Addressing the Need for Ultra-Low Power in Energy Harvesting Implementations

By Bruno Damien, ON Semiconductor & Andrea Colognese, Canova Tech

 

The subject of energy harvesting has gained a lot of interest within the electronics design community over recent years. It is through this process that small quantities of energy can be captured, collected and then utilized by items of electronic equipment, allowing simple tasks to be accomplished without the need for incorporating a conventional power source in the system design. In order to do this effectively, however, the system needs to operate with the highest possible levels of efficiency, both in terms of the constituent parts that are specified and the way the system is laid out. The following article will discuss several technical challenges and show how innovative digital, analog & power management semiconductor technology is playing a key role in overcoming them.

 

Applications that are now utilizing energy harvesting (or scavenging) include building automation systems, remote monitor/data acquisition devices and wireless sensor networks. As harvesting does not rely on conventional forms of power source it has two key ecological benefits. Firstly it does not result in any depletion of fossil fuel reserves and secondly it does not add to pollution levels (as there are no resultant carbon emissions, nor disposable batteries). In addition to dispensing with the need for wiring or cabling and the convenience thereby derived, the real advantage of this sort of implementation for OEMs and system integrators is that, once it is in place, it effectively has no day-to-day running costs, as there aren't utility bills or costly call out trips to replace batteries, etc.

 

Extracting the required energy

The harvesting of energy from the environment can be done in a variety of ways (depending on which proves most suitable for the specific application setting), with power levels normally in the region of 10µW to 400µW being generated. Among the mechanisms used are temperature difference, kinetics (normally through vibrational movement), solar power, the piezo-electric effect, the pyro-electric effect, and electro-magnetic. However, with the possible exception of solar energy, the perception that energy harvesting is ‘free’ energy is not totally accurate. Sources based on vibration or thermal gradients make use of considerable energy waste from the system. As a result repair and maintenance costs do need to be factored in.

 

 

 

Figure 1 : The power Range scale of real world applications

 

The power that is generated through the harvesting process can be used in many ways, for example:

Switches (building automation) - Here the mechanical force applied to move the switch ON or OFF is enough to generate a few milli Joules (mJ) worth of energy to run a wireless transmitter. This sends an RF signal that actuates a door latch or a light. As no wiring is needed there are both logistical and aesthetical upshots to this approach.

Temperature sensors (building automation) - The temperature difference between the ambient air and a heater can provide the power needed to send temperature data back to regulation system wirelessly.

Air conditioning (building automation) - The vibration of the air-conditioning duct can be employed to create an electrical signal via electromagnetic induction. The air conditioning can be controlled through this signal.

Remote monitoring (industrial/environmental) - This could be in the form of an unmanned weather station, a gas sensing system in a chemical plant, a Tsunami warning system. A solar cell or a small wind turbine can provide the energy required.

Medical implants (healthcare) - Such as blood glucose monitors, where heat or body movement allow a low power wireless transceiver placed on the patient’s skin to feedback data to a hub without the need for inclusion of a battery (thereby improving the patient's comfort and reducing the inconvenience experienced

Watches (consumer) - Where the use of either solar or kinetic energy can be used to run a battery-less timepiece.

Tyre pressure monitoring (TPMS, automotive) - Using surface acoustic wave (SAW) sensing technology, it is possible to circumvent the issues arising from mounting the battery and complicated electronics needed to support temperature/pressure sensors on each of the vehicle’s wheels, thereby reducing bill-of-materials costs and the engineering resource needed.

 

System Design Considerations

With only µWs of power to play with, it is clearly vital that everything possible is done to utilize it to the fullest. Engineers need to work hard so they can avoid wastage. This involves both hardware and software considerations and can be done through implementation of highly efficient component parts, as well as ensuring full design optimization. It is imperative that the electronic system consists of low voltage circuitry made with smart power management. Energy storage may also need to be considered, as the sporadic nature of these systems' operation means that in many cases there is no direct relationship between the time when energy is harvested and the time when it is subsequently utilized. The storage method used must be low voltage, with a high charge current capability, moderate discharge capability and possibly no self-discharge capability at all. The digital IC at the heart of the system must be able to offer more than adequate processor performance to carry out the system’s tasks while simultaneous being able to support low voltage operation, so that the power budget is not exceeded. Furthermore this IC must be cost-effective enough that its implementation does not impact too greatly on the overall expense associated with the system - otherwise the system will have too high a price tag to justify deployment in many of the energy harvesting applications already discussed.

 

 

 

 

Figure 2 : Cautious use of Available energy

 

Normally if there is a need to enhance performance levels, achieve greater optimization or raise the degree of integration, OEMs will look at taking a customized approach and engage with an ASIC vendor from the beginning of the project. Unfortunately this way is not always possible, as it requires large upfront financial investment to cover the NRE costs. This must subsequently be followed high enough unit volumes to recoup the investment. Many energy harvesting applications do not constitute large enough unit volumes to take this approach, but conversely engineers going down the route of just bundling together off-the-shelf components are unlikely to maximize their system’s efficiency. To make matters worse the development process is likely to require a great deal of time and engineering resource.

 

A third option is now available to the design community, offering the favorable technical attributes of an ASIC but without the investments and time-to-market drawbacks. This approach combines an ultra-low power microcontroller with an efficient, ready to customize and predefined IC integrating critical and must-have blocks like the harvesting interface and power management functions, sensor and actuator interface. Canova Tech’s ETA Platform provides an example of this.  Based on the LC87F7932 ultra-low power microcontroller unit (MCU) from ON Semiconductor, and Canova Tech ETA Platform , this new development kit gives engineers an industry-proven development kit that can be customized ( hardware and Software ) in order to suit specific application requirements and thus augment the system’s power/performance characteristics. The ETA platform is fully configurable and it can be interfaced and matched with most of the common energy harvesters in the market, handling DC and AC inputs larger than 0.9V or, with the use of an external transformer, larger than tens of a millivolt. The collected energy can be transferred / stored in various storage elements such as chemical batteries, capacitors and super capacitors. Through it the system can manage the accumulated energy efficiently, regardless of erratic delivery patterns, so that it can implement power saving strategies, like the use of the embedded ultra-low power configurable analog front end, in which the acquisition and conditioning of signals from the system’s sensors can be carried out without the supervision of the external MCU.

 

 

 

 

Figure 3 : Block Diagram and layout of the Eta Platform

 

 

 

The LC87F7932B MCU is an 8-bit device based on CMOS technology. It has a central processing unit (CPU) running at a 250ns (minimum) bus cycle time. The IC integrates 32 kBytes of on-board programmable Flash memory, 2048 Bytes RAM, an on-chip debugger, an LCD controller/driver, a 16-bit timer/counter and a real time clock. Its 12-bit, 7-channel low power analog-to-digital (ADC) converter transforms the acquired signal after conditioning has been completed by the front end. This digital signal can then be transferred wirelessly or stored for extraction at a later stage depending on the application.

 

 

 

 

Figure 4 : Example of selected Active block (in red)

 

 

In conclusion, there are a number of major obstacles and challenges involved in the design of energy harvesting systems. Engineers need to boost processing performance as much as possible, while keeping overall power budget to a minimum and not accruing heavy expenditure in what can prove to extremely cost-sensitive applications. Every effort must be made to employ the best optimized components and to ensure that the development process is totally streamlined. By employing the development platform detailed in this article, based on an ultra-low power MCU architecture and a configurable and customizable device, engineers can overcome these obstacles and thus realize more effective implementations.

 

 

 

 

Canova Techs ETA Platform

 

 

 

 

Functional Block Diagram for ON Semiconductor’s LC87F7932B MCU