Safety Standards in Appliance Motion Control Made Easier with Digital Control
The design of an advanced energy-saving, inverterized motion control system requires designers to deploy various skill sets in order to be able to address the wide range of technical challenges needed to complete the project on time and on budget. Not only must the designer select efficient power semiconductors and choose the appropriate microcontrollers and control algorithms, he must make sure with the utmost certainty that the system provides safe and reliable operation, meeting stringent safety standards like IEC60730 and IEC60335.
Allowing designers with digital control chipsets to be certified under the 60730/60335 standards greatly simplifies the engineering effort needed to meet the safety standard of appliance motor drives (i.e. in washing machine, dishwashers or air-conditioning systems), a mandatory requirement for today’s home appliance goods.
The IEC60730/60335 addresses the specific set of anomalies or abnormal operation conditions that can affect among others, the mechanical, electrical or electronic operation of AC appliances.
The design of an automatic control system for an Appliance product cannot be done without a predefined choice of the target safety classification. IEC60730/60335 divides the categories in :
- Class A: Products not intended to be relied upon for the safety of the equipment.
- Class B: To prevent unsafe operation of the controlled equipment.
- Class C: To prevent special Hazards.
In our specific discussion, because the Class A type of controls is deemed not to be hazardous if the software control malfunctions, the IEC norm does not require the designer to implement system checks and the development of the automatic control system is simplified.
Class C products are typical of gas heater or products containing flammable/combustible material hence the require redundant systems (i.e. additional sensors) to prevent hazardous situation, so also in this case the automatic control system can rely on the support of a mandatory redundant sensor.
However the vast majority of products are required to comply with the Class B Level. So the automatic control system has to include in its code all elements to prevent unsafe operation, without relying to the presence of an external redundant sensor or independent circuit.
In washing machines and dishwashers, the typical main problems with complying with the norm relate to the motor overheating and to a reliable motor speed signal monitor. The safety implications for the end products are obvious, whether it be a motor overheating or a motor driven at the “out of control” speed limit .
The speed control loop requires the reading of current sensors signal in the power inverter circuit shown in the figure.
In order to comply with the Class B requirements, the automatic control system code has to detect any conditions of malfunction for the current sensors, like open or short circuit of the element.
Because the demands for high washing efficiency, high energy efficiency, and low acoustic noise are a must for modern appliances, this leads to the use of a sinusoidal permanent magnet motor and a sinusoidal inverter control.
Sensorless control of the main circulator pump is therefore becoming more pervasive as a standard method to address satisfactorily the efficiency issue and the noise issue. Also, the elimination of Hall-effect Sensors also increases the reliability. In fact, at high temperature, Hall-effect sensors are more prone to reliability problems. However, the complete elimination of Hall-effect sensors introduces other challenges to the automatic control system to address the safety issues from the software point of view. For example, the user may open at will the dishwasher main door during the washing cycle to add or remove dishes. The inverter circuit controlling the main circulator pump needs to have a reliable motor pump speed signal to ensure that, by the time the door is open, the pump speed is reduced to avoid extremely hot water getting out of the washer. Since the speed signal is obtained by software algorithm based on inverter currents read by the current shunt, then the software must be Class B compliant.
Washing machines pose similar safety requirements to dishwashers. Modern horizontal washing machine program controllers need to rely on software to release the door lock to open the drum to access clothes. If drum is still rotating, this can pose a safety hazard to the user’s arm. In this case too, the software of the automatic system control must be Class B compliant.
Another important example is using a software calculated thermal image of the motor (in washers and also in fans) to avoid over-heating that could cause fire hazard. Again, the software needs to be Class B compliant.
The IRMCK171 is a one-time programmable (OTP) ROM-based monolithic mixed-signal IC for sensorless sinusoidal motor control of appliances that meets the standards of IEC 60335-1 Edition 4.2 – Class B.
The IRMCK171, part of the iMOTION™ integrated design platform for variable speed motor control, incorporates IR’s patented Motion Control Engine (MCE™), an algorithm editor that eliminates coding. A co-integrated 60MIPS, 8-bit, 8051 microcontroller enables application layer software development, operating almost independently of the MCE™ and does not compete for system resources such as interrupts or internal registers. The embedded Analog Signal Engine (ASE) integrates all the signal conditioning and conversion circuits required for single current shunt, sensor-less control of a PM motor
IR has developed specific source code level libraries to enable users to design automatic control system in compliance with the safety standards.
For example, the IRMCK171 Self-Test Library provides a set of function calls implementing the 8051 power up and periodic self-tests (safety checks) required for IEC 60335-1 Annex R and IEC 60730-1 Annex H Class B software compliance.
The library is provided so that end users can implement an IEC-compliant 8051 application with minimal effort.
The IRMCK171’s motor control engine (MCE™ processor) also implements power up and periodic self-tests in firmware.
These tests are also required for IEC 60335-1 Annex R and IEC 60730-1 Annex H Class B software compliance and run in conjunction with the functions of the 8051 library.
For added safety, the MCE™ self-tests are built into the firmware rather than being supplied in library form since the MCE™ firmware is not user-modifiable, so no risks for possibility for programming errors.
The 8051 self-test library functions control and manage the MCE™ self-test functions automatically.
Power up tests execute once at system startup (after power up or reset) to validate basic functionality of the 8051 and MCE™ processors and memories.
Periodic tests execute on a regular basis during normal runtime operation to monitor proper operation of system components, firmware and application software.
The designer that uses the IRMCK171 in automatic control system for motor drives in appliance or industrial applications can use the self-test libraries and firmware pre-developed to simplify and shorten the cycle to obtain Class B certification from the regulatory bodies. This dramatically reduces development time and shortens the time-to-market release cycle.
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