The challenge was to create a high-performance test bench to test HVAC compressors over a range of voltages and frequencies while enabling remote commissioning and operation via VNC and web browser. Unitronics’ UniStream controllers were selected.
An application success story from Unitronics
A client of Grupo Electrontecnial Galiana based in Madrid, needed a high-performance test bench, capable of testing HVAC compressors for the global market over a wide range of voltages and frequencies. The client also expected Remote Access capabilities so that operators would be able to remotely access and control the testing process. For this complex machine advance features were required. The UniStream® USP-104-B10 PLC + HMI, a UniStream® USC-P-B10 PLC, and a selection of Uni-I/O® from Unitronics were integrated into the solution.
The Application Challenge
A test bench machine was needed to test each HVAC compressor for the global market before it leaves the factory. Due the reality that countries have different power supply standards, the compressors must function at a wide range of frequencies and voltages. Consequently, the test bench required the capability to simulate any power supply in the world.
The bench had to be capable of generating from 12 VDC to 460 VAC in 50 or 60 Hz with a maximum power of 250 KVA to that it could simulate power supplies world-wide.
Voltage Control: 3 power lines had to operate simultaneously.
UniStream had to provide precise voltage at each output, by managing each single line through separate sequences, commanding the speed drives, switches, contactors, autotransformers and many other components.
The operator initiates a sequence by selecting the line frequency of 50 or 60Hz and the voltage to be set in the output. Then, UniStream runs a sequence of steps: Closes some contactors, starts the speed drives if necessary and closes the corresponding output of the transformer.
Once there is voltage in the output, UniStream automatically fine-tunes, controlling the voltage as close as possible to the set point. Depending on the load and the type of test, this voltage might change during the procedure, so the PLC continuously regulates the transformer to be able to remain as close as possible to the set point. Data: There were also requirements for acquiring data to be able to analyze it after the test. He utilized UniLogic’s built-in data functions to create timestamped datalogs of the tests, and then export them to .csv files for analysis.
The operator must be able to run tests while sitting in his office or somewhere else. UniStream’s built-in VNC Server feature made this simple, including commissioned the machine off-site, via VNC. The biggest obstacle was using a single controller to handle an extensive list of different requirements and tasks. Added to controlling the actual testing tasks, the PLC needed to handle communications – Modbus RTU and ModbusTCP, data logging and file handling, and well as enable remote operation.
User Defined Function Block
There were specific challenges that their engineer met by simply building his own function by using the User Defined Function Block (UDFB) feature. For instance, he needed to regulate the autotransformer and could not use the built-in PID function because the autotransformer is not an analogic actuator. It works by sending a digital command to increase or a digital command to reduce the voltage. Luckily, he was able to easily create his own PID function, which sent pulses to the transformer according to the distance between the set point and the output voltage value. When the distance between the set point and voltage is decreasing, the pulse width decreases. When the distance is great, the pulse width increases, continuously regulating the voltage to be able to reach set point.
Furthermore, he also created functions to enable operators to operate individual system components for easy system maintenance. He built function blocks for the contactors and the autotransformers, integrating auto/manual, interlocks and fail-to-start features. Then he made a template HMI screen for all these objects; this allowed him to create unique instances simply by indexing the structure to be displayed in the screen.
He had another challenge regarding Modbus—specifically, with the response speed of the 5 Modbus RTU slaves in our application. He solved it easily, by changing three of them to Modbus TCP. Afterwards, they had Modbus RTU and TCP working together—this turned out to be a good solution. The company was able to significantly increase network performance just by making a few changes in the PLC.
Advantages with Unitronics:
- Communication configuration is very easy with UniLogic. They can communicate through Modbus RTU and change some slaves to TCP easily and have both working with excellent performance
- Function blocks are easy to build and re-use.
- HMI screens: It’s very easy to develop a single template screen for function blocks which allows every component to be operated individually.
- Data, logged and exported to CSV files with the time and date of the tests for the operator.
- Remote Access & Control – UniStream’s VNC server was set up to perform the commissioning remotely, and to allow the operator to control the testing from home or the office.