Protecting small autonomous robots

Author:
By Barry Brents, TE Connectivity

Date
07/05/2014

Categories:
Automation, Motors & Motion Control, Power Supplies, Safety Standards

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Even a robot vacuum needs power safety

In recent years the development of information technology, artificial intelligence, sensor techniques and mobile-robot technology has led to intelligent systems being applied to smaller household appliances. One of the most representative products of this trend is the robot vacuum. Since the launch of the very first commercial sweeping robot in the U.S. in 2002, robot vacuums have become increasingly popular around the world (see Figure 1).

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Figure 1: robot vacuums have become increasingly popular around the world

Because a robot vacuum operates independently, it may be used unsupervised. Consequently, in the event of mechanical or electrical failure, the components must be protected by a timely shutdown to avoid possible damage. For manufacturers of vacuum manufacturers, ensuring reliability by protecting the motors, ports and batteries is a primary concern. Moreover, designers of these applications must comply with various safety standards, including requirements specified in the IEC/EN60950, UL1017 and EN 60335-1.

Principles of Operation

Robot vacuums avoid obstacles in their path by using an ultrasonic ranging sensor which emits ultrasonic pulses in the direction of travel and receives corresponding return acoustic pulses. Ultrasonic emission and reception are controlled by devices with either a microcontroller or a DSP (digital signal processor) as their core. The robot’s control system uses this data to determine an optimized path; it then engages the two-step motors and activates the drive wheels, initiating the travel function. As the robot follows this optimized travel path, the onboard cleaning units are activated to perform dust removal and floor cleaning.

In general, a robot vacuum consists of a travel mechanism, a sensor system, a control system, a sweeping system and a power supply unit. The travel mechanism occupies a large part of the body of a robot vacuum, and its size determines the amount of operating space the robot requires. Wheeled systems are generally employed for home-use robot vacuum. Ultrasonic sensors, contact and proximity sensors, infrared sensors, etc., are utilized to allow the sensor system to gather information about complex environments. The control system analyzes this data from the various sensors in order to control the operation of the robot, allowing it to navigate correctly and carry out its cleaning functions.

The sweeping system generally consists of master floor brushes, side brushes and vacuum cleaners. Master floor brushes and side brushes are used to sweep up dirt and debris using mechanical force, while vacuum cleaners are used to remove smaller dust particles from the floor.

The power supply comprises the components that provide power to the various parts of the smart sweeping robot. Since the sweeping robot operates autonomously, the power supply is a rechargeable battery. This not only allows for unmanned control but also improves the equipment’s functionality and flexibility. When fully charged, the robot can operate nonstop for several hours.

Preventing a Stalled Motor

While floor cleaning is in progress, the motorized parts of the robot vacuum can become entangled with debris, causing the motors to stall. When this happens, voltage is still being applied to the motor, but the motor cannot turn, so a high stall current will flow, which causes the temperature of the motor to rise rapidly. When the temperature exceeds the rating of the motor coil, the coil can burn out and the motor can fail. This not only inconveniences the consumer, it can also add to the cost of warranty repairs incurred by the manufacturer.

Polymer positive temperature coefficient (PPTC) devices are well suited for protecting motors during overcurrent events. Like traditional one-use fuses, PPTC devices limit the flow of dangerously high current during fault conditions. Unlike fuses, however, these devices reset after the fault is cleared and power to the circuit is cycled. With this approach, once the cause of the stall has been removed, the customer can expect that the robot will return to normal operation with no further maintenance requirements. PPTC devices help enable this functionality since their resistance can be restored to a low value after a motor malfunctions. Moreover, compared to many dual-metal breaker products, PPTC devices enable greater design flexibility, longer service life and reduced electromagnetic interference (EMI).

Design requirements
The design requirements for robot vacuum’s drive system are as follows. The maximum working current of a driving-wheel motor is 0.3A. The minimum temperature of the environment around the PCBs of the vacuum is 10°C, and the maximum operating temperature is 50°C. The maximum voltage of the charged batteries is 22.5V. Each motor requires protective components, attached to the motor surface, capable of providing protection by shutting off the current within 10 seconds of fault detection.

Figure 2(a) illustrates the real-time response of a PolySwitch surface-mount PPTC device (miniSMDC050F) being used to protect a stalled wheel motor. As shown in the graph, the protection response time is 3.0 seconds, which is far less the 10 seconds generally required in these applications. Figure 2(b) shows a PolySwitch radial-leaded PPTC device (RUEF400) being used in a dust-removal motor stall. The real-time response time in this graph is 6.0 seconds; again, much lower than 10-second response-time requirement. 

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Figure 2(a). Real-time response time of a surface-mount PolySwitch PPTC device protecting a stalled wheel motor.

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Figure 2(b). Real-time response time of a radial-leaded PolySwitch PPTC device protecting a stalled dust-removal motor.

In the same way, the appropriate PPTC devices in the PolySwitch family can be selected and attached to the respective motor surfaces to protect the master floor-brush motors and side-brush motors from damage from overcurrent conditions.

Protecting Power-charging Ports and Rechargeable Batteries

Since a robot vacuum operates autonomously, it uses a rechargeable battery for its power supply. This allows for unmanned control and also improves the product’s functionality and flexibility since, when fully charged, a robot vacuum can operate nonstop for several hours. Although convenient for the user, having the charging ports exposed means that designers must provide circuit protection to help protect against external short circuits and other failures.

Resettable PPTC devices help provide an effective port protection solution in robot vacuum applications. Figure 3 shows the charging circuit in the robot vacuum. Since the vacuum’s rechargeable battery will not exceed 30V, PolySwitch devices with voltage ratings equal to or greater than 30 Vdc should be used. These low-voltage-rated PPTC devices can be placed on the AC mains charger for secondary-side protection. Based on the working current levels and ambient operating temperatures for robot vacuums, the PolySwitch RUEF400 PPTC device generally can meet the relevant requirements.

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Figure 3. Circuit diagram showing a PolySwitch PPTC device in a charging port of a robot vacuum.

Rechargeable batteries provide power for robot vacuums. Various types of batteries can be used, including nickel-cadmium, lithium, and polymer batteries, all of which require protection against electrical surges or failures. PPTC devices are widely used as secondary protection devices in these applications, providing both overcurrent and over-charging protection. The overcurrent function helps protect against abnormally high-charging or discharging currents to prevent damage and ensure battery safety. The over-charging protection function monitors the battery core temperature to help prevent over-charging, enabling disconnection from the charger if such a condition occurs.

Conclusion

Resettable PPTC devices are well-suited for helping to protect the equipment’s motors, batteries and charging ports from damage caused by stalled motors, overcharging and other failures that can be encountered during normal operation. The PolySwitch family of PPTC devices offers various options for low resistance, rapid response time, small size and reset functions, which can help designers develop safe and reliable products.

TE Connectivity 

 

 

 

 

 

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