Controller process inputs are factory calibrated to the stated accuracy within the product specification. This calibrates the controller as a discrete instrument, but not the overall process where the controller is fitted. Users therefore often need to calibrate the controller in combination with other parts of the system it serves.
The right calibration can ensure that industry or application accuracy requirements are met to achieve the required quality, as well as providing additional benefits, such as reducing waste and driving higher throughput in the process by reducing measurement errors in the complete control system.
The following hopefully will provide a useful guide for those that have not used input calibration before and will explain the basics. It will also reveal the deeper level of benefit that it can bring to the system for more experienced users.
Exterior influences
There are many exterior influences that have an effect on overall system accuracy, for example, sensor factors such as variances within stated tolerance and mounting position along with cable type and length can all introduce errors. To achieve the required overall system accuracy you can eliminate some of those errors by using the input calibration feature on the controller.
In some applications a non-calibrated controller in a system with even a relatively small error can have a serious effect on product quality. In the aerospace industry, which is highly regulated due to the extreme need for safety, precision is vital, and the success of a manufacturing business depends on meeting quality control requirements.
In order for aviation manufacturers to meet requirements when heat treating metal components to the Nadcap (National Aerospace and Defense Contractors Accreditation Program) AMS2750 standard, it is necessary to establish a thorough quality system, including well-documented process instructions and complete records for all production batches, including time and temperature data.
To guarantee quality, system accuracy tests are carried out regularly to ensure the complete system is correctly calibrated within the allowable parameters. Sometimes, the need to maintain tight temperature tolerance is driven not by quality standards but by the need to maximise efficiency in certain processes.
No manufacturer wants to keep running product through a process multiple times to get the right result, or suffer process downtime because output on the production line is of poor quality. By ensuring that your system is accurate you will reduce the likelihood of these situations occurring.
A controller displays a measured process value from a sensor that is positioned as closely as possible to the product within the process equipment. The sensor provides an analogue signal that the controller converts to digital for display. To calibrate the controller the value displayed on the instrument is compared to a calibrated temperature measurement source at the product in order to determine the error.
Positioning of a sensor can be very important. The ideal place to position a sensor should be precisely where you want to measure the object; however, this is not always possible. If you have to locate the sensor further away, then it is necessary to compensate for this in the system calibration.
A frequent source of problems is the length of cable being used. Wherever there are long runs of cable, there are corresponding signal losses, which cause measuring error. For accurate measurement, appropriate cables must be matched to the sensor type. For example, the cold junction of a thermocouple can be created at cable joints instead of the controller, meaning an error is introduced.
Sensors may be subject to significant amounts of heat over their lifetime, with corresponding deterioration in terms of measurement precision. An accumulation of material on the sensor face will also affect its accuracy. In these cases, it is advised to recalibrate a controller regularly in order to avoid subsequent measurement errors.
There are two ways to calibrate temperature sensors. One is single-point, or zero shift, calibration, and the other is two-point calibration. Single-point calibration is used in situations where you have an error value that is common across the required measurement span.
It uses a single offset parameter setting that is added to, or subtracted from, the uncorrected measurement to display a calibrated value. At the mid-point or perhaps the most critical point of your operating range, the controller's measured value should be compared to a calibrated temperature source to determine the offset needed.
Where the error level is not constant across the range, two-point calibration should be used to achieve the desired accuracy. Two offset corrections are made to the non-calibrated device, one at the lower end of the operating scale and the other at the upper end. The user calculates the offsets for the controller to apply at these points.
When calibrating a controller input within a system you should always work inside the temperature band at which you want the machine to operate. For example, if your machine will always operate between 200 degrees and 400 degrees, your minimum calibration reading should be taken at 200 degrees and your maximum reading should be taken at 400 degrees. There is a linear relationship for the displayed value between the two calibration points.
Some processes require certification of calibration – if so, you may have to ask an approved engineer to perform independent calibration services; this will provide evidence that the process temperatures measured fall within certain accuracy.
In the UK this service is provided by calibration specialist companies accredited by UKAS (United Kingdom Accreditation Services), while other territories use their own accreditation body; for example, in the USA it is NIST (National Institute of Standards and Technology) and in Germany it is DakkS (Deutsche Akkreditierungsstelle).
Following calibration, the accredited calibration specialist will provide you a certificate to show that your system has been calibrated and meets a stated tolerance.
With the right calibration, you will not only ensure that industry requirements are being met, but also enjoy a range of benefits, from reducing waste to driving higher throughput on the production process by optimising the entire control system.
Ian Collins is a product manager at West Controls Solutions