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NP4000 InstrumentationFrequently Asked Questions |
How to get signals into my computer.As you probably understand, computers do not directly understand what an input signal is. First of all the input signal has to be converted into a format for which the computer understands. For analogue signals this is carried out by a device called an analogue to digital converter (ADC). The ADC reads the input signal and converts the varying analogue input into a number that the computer understands. The number that the ADC supplies to the computer system is proportional to the parameter being measured such as pressure, temperature voltage etc. Some digital signals can be connected directly to the computer but most often they require some conversion to the correct signal levels. Digital signals have only two states, they are high (on) and low when they are considered off. Let us assume that you are collecting data via one of our NP4000 instruments. All you will need will be connect the output from your sensors directly to the signal terminal connectors. The NP4000 instruments will then pass data to your computer system via one of the data transmission networks in a format that your computer can understand. An important part of most measuring systems is signal conditioning. The signal conditioning is used to convert signals generated from sensors into voltages that can be measured by the ADC and also sometimes to provide power directly to transducers such as strain gauges. The data acquisition systems used in the NP4000 instruments can directly accept voltages in a range from ± 0.3 mV up to ± 5V but this range can be tailored to suit most other applications upon request. Voltages as high as 300 V can be measured for specialist applications. Additionally signal conditioning can be used to provide isolation to protect the data acquisition circuitry from potentially dangerous signal levels. The NP4000 instrumentation is supplied with isolation for all of its inputs as standard ensuring that no accidental damage can damage your measuring system. The isolation is built directly onto many of the input boards, apart from connecting the signals to the signal terminators mounted on the front of the NP4000, no other ribbon cables, signal conditioning units power supplies are required. What distance can the measuring system be from the signal source?As the distance between the data acquisition system and the signal source is extended, you will observe an increase of noise in the signal under investigation. The increase in noise is often caused by electromagnetic pickup on the signal wires connecting the sensor to the ADC. There increase in signal noise depends on a range of factors such as, signal amplitude, cable type, noise level, noise characteristics (white pink). Noise is most likely to effect the representation of low value signals where the analogue signal is only moderately above the background noise level. It is generally possible to extend the distance between the transducer and the measuring system to several hundred feet if the signal characteristics under investigation is of low frequency and of substantially high amplitude compared to the background noise. The source of the signal is a varying current instead voltage source. The type of cables used to attach signals from the sensors to the terminators will also effect the mount of noise seen on an analogue signal. Where possible it is recommended that screen or twisted pair cables should be used connect signals to the monitoring system. Generally the distance between the terminators for the ADC system and the signal source should be kept to a minimum. If you require to transmit signals over a large distance due to the placement of the sensors, then it is recommended that operation be undertaken via 4-20 mA current loop system. It must be pointed out that pick-up contamination of a signal is generally considerably greater for a voltage source (signal) than for a signal transmitted at a fixed voltage and varying current level were the varying current does so in relation to the signal being measured. A signal transmitted as a current source can be converted back to a voltage again within the NP4000 input cards. Isolation and Protection . Is it necessary.
A very important feature for most data acquisition systems is the ability to protect the computer system from dangerous over voltage conditions which could cause expensive and irritable damage to the expensive circuits contained within a measuring system. Overvoltage conditions generally occur following user error i.e. mains voltages being accidentally presented to the input circuitry during installation of equipment during system commissioning, or the wrong input levels being presented analogue boards caused by the failure of external circuitry. Any problem could cause damage to the equipment and even worse harm an operator. The NP4000 instrumentation includes up to isolation for all of its analogue and digital inputs. This isolation is built onto the circuit boards provided as part of the NP4000 instrumentation. No extra signal condition is generally required for use with our systems making them small and compact compared to most opposition units. Should a problem occur with any of our input boards individual channels can be removed and replaced. Custom isolation and protection systems can be produced to meet most user defined specifications. The Keynes Controls products contain 500V continuous overload protection as standard. The NP4000 instrument operates externally to the data processing systems and so will be the only unit to fail should large extraneous spikes be passed to the data acquisition system. Another important reason that you should install isolation into your monitoring system is that isolation will enable the measuring of signals to be examined when a floating ground exists. This feature generally occurs on large scale plant operations especially on offshore platforms or large scale operations around industrial plant. Floating grounds generally exist when there is no common earth link between the data acquisition system and the signal source. How to reduce noise for analogue measurements?Any measurements will contain a certain amount of unwanted noise. The ability for a monitoring system to function in the face of noise will depend directly on the signal-to-noise ratio required for the particular application. Most noise incurred problems can be overcome by proper installation of the project cabling. Often noise problems can reduced by applying suitable grounding and shielding principles. A common error discovered in data acquisition systems is to assume that the signal ground and return lines are the same. This often leads to multiple ground links at separate points causing earth currents to flow due to the different parts of the system. The earth currents can be a major contributor to overall system noise. All the equipment connected within a system should all be terminated to one central earth point. The return path for signals to ground should have as low an impedance as possible to reduce the effects of EMI. The use of proper signal wiring to connect the various transducers to the data acquisition system is essential. Balanced cables are helpful for rejecting ground difference potentials. Coaxial (shielded) cable and twisted-pair wire is most suitable for interconnections when attempting to measure low level signals. With twisted pair cable, interference is induced on both cables simultaneously, and so effectively cancels out most error incurred the measured signal potential. You may have to use filtering to remove noise from an AC signal. This action can be carried out with hardware dedicated filter packs or via software when post processing the acquired data. The most common type of noise to instrument systems is that of pick-up of the mains power supply i.e. a component of 50/60Hz depending upon your local power supply arrangement. Filtering is often used to remove noise components that cannot be prevented by grounding or shielding. The NP4000 instrumentation has been carefully designed to remove directly contamination by mains (50/60) Hz pickup. This feature is selectable within the driver software and is one of the few instructions that requires selection when configuring one of our instruments. The method for which this operation is carried out is described later in the signal enhancement operations section. Are there advantages for using a visual software packages for system creation?Application packages that perform specific tasks such as data chart recording, logic analyser and oscilloscope operations are available from many different sources. These packages have been developed to present data simply and easily in user friendly environment and cater for a wide range of hardware to provide the interface to the real world signals. Most packages provide a simple to use system for the creation of run time and user interface screens such as the DasyLab software from DASYTEC. These packages are very flexible and allow the creation of most types of application software very quickly without the use of dedicated programming staff. These types of packages are generally provided for engineers to create and operate dedicated control systems. If you are an experienced software developer or that you have a project that has to utilise company specific hardware then there some advantages for developing your own code. Code is often developed using a compiled language which generally is small and operates faster than that provided by a graphical analysis package. Speed may be important if there are time critical tasks that have to be undertaken in real time. If the cost of the development is of concern, then the small added investment for a graphical software development package will have many advantages. No matter what your level of support, using a graphical control package such as DasyLab will dramatically reduce your development time and reduce costly installation times. Is 24-bit resolution worth the cost?An important part of any data acquisition system design is the resolution of the ADC component. To low a resolution and the signal being examined can be difficult to distinguish, to high a resolution then you may be paying a premium for components that are not required. Many transducers are only accurate to 16 or fewer bits and so the using a 24 bit ADC will be a needless overdesign. The NP4000 instrumentation has been designed to use 16 and 24 bit resolution. The 16 bit systems are ideal for sensors such thermocouples where the required sample rate is low and the sensors are only rated to about 0.1 % accuracy. The 24 bit system has been designed to ensure the highest possible dynamic range is achieved when examining analogue signals. For low speed applications were the dynamic component is of negligible value you can statistically improve the static signal component by a repeated number of measurements and then ensemble averaging the results. Keynes Controls developed the 24 bit data acquisition system to remove many of the ambiguities that may be misinterpreted in data analysis operations. Amplifying a signal to enable a data acquisition system to record a measurement does not always provide the solution for a measuring system. For example if we are looking at signals from a strain gauge which by nature of the operation are very small, then amplification of the signal will not only improve the range of the desired signal but it will also increase the overall noise component i.e. cause a reduction in the signal to noise ratio. For application where the signal amplitudes are small to start with then any reduction may have catastrophic implications. Detecting the signal at source without distortion caused by conditioning equipment is by far the best way to proceed when designing a system. The use of 24 bit data acquisition system will provide the optimum SNR for monitoring signals. Should I use a 24- or 16-bit board?The NP4000 instruments can be supplied with 16 or 24 bit resolution data acquisition boards. The 16 bit system can be used to gather signals at rates up to 50 samples a second while the 24 bit ADC boards can gather signals up to 1000 samples a second. This discrepancy in sample rate is currently due to the component manufacturers of the individual ADC chips. Both the 16 bit and 24 bit ADCs used in our instruments utilise Sigma Delta converters and are the fastest of their type currently available. The main advantage for utilising Sigma Delta converters for analogue measurement operation is that negate the requirement for using external anti-aliasing filters. The anti-alias filter characteristics built into the ADC chip has their pass band characteristics changed automatically to match the maximum pass band allowed by the Nyquest rate defined by the instrument sample rate. Is it OK to leave the NP4000 connected to a network when switched off?There is no problem leaving a NP4000 instrument in a powered off state connected to a local area network via one of the Ethernet interfaces. It is recommend that you do switch off any units that are not required in order to reduce the network bandwidth used to transmit data to any attached work stations. The RS-232 interface can only be used for single instrument operations and so cannot be used unless the instrument is switch on. Some possible network reliability reduction has been observed on leaving the instruments connected a multi-drop RS-485 network. No cause for this degradation in network reliability has ever been identified to have being caused by our instruments so care should be taken when using third party equipment along with the NP4000 units. Can I mix 16 & 24 bit results on the same network ?Yes you can mix 16 bit and 24 bit NP4000 instruments on the same network and pass data obtained from each unit directly to third party application packages for analysis purposes. For example should you have 5 NP4000s on a network, three instruments containing 24 bit ADCs and two 16 bit ADCs then you can pass data from any channel of any Pod to an application package for analysis. You may pass 3 of 24 bit ADC channels from one instrument and 9 of 16 bit channels from the other instruments to a package for analysis. There is no limit to how you mix the data. You cannot mix 16 bit and 24 bit ADC cards within the same instrument. This is the only limitation as to the performance of our instruments. Is it safe to plug new units into a system when logging operations are under way.Yes you can plug new units onto the Ethernet networks while logging operations are underway. However the system will not immediately see the new device. Run the NP4000 manager software to ensure that the new device has been seen. Be careful when connecting a new instrument to the 10Base-2 networks to ensure that when you are connecting a new instrument you do not crash other peoples systems already connected. Follow the system managers instructions for network connection at your site. We have not yet had any complaints that the NP4000 instruments have caused any data control operation failures when being used on an RS485 network. Should we receive any reports then we will update this page accordingly. Any problems with the system then please get in touch. Can I trigger control operations directly from one of the analogue or digital inputsYes you can trigger data acquisition and control operations directly from any of the system inputs. This is not a feature of the standard system driver but if you contact technical support we can advise how this operation is undertaken using the developers toolkit. You could for example monitor vibration in part of a plant with one instrument and use the digital control interface within a second instrument to close down a machine if the detected vibration level above a critical level. |
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