Original equipment manufacturers (OEMs) of mobile robots and the companies investing in their deployment have two primary options for charge coupling design: wireless and contact-based. Both approaches come with inherent advantages and disadvantages.
Wireless charging better supports fully automated workflows but at higher costs, lower energy efficiency, less on-board space, and overall design complexity. Contact-based designs generally require some manual intervention but result in less wasted energy and a lowered cost burden.
Ultimately, OEMs must find a reliable partner for developing charging systems and determining whether the benefits of fully autonomous operations outweigh the constraints of wireless charging.
To begin examining these tradeoffs, there are seven notable categories to consider.
1. Charge Output Power/Voltage/Current
Choosing a charge coupling design for your mobile robot is influenced by how much power the charger must be able to output, which is a product of the charger output voltage and the ideal rate at which that voltage is applied (current). Charger capabilities—and their coupling designs—are substantially impacted by the following factors:
- Battery pack voltage – How much energy must the battery be capable of outputting to provide sufficient torque (or drive power) to all connected systems and components?
- Energy storage capacity – How long is the robot expected to run continuously before the next charge?
- Targeted charge times – How quickly must a robot’s battery be recharged to resume task completion during standard operations?
The ideal relationship between these three factors is a major consideration for OEMs and their partner firms that design, develop, test, and produce chargers.
For example, higher voltage charging transmissions reduce I2R loss (i.e., the heat created from inefficient electricity transfer that is common with copper wiring) at lower current levels, but the more robust insulation will raise costs. In contrast, wireless coupling is inefficient and costly (covered further below), but it may be better suited to contactless operation for high voltage applications or low voltage, high current charging that requires larger and more expensive physical connectors.
To determine the best solution for all stakeholders’ needs, OEMs and their charger suppliers must consider these factors when collaborating.
2. Energy Efficiency
Wireless charging is inherently less efficient than contact-based couplings and becomes even less so as the distance between the induction coil and robot increases. Below are three main topologies used for wireless charging and their respective efficiencies:
- Small gap DC/DC – 95-98% as efficient as contact-based coupling designs
- Large gap chargers – 90-94% as efficient as contact-based coupling designs
- Large gap AC supply – 75-90% as efficient as contact-based coupling designs
Wireless charging inefficiencies result from the energy transfer through the electromagnetic field (that couples the transmitter and receiver) and from I2R loss due to the copper utilized for the coils. Lower efficiencies will impact an OEM’s market competitiveness, but many mobile robot fleet owners will trade off a 2-10% reduction for the convenience of wireless.
Wireless charging technology is continually improving, and while it is currently less efficient than contact-based coupling, it may be equally effective within the next five to ten years.
3. On-board Space Constraints
Lithium batteries are smaller than traditional lead-acid batteries. Their greater power densities are capable of storing more energy. Although smaller battery sizes create more on-board space, wireless charging’s sophisticated components will fill much of what lithium technologies have made available.
Adding on-board components increases weight for mobile robots, leading to potential tradeoffs with other functionalities. For example, if a mobile robot weighs more:
- It will require more power to operate and move
- Sudden or reactionary movements (based on AI systems) can break equipment instantly or via increased wear and tear over time
In contrast, contact-based chargers will generally create little to no added restrictions or weight.
Traditional off-board chargers that plug in will not affect these space and weight considerations, although guided, autonomous contact-based systems might, based on their design.
4. Physical Operation Environment
Wireless charging may become necessary depending on where the mobile robots are operated.
If you work around flammable chemicals and gasses, engaging the connection for contact-based charging may create a spark that ignites, causing severe injuries. If the mobile robots are subject to corrosive conditions (e.g., salt water, chemicals), contact-based coupling components are more likely to suffer than wirelessly charged units.
5. Staffing Availability vs. Full Autonomy
The only way to achieve a fully autonomous mobile robot fleet is with wireless charging or sophisticated contact-based coupling designs utilizing guided connections.
Traditional contact-based charge coupling still requires someone to connect the unit or its battery to a charger. However, this is less of a consideration than it might appear.
Deploying a fleet of mobile robots requires management personnel to perform maintenance. Therefore, dedicated personnel will already be present to manage the robot fleet. They can also ensure proper connection and charging without adding a significant burden.
6. Telematics and Charge Cycle Responsiveness
Regardless of which charge coupling design is chosen, the communication of telematic data from battery management and charging systems is necessary for optimizing operations. There are two primary considerations regarding the communication of telematic data from a BMS:
- Transmission of data is faster with wired connections than wireless, as exemplified by ethernet cables still outperforming WiFi speed, reliability, and data security.
- The communications link transmitting telemetry data must be able to deliver the reliability and speed that meets or exceeds system requirements.
The highest speed and most reliable telematics data transmission are achieved through a wired data connection.
Contact-based charge coupling solutions provide more rapid voltage and current feedback to the charger and allow very fast adjustments to the charge cycle.
In contrast, wireless systems separate the functional blocks responsible for quickly adjusting charge cycles and, as a result, require very tight feedback loops to ensure safe charging. System requirements may be met with the latter, but they are virtually guaranteed with the former.
7. Production and Procurement Costs
Wireless charge coupling is more expensive than contact-based solutions—as much as a 300% increase. Similarly, developing sophisticated guidance for an automated connector system that ensures autonomous contact-based charge coupling will require significant research, development, and production costs.
When OEMs target market competitiveness and mobile robot fleet owners look for the best ROI, traditional contact-based charge coupling introduces the least cost.
For a “happy medium,” OEMs should discuss guided plate-to-plate connections with their charging system partners.
Evaluating the Tradeoffs of Wireless and Contact-Based Charging
Neither contact-based nor wireless charge coupling is inherently better than the other. Determining which solution to use for mobile robotics entirely depends on use cases and costs. As a result, OEMs developing mobile robots must collaborate with their charging partners and perform extensive analysis to identify the best coupling design for their target market.
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