1. Isolation
2. Current Loops
3. Ground loops
4. Input Impedance
5. Frequency Response
6. Noise
7. Temperature
8. Accuracy
9. Expense
10. Intrinsic Safety
1. Isolation
Isolation is one of the most critical issues in process control.
It is used to prevent unwanted current loops, ground loops, protection
of delicate equipment and ensuring the safety of human operators when
high common mode voltages are to be expected.
Isolation is the electrical separation of two circuits such that there is no electron flow between the two circuits. The isolation breakdown voltage defined for such equipment, is the voltage required to cause flash-over or a breakdown in isolation, in such a circuit. Isolation in general purpose analogue circuits is usually achieved by converting the input signal to a frequency, then passing it to the output through either an opto-coupler or transformer, before reconverting the frequency to the required signal.
There are three isolation paths to be considered, supply to input, supply to output and input to output.Signal powered isolators provide isolation for the input/output and output/supply paths, but not for the input/supply path (as the signal is the supply).
Loop powered isolators provide isolation for the input/output and input/supply paths, but not for the output/supply path.
Only an isolator which provides true 3-way galvanic isolation will isolate all three paths. (a 4-wire or auxiliary powered transmitter).
Note: Dotted line shows the current path (The second receiving instrument input is bypassed).
2. Current Loops
A current looping problem will arise whenever the path of least resistance for the current loop is not the intended path. Fig 5 shows a classic example of this, the two receiving instruments are supplied from the same 24Vdc supply and have a common input and supply negative. In this illustration the current flows into REC 1 where it finds the path of least resistance to return to the supply ground is return via REC 1's negative supply rail, thus bypassing REC 2's input. Note: this would have been eliminated by isolating the 24Vdc supply from REC 1's input.
Fig 5 Example Of A Current Looping Problem
3. Ground loops
This type of fault occurs whenever the return path of least resistance for the signal is via the earth or ground. Fig 6 provides an example of this type of fault. Note: a transmitter with input/output isolation would have eliminated this fault.
Fig 6 Example Of A Ground Loop
4. Input Impedance
Various sensors use an electro-chemical reaction to produce an output, and thus have a limited life. This life span can be reduced if the transmitter input does not have a sufficiently high input impedance. For pH electrodes this impedance must be in the 100's of meg ohms.
In a current loop the loop can only drive into a finite impedance, e.g. a 4-20mA loop with 18V drive can source 20mA into 900 ohm maximum. Current loop limiting will occur if this maximum is exceeded, with the result that the output will not reach 20mA.
Noise can also be induced if the impedance is too high, without utilising the proper screening techniques, especially for frequency inputs where induced noise can be detected as information.
5. Frequency Response
The frequency response of a circuit can be defined by the T90 time. That is the time taken to reach 90% of output for an input step change of 0-100%. The response may also be defined as frequency {Hz), with the frequency response being defined as the frequency which can be applied to the circuit which would be attenuated by 3dB (71%) at the output. For fluctuating signals such as AC current, or normally slow signals such as temperature, a slow, even damped response is commonly required, to eliminate spurious noise and spikes. Conversely a very fast response is required to monitor the impulses from impact testing, using pressure transducers.
The response of particular circuits must be considered especially in open loop control applications to prevent oscillation.
6. Noise
Noise and interference can originate from any number of sources: lightning, RFI, thyristor switching (VSD), etc. When monitoring low level signals it is essential to follow proper shielding, installation and grounding techniques to prevent noise interference. The increasing use of high power walkie-talkie's around unprotected signal conditioning equipment is one of the new trends causing unwanted problems. The 'problem' usually goes un-noticed in normal operation, housed inside a grounded steel cabinet. However, when the maintenance officer, in the course of routine checking opens the cabinet, the Faraday cage is destroyed and RFI noise can be induced. For critical applications RFI protection must be considered even inside cabinets.
7. Temperature
Most analogue equipment is designed to perform over the range -10...+60°C, with a wider range for the Military Specification versions (at much higher cost). The effect of temperature must be considered for both the sensor used and the transmitter. Some parameters notably conductivity and pH, are dramatically affected by temperature, conductivity by up to 2% per°C. Thus the option of temperature compensation, automatic or otherwise, must be considered in situations where a stable temperature cannot be maintained. The effects of temperature also affect the transmitter. All circuits will have some temperature drift, some more dramatic than others. General purpose units would typically have a temperature effect of 0.02% per°C, or a 1% drift over the range 0-50°C.
8. Accuracy
When choosing the sensors and transmitters for a particular application, the accuracy of the sensor and the transmitter need to be considered. For example it is no use trying to achieve an overall system accuracy of 1.0%, using a sensor with an accuracy 1% and a transmitter with an accuracy of 0.5%, as the accuracy of the separate items must be combined. Also the use of a general purpose sensor with a high performance transmitter (or vice versa), is a waste of the features (and extra cost) of the high performance unit.
9. Expense
The feasibility of a project's viability is directly related to the expenditure required. When applying this idea to analogue signal conditioning the following considerations apply:
- Isolated circuits will be the best, design-wise, but are more expensive. Conversely, the choice of a non-isolated circuit for purely cost cutting concerns without design considerations invites extra future expense, through hardware and labour costs.
- Cost savings can be made by utilising loop powered (2-wire) equipment over auxiliary powered (4-wire) where possible, not only as the loop powered equipment is generally lower cost, but also through lower installation costs (less connections and cabling)
- The overall accuracy of the system will also affect the end price. If a 0.5% accuracy is required for the system then the extra performance (and expense) of a unit with 0.1 % accuracy would be wasted, unless the receiving unit is capable of utilising the extra performance.
10. Intrinsic Safety
For applications where there is a danger from explosion or other safety hazards if an electrical spark should occur, then normal conditioning circuits must be supplemented with Intrinsically Safe (I.S.) equipment, or have I.S. barriers installed to eliminate the potential hazard. I.S. circuits are designed such that there can never be enough power on the hazardous side to produce a spark capable of igniting a combustible gas.