Calsoft Labs Case Studies
Product Development

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Superheat Controller (SHC)


This case study showcases the development of Superheat controller for driving revolutionary refrigeration systems based on Silicon Expansion Valves (SEV). Calsoft Labs architected and developed the entire firmware running on PIC microcontroller based hardware board and PC based GUI for status monitoring and configuration of the entire system.

Client

A leading US company that develops and markets innovative silicon MEMS (Micro-Electro-Mechanical Systems) valves and electronics and applies them to standard industrial applications, thus enabling cleaner and energy-efficient solutions.

Challenge

The client had an existing PIC micro-controller based hardware. The challenge was to develop production quality highly optimized feature rich controller firmware and PC based GUI for configuring and monitoring the SEV control algorithms and related parameters in an aggressive timeline. In addition, the control algorithm specifications were provided in the form of a simple description.

Solution

A refrigeration unit runs at highest energy efficiency when the superheat of the refrigerant is maintained close to 0 degrees C at the exit of the evaporator of the refrigeration system. The goal of the superheat controller is to control superheat at a given target set point, often near 0 degrees C, by modulating a variable orifice silicon expansion valve (SEV).

The 7 member team from Calsoft understood the entire system and provided suitable algorithm optimizations and production essential features such as different levels of diagnostics (automated as well as manual) and sensor calibrations at end of line of manufacturing phases (pre-wire bond as well as post wire bond).

Calsoft developed modular architecture for the firmware based on structural decomposition of various system functions. The system consisted of scheduler, control algorithms to drive the SEV valve, configuration features, propriety host communication protocol and device driver modules for RS232/485 communication and various interfaces and system features. The entire system was developed in highly optimized modular -C language- implementation on a compatible compiler tool chain. The PC based GUI was designed with end-user in mind for the ease of operation albeit providing advanced controls for the mature users. In addition, the tool provides complete automation for different levels of system tests during manufacturing and bulk node addressing feature to communicate with networked (RS485) superheat controllers on the field. Software Failure Modes and Effects Analyses (SFMEA) were performed as per the industrial standards to arrive at tweaks for robust software system design.

The various components of the system are:

  • Scheduler: The SHC controller system software functions are achieved by a Timer based scheduler working at pre-fixed timer tics and this interval is referred to as the system’s scheduler time base interval. The time base interval was chosen based on the requirement that SHC controller software needs to execute an iteration of control algorithm in this interval. The scheduler is a state based activity scheduler which schedules a particular function that needed to be performed between the Scheduler time base intervals.
  • Interrupt routines: Various device interrupt routines (UART, Timers, etc) were designed to be highly optimized consuming less than 0.1 % of one scheduler iteration period. This ensured no impact on timing of main system tasks and thus ensured system design to be safe and robust.
  • Control algorithms and system features: Modular implementation of the system control algorithm and system features ensured correct functionality of the system and the easy maintenance of the system software.
  • Diagnostics: Power-on as well as operational diagnostics of various sensors and system conditions were implemented for safe operation and early notification of the system error conditions.
  • GUI tool: Highly interactive and easy to use GUI featured status monitoring of the system and configuring the system for various modes of operation and critical system parameters. The GUI tool communicates with the hardware board on RS232 / RS485 communication interface.
  • Prescriptive control: Highly configurable logical conditions are implemented to configure the system for various exceptional system conditions such as drive the system to a safe mode when silicon expansion valve is faulty.
SHC controller

Technology

  • OS: GUI application was developed and tested on Windows XP system. The OS-less firmware was developed on timer based scheduler implementation
  • Software/Tools: PIC C compiler tool chain for firmware, Microsoft Visual basic framework for the GUI tool
  • Hardware: Customer supplied hardware based on PIC 18X microcontroller with analog sensor inputs such as pressure sensor, internal and external temperature sensors etc., digital I/O’s, PWM output and UART port etc.
  • Languages: C
  • Technologies/Framework: Embedded and Microsoft Visual Basic framework
  • Protocols: Proprietary protocol for host to PC communication over RS232/RS485

Benefits

Calsoft helped the customer to:

  • Accelerate development of their product with skilled engineers
  • Reduce time to market with proven processes and methodologies
  • Increase product development productivity with a flexible, proven partner approach
  • Achieve their overall objective at a lower cost and with guaranteed support from a proven