APCS the Signal Conditioning Specialists

Find a Product Function ⇒ Input ⇒ Output

Select products with a transmitter and or alarm function.

dc Input Products
 

ac Transducer

An ac transducer is used to change an electrical quantity such as voltage, current, power or frequency into a proportional dc output.

By means of a transducer, a complex electrical quantity, such as watts, can be measured at a convenient location and converted into a load independent dc current signal for transmission over two wires over any distance for display, recording or control.

For remote indication of watts or vars, a transducer can reduce the number of signal wires to be laid between source and indicator from as many as nine to two.

Transducer output wires need only be insulated for low voltage and have small cross-sectional area. Such lines are easily run and effect savings in terms of cable costs and space occupied on cable trays and the connecting and terminating elements required.

Active Transducers
Active Transducers need an auxiliary power supply.
Passive Transducers
Passive Transducers are signal powered no auxiliary supply is needed.
Class 0.5 means
  • inaccuracy of the transducer is less than 0.5% of the span.
  • non-linearity is less than 0.5% of the span.
  • ripple (peak-peak) is less than 0.5% of the span.
  • response time is less than 500 msec.
 

Typical Features of APCS ac Transducers

  1. Australian designed and manufactured (for local support).
  2. Fully meets standards AS1384, BS6253, IEC688.
  3. Prompt delivery of standard items or specials which can be calibrated to order.
  4. All solid state with precision components for long term stability.
  5. Isolated input  output  power supply.
  6. Moulded, flame retardant housing - DIN rail or panel mount.
 

Information required for Transducer Manufacture

For a Power transducer we need for example:

  • Power Supply. 240V,50Hz.
  • System configuration. 3 phase 3 wire balanced load.
  • Frequency. 50Hz.
  • Input current and external CT (if used). 0-5A from 100/5 CT.
  • Input voltage and external PT (if used). 0-110V from 11kV/110V PT.
  • Output 4-20mA.
  • Calibration information:nominate if calibration is for rated current or a nominated watts.

For a 3 phase 100/5A CT and a 11kV/110V PT

  • W = √3 × 11kV × I × 100A = 1.905MW
    if the transducer is calibrated at 5A input.

  • Alternatively we can calibrate at 2MW. In this case the current calibration is
    I = ( 2M ÷ ( √3 × 11kV ) ) × ( 5 ÷ 100) = 1.905MW


Alarm

The alarm module measures the process variable using the appropriate input circuit for the probe or signal type and compares it with a trip point value to determine when to switch.

Main factors to consider when choosing a alarm module are;

  • Power supply used to power the module.
  • Input measurement from probe, sensor, voltage, current or process signal.
  • Trip action. A direct trip action (DIR) will cause the output relay to energised above the trip-point and a reverse (REV) trip action will cause the relay to be energised below the trip-point.
  • Reverse action is usually used in fail safe situations to ensure the correct state of the relay contacts when power fails on the alarm module. Trip-point (TRIP) is usually adjusted by the 15-turn trim potentiometer from the front of the module or set using software.
  • Contact rating and type, normally open (NO), normally closed (NC) and change over(CO). Change over contacts give the option of connecting the relays as normally open or normally closed as required. These relay states are when the relay is de-energised or when no power is applied to the module.
  • Trip status is indicated by LED to give you a visual indication of the relay status.
  • Dead band is used to set a window around the set-point to prevent the relay from continuously switching when close to the set-point. This setting could be factory configured, set with a trim potentiometer(dB) or set using software.
  • Switching delay. This could be accomplished by filtering the input signal to give a slower response. In more advanced systems the on delay (continuous trip time before the alarm energises) and off delay (continuous below trip time before the alarm de-energises) can be set independently. This setting could be factory configured, set with a trim potentiometers or set using software.
 

Alarm Dead Band

Many trip alarms have a dead band adjustment. Dead band (DB) is a common term for relay hysteresis and is usually expressed as percentage of span (%DB).

Dead band does not interfere with the ON switching of the relay. Looking at the direct action example below, we see that dead band is the change of input required to turn the relay OFF.

Alarm Dead Band

Alarm Dead Band

Lets follow the input path on the diagram. The input begins below the set-point adjustment. The output (relay) only switches ON after the input goes above the set-point.

The input continues to vary above and below the set-point however the output (relay) remains ON until the input falls below the dead band adjustment.

The input now varies above and below the dead band adjustment, however, as we can see, the relay can only be switched ON if the input returns above the set-point.

Dead band may be easier to understand if units are introduced. Lets use an example process where the level of a water tank is monitored.

The set-point is set to comfortable operating water level of 10m. The dead band is set to a minimum water level of 8m. With these settings the outlet valve controlled by the alarm allows water to exist the tank only open if the water level rises above 10m, and will close if the water level falls below 8m. If we wanted to reduce the minimum water level to 6m then we would need to increase the dead band.


Isolator

dc Input Transmitter

A Signal Isolator is a Signal transmitter with Isolation

Isolation is the electrical separation of two circuits such that there is no electron flow between the two circuits.

 

Signal 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 passing signal over a barrier using magnetic or optical coupling then converting the signal to the required output type.

There are usually 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 a 4-wire or separately powered transmitter will provide true 3-way galvanic isolation will isolate all three paths.
 

The need for Isolation

In most processes there are pieces of electronic measurement and control equipment from many different manufactures. The signals from these instruments are interconnected to each other and to sensors, transducers and output devices connected in the process loop. In any such measurement and control system there are several problems that are likely to occur, all of which can be solved by incorporating the appropriate isolation between signals.

Proper isolation should be of the highest priority in process control systems. Used correctly isolation will prevent unwanted current loops, ground loops, damage to delicate equipment and ensure the safety of human operators when high common mode voltages are to be expected.

 

Common Ground Loop Problems

This type of fault occurs when the return path of least resistance for the signal is via the earth or ground. An example of this type of fault is when measuring the pH of a liquid and the liquid has a return path to a grounded power supply.

It must be remembered that if the measurement is not isolated the return path could be through any one of the instruments connected to the same plant.

Common Ground Loop Problem

Common Ground Loop Problem

 

Common Current Loops Problems

Common Current Loop Problem

Common Current Loop Problem

A current looping problem will arise whenever the path of least resistance for the output loop current is not the intended path.

A classic example of this is shown below, 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 Inst. 1 where it finds the path of least resistance to return to the supply ground is return via the Rec Inst. 1 negative supply rail, thus bypassing the input of Rec Inst. 2.

 

Common Floating Voltage Problems

Dangerous high voltages can be present at sensor level, as in the case with a dc shunt. This voltage will be conducted through a non-isolated transmitter creating a threat to humans with a floating load or damage to receiving devices.

Common Floating Voltage Problem

Common Floating Voltage Problem

 

Common Mode Noise

Often the signals from sensors and transducers are small voltages and susceptible to noise pick-up from motors, variable speed drives and general switching spikes carried by the electrical reticulation system.

If there is isolation between input and output, the common mode noise (present on both input terminals with reference to ground) will be prevented from passing to the output even if it is relatively high in magnitude.


Transmitter

Auxiliary Powered Transmitter Concept

The signal transmitter converts a low level signal from any sensing device into a standardised process signal. These transmitters sometimes incorporate electromechanical devices such as strain gauges for measurement of physical values - pressure, flow, level, etc.

Signal transmitters can;

  • Convert one specific type of signal into another type of signal
    Input: Thermocouple type J 0-800°C
    Output: 4-20mA.
    May provide galvanic isolation from one signal (input) to another (output).
    Note that APCS defines a Signal Isolator as a transmitter with isolation.

When choosing a a transmitter please consider the following;

  • Input signal or measurement.
  • Output and what powers the signal conditioning module are a major determining factor when choosing a transmitter.
  • Is isolation required?

Calibration, Offset and Span

Most transmitters have two adjustments in common which are termed OFFSET (zero) and SPAN (gain). These adjustments allow the output signal to be varied considerably, generally 20% of the transmitter range.

The principle used in the calibration of transmitters is that if two points on a straight line are established then the line itself is established.

By examining Figure 2 it can be seen that at low magnitudes of input signal the effect of span adjustment is very small. Consequently the first calibration point should be selected close to the bottom of the input range. The offset adjustment is then used to give the required output signal for the input signal being fed to the transmitter.

Because alteration of the span results in a higher output deviation at the top end of the input range it follows that the second calibration point should be at this top end, say in the top 25%. Thus the calibration at this point is achieved by imposing an input signal of suitable magnitude and adjusting the span potentiometer to give the correct output.

In practice it is sometimes difficult to eliminate the interaction effects between offset and span so it is recommended that the procedure for setting these two points be repeated until both points are obtained without the need for further adjustments.

The Offset Adjustment sets what is in effect the value of output when the input is zero.

Figure 1 shows the effect of altering the offset of a transmitter.

Fifure 1 Offset Adjustment

Fifure 1 Offset Adjustment

Figure 2 Span Adjustment

Figure 2 Span Adjustment

The Span Adjustment alters the slope of the relationship between input and output signals.

Figure 2 shows the effect of altering the span of a transmitter.

The principle used in the calibration of transmitters is that if two points on a straight line are established then the line itself is established. By examining Figure 2 it can be seen that at low magnitudes of input signal the effect of span adjustment is very small. Consequently the first calibration point should be selected close to the bottom of the input range. The offset adjustment is then used to give the required output signal for the input signal being fed to the transmitter.

Because alteration of the span results in a higher output deviation at the top end of the input range it follows that the second calibration point should be at this top end, say in the top 25%. Thus the calibration at this point is achieved by imposing an input signal of suitable magnitude and adjusting the span potentiometer to give the correct output.

In practice it is sometimes difficult to eliminate the interaction effects between offset and span so it is recommended that the procedure for setting these two points be repeated until both points are obtained without the need for further adjustments.