllable between shared network/platforms, cheaper & global availability of spare equipment, Dynamic range (‘easier’ to avoid being limited by ‘self’ noise), Continuous processing (no inherent band-width limits), hard to modify, available diagnostic instrumentation is good.
- Cons Analog control: each component requires individual wiring, installation costs, difficult to do accurate design (tolerances of electronic components), hard to do complicated controls, interference, hard to build in comparative logic (intelligence), hard to ‘back-up’.
- Pros Digital control: easy repair of wires in case of damage, flexibility (can do really complicated controllers, easy to modify, quick to develop, easy to make back-up), can build in lots of logic (intelligence for switching states, locking logic, adaptive filtering), diagnostics (in particular, remote off-line testing and diagnostics), error detection, signal generation, control interfaces are more precise.
- Cons Digital control: each component requires individual wiring, easy to break (backup, backup, backup!), installation costs, software interfaces are currently not as user-friendly as real knobs and switches, software instrumentation not as advanced as analog.
An AS-i network is very simple and needs only one cable to connect the input and output modules from any manufacturer. AS-i users do not need deep knowledge of industrial systems or communication protocols. Unlike other digital networks, the AS-i network does not need terminators or equipment description files. Simplicity is its strong point. AS-i systems are efficient and very fast, making them able to replace large and high-costs systems. Expansibility is very easy to get – just connect a module, address it and then connect the network cable. Check if the power supply LED is connected and the connection to the next module is enabled. The AS-i network supports any cabling technology: star, bus, tree, ring or other configuration up to 100 m of cable. Or else, by adding repeaters it is possible to expand the system up to 300 m. The AS-i network is easy to install, since it needs no terminators at the ends. AS-i networks typically reduce cabling and installation costs by 50% in comparison to other conventional networks. The use of a single cable for connection to discrete devices reduces the need for cabinets, conduits and trays. The savings obtained in the network are really significant, since using few cables brings down installation and commissioning costs and engineering time.
Figure 1: b) AS-i network
Pros and cons of AS-I control
- Pros Analog control: less cabling and installation, bus-system, serial data combined with 24V supply, both inputs and outputs, analog and digital
- Cons Analog control: global spare availably, programming-tool and -knowledge required on the operating crew, individual field equipment is commonly higher priced
Digital (analog) vs AS-i
The above is a brief explanation and comparison between multiple control system variations. In case you prefer to receive any more detailed clarification, education or advise on this subject, please tell us about your project via our contact form: http://www.lionbulkhandling.com/contact/