Adding analog inputs or signal processing functionality to a machine or robot control system often can increase costs and complexity substantially. There are two main ways for OEMs to address this issue: with external signal conditioners or with signal conditioning input modules.
External signal conditioners can accept analog input signals, and send a discrete output to the PLC or other controller. "If an OEM is using a PLC, adding analog modules can increase total cost greatly, and also can degrade overall control system performance," says Daniel Liu, business development manager of data acquisition and control at Moxa Technologies. He prefers Moxa's standalone ioLogik microcontroller. "Our E2242 has four analog inputs and 12 configurable discrete outputs, and it can be used to condition or process analog signals, and to translate these signals into discrete outputs that can be fed to a low-cost PLC," Liu says. "It uses simple IF-THEN-ELSE programming for setting analog thresholds to trigger the discrete outputs."
When operating conditions are harsh and demanding, external signal conditioners often are found to be the best solution. "A wood firing system OEM of industrial boilers and kilns needed to increase the accuracy of signals from a specialty oxygen sensor," explains Michelle Goeman, product manager for terminal blocks and electronic interface at Wago. In this application, the signal conditioners had to meet the OEM's five-year warranty, a condition vital for this large exporter to reduce warranty claims. Another complicating factor was that all the conditioners had to withstand routine operating temps of 70 °C (158 °F).
"The OEM specified our Jumpflex mV signal conditioner because it can be fine-tuned for millivolt signals via DIP switches, or via software with our FDT/DTM configuration tool," Goeman says. "The millivolt signal transmission error is <0.1%, even after adjusting signal ranges, and recalibration is never required due to factory-calibrated, laser-capable resistors within the DIP switches."
A U.S.-based machine builder of lithium-ion systems needed its automation system to receive analog signals of varying voltage ranges, and these signals were very susceptible to noise and difficult to monitor.
To better manage the voltage ranges, the OEM worked with Rockwell Automation to implement a versatile current/voltage signal conditioner. The signal conditioner is DIP-switch-programmable, and converts the customer's varying signal ranges to a standardized 4–20 mA signal.
"The OEM standardized its bill of materials by using one signal conditioner for all its applications," comments Christopher Welker, product marketing engineer at Rockwell Automation. "Because all signal conditioner outputs are 4–20 mA, the OEM was able to standardize on PLC input modules. The signal conditioner also helped provide isolation on the analog loops, which helped reduce noise and eliminated the possibility of ground loops".
Weighing and scaling can be a particularly difficult application for analog signal processing because measurements often need to be made in areas with vibration and other influences that can cause inaccuracy. An expensive way to solve this issue is with an external weighing instrument. A more cost-effective means is to use an analog input with built-in signal conditioning features.
"One of our customers is using our Snap load cell module in an application that involves weighing grain on a large platform-type scale," says Ben Orchard, application engineer at Opto 22. The customer needed to obtain a more accurate measurement that would compensate for the oscillation and interference caused by workers treading on the scale, and by scale movement and vibration as loads were being brought on and off.
"When they configured the load cell module with our PAC Manager configuration software, our customer used the analog filtering functionality to condition the raw load cell signal," Orchard says. "With a little tweaking and experimentation, our customer succeeded in reducing the multiple readings they were obtaining due to scale vibration, and they were ultimately able to acquire a more accurate measurement."