Realising the wireless switch

30 June 2011

John Corbett looks at the options for harvesting motion, light and heat for wireless monitoring and control

Line powered wireless systems have been available for a long time but, over the years, battery operation prevailed as a more attractive means for wireless monitoring and control. Now, however, battery replacement and disposal costs associated with many building control applications prohibit their use so alternative means of powering products, such as energy harvesting, are coming to the fore.

The concept of harvesting from the immediate environment is not new; wind and water have been such sources for hundreds of years. The concept of making a wireless, batteryless solution has, however, only recently been achievable. Typical uses of these devices are in the sensor and control market and, as such, the principle energy sources available are kinetic energy, light and heat.

Harvesting these sources of low energy poses a problem if you want to maximise wireless range as power is ratio of energy over time and some devices – like kinetic harvesting switches – do not operate over long periods.

Fast reliable transmission protocols are vital and these have been effectively incorporated into the EnOcean Wireless standard.

Motion
Linear Motion is the most obvious solution for any switching device but finding a suitable and sustainable method to extract energy is no simple task. Piezoelectric and inductive generators are the most common. Piezoelectric devices can give a small footprint per column of energy but can also be inefficient and mechanically unstable. Inductive solutions are larger but are lower in cost and more efficient.

Clearly when using an inductive system for a switch there is little mechanical movement and only a few Newtons in the switching action. This means that sophisticated techniques are required to generate enough energy to power a wireless switch - this can be achieved in the energy harvesting design shown below and is used in EnOcean’s self-powered range of devices.

With sufficient expertise it is possible to design a product in combination with the electronics to transmit three data packets or telegrams per button push allowing on/off and dimming commands to be transmitted.

Light
Indoor solar cells are an ideal energy source for most areas and a small eight cell design can deliver 11-14 uA at 3-4 V. Light outputs can be a variable in most buildings and, of course, can be absent at night so suitable energy management and storage schemes need to be employed to avoid shutting down devices.

Whilst NiCd or Lithium batteries are low cost their life span can be short, depending on energy usage, and they require complex charge circuits. If a maintenance free system is required for 10+ years a better approach is to use a PAS Capacitor. A PAS or Polyacenic semiconductor is smaller than a double layer capacitor and has a high capacity per mm2 with low self discharge rates. It is also environmentally friendly having no Cadmium, Mercury or lead. A PAS coupled with an effective energy management scheme makes batteryless PIRs, Thermostats and CO2 sensors simple to implement at minimum expense.

Temperature differences
Differences in temperature can occur in many locations and can be used to power a number
of remote applications. A thermoelectric device creates a voltage when there is a temperature variant on two junctions of two metals, a property discovered by Seebeck in 1821. Conversely when a voltage is applied, it creates a temperature difference (known as the Peltier effect). There are a number of low cost Peltier elements available and these can be used ‘in reverse’ as generators for small wireless monitors. The voltage generated from these elements is very low and it requires innovative DC/DC conversion to get the voltage to a level for use by a typical wireless controller. The most suitable device for this purpose is EnOcean’s ECT310, a lowcost ultra low voltage DC/DC converter that uses a blocking oscillator design. Wireless sensors and even actuators can be operated given suitable charge management.

With a two Kelvin temperature difference and a standard low-cost Peltier element the DC/DC starts operating at around 20 mV giving an output that depends on the actual temperature difference of the Peltier element. An input voltage range of 20 mV to 50 mV corresponds to an output voltage range between 3 V to 4 V. A typical thermodriven sensor consists of a sensor element, a small Peltier element, a DC/DC converter and a radio module as shown above.

Implementation of an energy harvesting wireless system
Implementing a wireless system using harvested energy constrains a microcontroller design and a balanced approach is required to ensure energy is available for sensing control. The start-up time of a microcontroller plays a strategic role and is usually influenced by oscillator delay. Crystals and ceramic resonators can take several milliseconds to stabilise.

RC oscillators, by contrast, provide fast start-up but generally suffer from poor accuracy over temperature and supply voltage. To save time it is advisable to use a microcontroller that can start with an RC oscillator and subsequently switch to a crystal oscillator.

Rapid switching of a sensor saves energy and is a particularly effective approach when measuring parameters that change slowly. It is also possible to achieve an average current consumption just above the total current consumption of the continuously running processor blocks given suitable power strategies.

While several circuit blocks can be switched off, others must be operated permanently, such as threshold switches.

These activate electronics and timers that trigger periodic activities such as sensor readings. These circuit blocks rapidly dominate the entire energy requirements and must be aggressively optimised. The timers of typical harvesting modules should require only approximately 20 nA, so these are typically analogue and switch off all components during sleep periods. This can enable a power reserve via the PAS of up to one week with solar devices, even in complete darkness.

If there are highly dynamic processes that need to be analysed it is also worth preprocessing the data in the sensor, reducing the data to be transmitted and only transmitting measurements if there is a change from the last measurement. Simple well architected software performance reduces execution time saving more energy.

Another element to consider when waking a CPU is the oscillator start-up time and power down. These include factors such as the time to enter and exit the mode and the energy consumed by doing this. Finally, avoiding flash, EEPROM and other memory writes clearly saves more power.

In order to conserve power it is necessary to employ a number of routines to balance power demands. Besides an Off mode that clearly shuts everything down and one fully active CPU mode, a number of interim modes make sense for strategic use of harvested power. For example, four standby and sleep modes can use various timers and frequency sources allowing the user to select the best power savings and timing accuracy


  1. Deep Sleep Mode: used for weak ambient energy powered, event triggered transmit applications, where ultra low power consumption is mandatory. Only the ultra low power blocks are active.
    Flywheel Sleep Mode: used for high precision system timing in low duty-cycle networks. This timer has a programmable cycle time and is based on a wristwatch crystal oscillator.
  2.  
Short Term Sleep Mode: is used for interrupts which are significantly longer than the main crystal startup time (i.e. between RF sub telegrams) and is based on a Short Term RC Oscillator with moderate accuracy.

Standby Mode: timers stay running and all the content of registers and RAMs stay alive to ensure the fastest wake-up and highest timing accuracy in exchange for higher power consumption.

Powered Receivers and TCP/IP systems
Having designed a portfolio of energy harvesting devices, they normally need to communicate with a powered device such as a light or HVAC controller. As these typically require permanent power anyway a harvesting network can make use of them for communication and control. In a standard system a harvesting switch can transmit to a receiver that has a triac or relay. Powered devices can also act as repeaters to span greater distances and can also assist two way communications. As a harvesting device can only listen for short periods, the powered device can be a postmaster/mailbox for nonpowered devices that can then pick up commands after they wake-up again.

Lastly, taking this one stage further, the simple receiver can be replaced by a gateway/controller that can communicate to the main system infrastructure over TCP/IP.

Many building systems are already in place using BACnet, LONworks or KNX, and it is a simple process to add a bridge/gateway so the EnOcean Wireless Standard and incumbent protocols can simply interact with each other.

Rapid switching of a sensor saves energy and is a particularly effective approach when measuring parameters that change slowly. It is also possible to achieve an average current consumption just above the total current consumption of the continuously running processor blocks given suitable power strategies.

While several circuit blocks can be switched off, others must be operated permanently, such as threshold switches.

These activate electronics and timers that trigger periodic activities such as sensor readings. These circuit blocks rapidly dominate the entire energy requirements and must be aggressively optimised. The timers of typical harvesting modules should require only approximately 20 nA, so these are typically analogue and switch off all components during sleep periods. This can enable a power reserve via the PAS of up to one week with solar devices, even in complete darkness.

If there are highly dynamic processes that need to be analysed it is also worth preprocessing the data in the sensor, reducing the data to be transmitted and only transmitting measurements if there is a change from the last measurement. Simple well architected software performance reduces execution time saving more energy.

Another element to consider when waking a CPU is the oscillator start-up time and power down. These include factors such as the time to enter and exit the mode and the energy consumed by doing this. Finally, avoiding flash, EEPROM and other memory writes clearly saves more power.

In order to conserve power it is necessary to employ a number of routines to balance power demands. Besides an Off mode that clearly shuts everything down and one fully active CPU mode, a number of interim modes make sense for strategic use of harvested power. For example, four standby and sleep modes can use various timers and frequency sources allowing the user to select the best power savings and timing accuracy;

1. Deep Sleep Mode: used for weak ambient energy powered, event triggered transmit applications, where ultra low power consumption is mandatory. Only the ultra low power blocks are active.

2. Flywheel Sleep Mode: used for high precision system timing in low duty-cycle networks. This timer has a programmable cycle time and is based on a wristwatch crystal oscillator.

3. Short Term Sleep Mode: is used for interrupts which are significantly longer than the main crystal startup time (i.e. between RF sub telegrams) and is based on a Short Term RC Oscillator with moderate accuracy.

4. Standby Mode: timers stay running and all the content of registers and RAMs stay alive to ensure the fastest wake-up and highest timing accuracy in exchange for higher power consumption.

Powered Receivers and TCP/IP systems
Having designed a portfolio of energy harvesting devices, they normally need to communicate with a powered device such as a light or HVAC controller. As these typically require permanent power anyway a harvesting network can make use of them for communication and control. In a standard system a harvesting switch can transmit to a receiver that has a triac or relay. Powered devices can also act as repeaters to span greater distances and can also assist two way communications. As a harvesting device can only listen for short periods, the powered device can be a postmaster/mailbox for nonpowered devices that can then pick up commands after they wake-up again.

Lastly, taking this one stage further, the simple receiver can be replaced by a gateway/controller that can communicate to the main system infrastructure over TCP/IP.

Many building systems are already in place using BACnet, LONworks or KNX, and it is a simple process to add a bridge/gateway so the EnOcean Wireless Standard and incumbent protocols can simply interact with each other.

The author is Sales Director UK for EnOcean


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