InWheelSense™: Where the rubber meets the road

As the electric evolution of all motorized vehicles continues in the 21st century, electronic systems for safety, comfort, convenience and entertainment purposes, continue to be developed, with some automotive areas having been targeted earlier and with more functionality than others. But despite all the various existing electronic systems, it wasn’t until the 1980’s that an electronic system penetrated into the automotive wheel environment. Tire Pressure Monitoring Systems (TPMS) were initiated in the European market and have proliferated globally from there. Other than this system, there are no other widespread electronic systems implemented into the tire-wheel arena currently … if one excludes the self-inflating tire systems used by the military, some commercial vehicles and a few high end vehicles. But these typically use a TPMS as part of the overall system.


Throughout the history of the automobile, the implementation of electronics has lead the way in major innovation milestones. Electric “vehicles” themselves have been around since the mid-1800’s and electric cars late that same century which have now come full circle. One of the earliest electronic systems was the car radio (albeit with vacuum tubes) in the 30’s, followed by electronic ignition systems in the 50’s, Transmission Control Units (TCU), Adaptive Cruise Control, (ACC), electronic ignition, fuel injection and fuel pumps in the early 70’s and processor based systems in the mid 70’s and on and on and on. To the point where electronic systems now constitute >30% of the cost of a vehicle on average.

Due in part to the U.S. adopting the 2000 Tread Act, TPMS are now commonplace and these target occupant safety, avoidance of accidents and cost savings. Cost savings are realized by providing real-time tire inflation information in order to reduce poor fuel economy, poor handling and help prolong effective tire life by reducing tire wear associated with under/over inflated tires. The TPMS provides an indicator warning, via a transponder coil, to the driver when any of the tires are not inflated to an optimized pressure range. These systems are powered by a non-rechargeable, single use on-board battery.

However, where the “rubber meets the road” can be a source for much more information than just what the tire pressure is. But in order to implement more sensors and support electronics, a higher power generating on-board tire-wheel source is required. This environment has a unique problem that most other vehicle systems don’t have, i.e., how to provide sufficient power to a rotating device and still have it be light weight, small size, cost effective and not violate more than one or two laws of Physics. This is where our story begins.

InWheelSense™ introduction and technology overview

Imagine a system technology that would allow one to easily and accurately measure multiple parameters. One that was self-sufficient, all-inclusive and with the capability to allow outside-the-box thinkers to integrate their “futuristic” ideas into the tires themselves and allow the system to scale as new applications arise. What type of parameters? Besides tire pressure, possibilities are: tire wall temperature, road surface conditions, wheel alignment conditions, and many, many more. Now imagine that all of these can be achieved by an EXISTING single system solution.

Just now being introduced into the market, InWheelSense™ is a multi-faceted technology that consists of three main sub-systems: power, sensing, and connectivity. This innovative, piezo-powered sensor platform delivers data analytics, obtained at the wheel, directly to the vehicle and/or to cloud connected devices. Analytics supporting high performance driving, autonomous vehicles, safety, comfort and energy savings.

The standard platform comes with a set of pre-selected sensors. These were chosen to highlight just a few of the possible features and solutions using InWheelSense™ and ones that would enhance performance, driver safety and comfort. However, the system is flexible and allows the user to select and integrate sensors of his/her choice. The optimization of the power system is then adjusted to ensure that the system self-generates sufficient power for more sensors and/or more power hungry sensors.

InWheelSense™ Energy Harvesting (EH) Module

Figure 1:TDK InWheelSense™ EH module

Figure 1 TDK InWheelSense™ EH module

Power is generated by the energy harvesting module which contains a lead zirconate titanate (PZT) ceramic element that is classified as a piezoelectric (and ferroelectric) material. When an electric current is applied to the material, thus creating an electric field (voltage delta) between two surfaces, it causes a mechanical deformation of the material. This is actually called the “inverse piezoelectric effect”. The opposite is also true, i.e., by applying a mechanical force or movement of the material, a small electrical charge (thus current) can be generated and is called the “direct piezoelectric effect”. InWheelSense™ uses this latter phenomenon for power generation and also when the element is used as an actual sensing element. The amount of current generated is dependent on the element’s size and shape, force direction and the amount of mechanical deflection of the element and this allows the unique on-wheel system to generate enough power to be self-sufficient.

The EH module works with a range of wheel sizes from 16” to 21” in diameter. It is mounted on the wheel at the tire-wheel interface, yet does not impact the integrity of the air tight seal. This mounting scheme ensures optimum power generation and is repeated every revolution for each EH module mounted on the wheel. For example, when driving at 65 mph on 18” diameter wheels, each rotation will generate around 1mW of power per module. If more power is needed, the platform allows for power scalability by adding multiple EH modules. An example of a complete mounted InWheelSense™ system is shown in Figure 2.

Figure 2:Installed InWheelSense™ platform with 5 EH modules on an 18” wheel

Figure 2 Installed InWheelSense™ platform with 5 EH modules on an 18” wheel

The generated energy can then be used immediately, with some power conditioning, or can be harvested and stored in an on-board battery or an electric double-layer capacitor (EDLC) super-capacitor. The differences between these two energy storage devices are a battery, e.g. a lithium-ion battery (LiB), will store more energy and for a longer period but also takes longer to be charged while the ELDC is able to be charged very quickly AND deliver the energy more quickly. The InWheelSense™ system, in a standard 5 EH module configuration, can supply milliwatts (mW) of continuous power or 90mW in a burst mode for about 30 msec. This burst mode need is typical of sensors that provide intermittent functionality that comprises of both sleep mode, and data transmission mode and spend a majority of their time sleeping, then wake to measure and send out data.

The power generation of the patent pending EH module takes advantage of the energy harvesting methodology that occurs with each revolution of the tire-wheel and optimizes the conversion of the rotational energy into electrical power and provides higher power output than other coming-to-market solutions can. The EH module leverages rotational energy while most other systems make use of linear forces. The EH module is the product that will be manufactured by TDK.

The overall power system is flexible enough as to allow the user to distribute the generated power as needed, and the total amount of power required will ultimately depend on the sensing circuits and sensors implemented by the user. The sensing functionality is part of the InWheelSense™ Control Module (IWCM) and is discussed below.

InWheelSense™ Control Module (IWCM)

The IWCM is the heart and soul of the InWheelSense™ evaluation kit (Eval Kit) platform. It was specifically developed to feature just a few of the numerous solutions that are now possible with on-wheel sensors that support both on- and off-wheel metrics which the sensors can provide highly accurate data for. Off-wheel metrics meaning things like car door closures, gear engagement, etc. These will all be user defined but the key point is the EH module power generation will provide the needed power to implement more sensors than just the current TPMS or tire strain gauge.

The IWCM is securely mounted on a backing plate that is clamped onto standard lug nuts of the wheel. It is the preferred mounting location to reduce gravitational forces (G) subjected to the additional sensors inside the IWCM, thus preventing “saturation” of these sensitive devices. This avoids inaccurate data and, increases sensor life by reducing mechanical stresses.

Figure 3:InWheelSense™ Control Module” wheel

Figure 3 InWheelSense™ Control Module

Power Conditioning Interface

The power conditioning interface of the IWCM performs the system voltage rectification, step-down voltage level control, and smoothing of the EH module generated alternating current (AC) voltage waveform. This interface also provides regulated power directly to the load and is now in development to support the storage of power, including the battery charger circuitry, to the battery itself and system power monitoring. Currently the on-board battery is charged by an external USB source. The standard IWCM printed circuit board assembly is shown in Figure 4.

Figure 4:IWCM mother board with sensors

Figure 4 IWCM mother board with sensors

The on-board custom power management integrated circuit (PMIC) converts the EH module generated AC voltage to a rectified direct current (DC) voltage, and steps the voltage level down to a steady, ripple-free regulated 3.3 volts (V). There is an additional on-board battery with 2500mAh capacity for providing power to 3rd party components for testing purposes. Future plans are to use the regulated output and implement a direct battery charging stage.

Sensors Interface

Most of the TDK and 3rd party sensors reside within the IWCM. The ADC interface for the EH modules are also inside the IWCM. The presets in the IWCM allow the EH module to function as an energy harvester (EH gen) or, act as a sensor (EH sense). This setting is selectable using jumpers on the motherboard. In future designs, this will selectable on the fly using software controls. The Eval Kit includes sensors that were selected to showcase many foreseen applications and data needs and includes:

  • Multi-channel High Sensitivity InWheelSense™ EH sensor
  • 6-Axis inertial measurement unit (IMU)
  • Barometric Pressure sensor
  • Temperature sensor

Future development activities include:

  • 9-Axis IMU
  • 2 channel Microphone
  • Ultrasonic Time of Flight (TOF)
  • Hall effect switch
  • Shock/Wake sensor (EH sense)
  • Brake dust, tire micro plastic particle sensor

The above existing sensors are included solely for the purpose of enabling the Eval Kit platform and the user will have the flexibility to add or remove sensors as deemed necessary. These can be either TDK sensors or any other 3rd party sensor solutions with IoT platform capability. The Eval Kit provides an interface connector to allow daughter boards with custom sensors to be easily integrated on the IWCM.

Along with sophisticated predictive algorithm libraries, the Eval Kit sensors can be programmed to target specific applications and detect unique signatures. The signatures can then be used for machine learning (ML) which will help translate collected data into performance metrics. This includes real-time speed, road surface condition, slip detection, tread wear, pot hole mapping, tire sidewall temperature sensing, loose wheel indication, behavioral modeling, black box metrics for insurance purposes, robo-taxi safety, fuel efficiency sensing etc.

The InWheelSense™ Eval Kit interfaces with an open software platform that allows access to libraries and associated tools for streaming live data. This further allows visualization tools like cloud and PC based “dashboards” to be used to display data in a more user friendly format. Lastly, the IWCM platform provides a path for Edge Computing.


The IWCM platform currently includes low energy Bluetooth (BLE) for wireless connectivity. The BLE system allows for continuous real-time data streaming. The data is sent to the vehicle gateway and/or host computers and processed to provide driver friendly information. Processed data can then be shown on the aforementioned dashboard type displays, indicators or warning light notifications. Applications targeting mapping or supporting autonomous driving benefit from real-time data streaming.

Future plan is to add long range (LoRa) which is a low-power wide-area network (LPWAN) protocol. LoRa, being a spread spectrum technology, enables long-range transmissions (more than 10 km in non-urban areas) with low power consumption for battery operated devices. The long range/low power combination does limit the data rate that can be transmitted to around 50 kbps, and is dependent on the spread factor (SF). However, the maximum data rate does not suffice for current InWheelSense™ sensors’ raw output data to be transferred from the end-device to the gateway. Therefore, any LoRa data will need to be processed by the IWCM and/or limited in its scope and Edge Computing will allow raw data to only be transmitted by BLE or stored on-board for later download. Depending on the global area, operating frequencies fall within the license-free sub-gigahertz radio frequency bands such as: 433 MHz, 868 MHz (for Europe), 915 MHz (for Australia and North America) and 923 MHz (for Asia).

Within LoRa, the sensors in the system will be asynchronous and transmit data when it is available to be sent. Data is transmitted from the sensors in the wheel (end-node) to the gateway, which then forwards the data packets to a centralized host computer or processor unit. LoRa chipsets also have geolocation capabilities.

Lastly, the InWheelSense™ will soon incorporate an on-board “black box” that will provide data recording capability, via a memory module, for data that can be either saved for some predetermined period or for “data dumping” once connected to a gateway.

Installing the InWheelSense™ Evaluation Kit system

The above sections discussed the main sub-systems of the InWheelSense™ platform and what function each performs. The mechanics of the system are shown in Figure 5.

Figure 5:System hardware a) individual modules b) mounting hardware c) installed system

Figure 5 System hardware a) individual modules b) mounting hardware c) installed system

Starting left to right, in Figure 6a, the Eval Kit’s modules are shown with the IWCM in the center and the EH modules at 72º intervals around the center. Figure 6b shows the mounting hardware which secures the angular positioning of each EH module and provides the mounting features for attachment to the wheel at the tire-wheel interface. Ultimately, the mounting hardware will need to be selected depending on the number of EH modules used and on the wheel size to be mounted upon. And finally, Figure 6c shows the InWheelSense™ system completely mounted.

Due to the modularized approach of the Eval Kit, implementation is not as complex or time consuming as one may think. By mounting the IWCM at the center of the wheel and securing it by the lug nuts, simplifies the installation. Generally, installation of the Eval Kit is as follows. For full instructions on installation of the Eval Kit, contact TDK.

  • 1. Installation and verification of lug nuts/ log bolts along with lug bolt clamp fastener assemblies on the wheel
  • 2. Installing mounting hardware on wheel
  • 3. Installing the EH modules on the wheel at the tire-wheel interface
  • 4. Placement of the IWCM and securing it to the mounting hardware
  • 5. Connecting of the EH modules to the IWCM main board
  • 6. Placing protective hardware cover over the IWCM
  • 7. Balance the wheel appropriately using commercially available tools
  • 8. Perform a drive test to verify system functionality

This process is shown in the time-lapse pictures of Figure 6.

Figure 6:Installation process of the InWheelSense™ platform

Figure 6 Installation process of the InWheelSense™ platform

Placement and position of EH module is critical for power generation. As mentioned, the module sits on the outer rim of the wheel at the tire-wheel interface and must not interfere with the tire sealing. The correct positioning is shown in Figure 7.

Figure 7:EH module mounting position

Figure 7 EH module mounting position

As explained, no special skills or unique equipment is required to perform any of the above operations. The standard mounting hardware is limited to the size of wheels that can be supported. The hardware can support wheel diameters ranging from 16 inches to 21 inches. It is understood that for large vehicles, especially semis (big rigs, 18 wheelers, etc), farm equipment, and buses, the mounting hardware may need to be adjusted.


As addressed in detail above, it was shown in this paper that the TDK InWheelSense™ technology platform:

  • 1) Is a self-sufficient on-wheel modular technology capable of measuring and detecting multiple vehicle metrics both on-wheel and off-wheel
  • 2) Contains a one of a kind energy harvesting module which generates on-board power via piezo ceramic elements and that these same elements can be used as highly sensitive sensors
  • 3) Has a set of standard sensors but is highly flexible allowing for customer developed expansion boards to be easily incorporated by an interface connector
  • 4) Includes a control module that holds the power conditioning and power storage circuits and components, the sensors and the communication devices needed to send data to a gateway for further processing
  • 5) Utilizes an open platform architecture and Over-the-Air (OTA) updates with Edge Computing
  • 6) Has customized predictive algorithms, and visualization “dashboard” capability in place and in development standard set of software libraries and tools that will be available for users
  • 7) Provides connectivity via BLE, with LoRa for future development
  • 8) Includes on-board power storage and proposed on-board data storage
  • 9) Placement of sensors in the center of the wheel reduces exposure to high centrifugal forces
  • 10) Can be easily installed with no special tools in approximately less than 2 hours with included mounting hardware
  • 11) Potentially improves safety and collision avoidance, increases comfort, and provides services with key driving metrics … thus enabling a tire-wheel autonomous “revolution” with the InWheelSense™ platform technology being the center piece and allowing for future sensor integration
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