“After a busy week, household cleaning is one of the last things people want to do. So far, sweeping robots have been available for about 23 years. With the increasing level of intelligence and automation, people can focus on their own affairs while working.
After a busy week, household cleaning is one of the last things people want to do. So far, sweeping robots have been available for about 23 years. With the increasing level of intelligence and automation, people can focus on their own affairs while working.
Reference design and products of sweeping robot
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Many functions are integrated on today’s sweeping robots, such as the new mopping function and automatic dust removal. But for designers, this also means that more challenges will be faced when designing a reliable system. The small amplifier can help it quickly overcome many major challenges. The following lists the six challenges that designers will encounter in the design process, and the six solutions that small amplifiers can provide:
Design challenge 1: The motor life is shortened due to the delay in stall detection.
The power of the sweeping robot’s wheels determines its obstacle clearance ability. In order to be able to pass through thick carpets and over the threshold, its motor power needs to be at least 30W or higher. If a stall or overload event occurs, such as a wheel jammed by a wire, the motor winding current will rise immediately. Delaying detection of this condition can cause the motor to overheat and shorten its life.
Solution 1: Fast transient response current sensing in the motor control system.
To reduce the possibility of overheating, a low-side current sensing circuit can be used to monitor the current of the motor; see Figure 1.
Figure 1: Current sensing circuit in a motor control system
The key parameter used in this application as the current sensing circuit in an operational amplifier (op amps) motor control system is the slew rate. For example, when a stall event occurs, the winding current will rise from 0.5 A to 3.5 A, and the corresponding output of the op amp is 0.5 V to 3.5 V (50mΩ shunt resistance and 20-V/V gain). When using an op amp with a slew rate of 0.5 V/μs, the settling time of a step change is about 6μs, while using TI’s TLV905x op amp with a slew rate of 15 V/μs, the settling time of the same step change is only It is 0.2μs. Therefore, the use of TLV905x with a 30-fold increase in transient response speed will increase the controller’s margin for over-current protection.
Design challenge 2: The battery life is shortened due to the inaccurate charging voltage.
Expansion of battery capacity is an important design challenge faced by sweeping robots. Consumers expect the robot to complete a complete cleaning cycle before it needs to be recharged.
High output voltage ripple using low quality current sensing will produce unusable battery capacity. For example, if the battery accuracy at 4.2 V is ±3.5%, the usable battery capacity will be reduced to 40% after 250 charging cycles, and if the battery accuracy at 4.2 V is ±0.5, the usable battery capacity will remain at 85%.
Solution 2: High-precision voltage/current sensing in a constant current/constant voltage loop.
A common way to charge a battery is to use a discrete charging solution as shown in Figure 2. Voltage and current sensing circuits generate feedback voltage and current signals in the control loop. In order to achieve high accuracy and stability, offset voltage and temperature drift are the two key parameters of the operational amplifier used here.
Figure 2: Discrete battery charger circuit
Design challenge 3: The battery overheats due to the error of the negative temperature coefficient (NTC) thermistor.
Monitoring the temperature of the battery pack is a major safety issue for robot vacuum cleaners. Compared to the temperature sensor solution, a cost-effective way to monitor the temperature of the battery pack is to use an NTC thermistor sensing circuit. Inaccurate temperature sensing may cause the battery pack to overheat or burn out.
Solution 3: Use NTC for high-precision temperature measurement.
One way to measure temperature is to use resistors and thermistors to distribute power, and connect the voltage divider output directly to the analog-to-digital converter (ADC) pins inside the system controller. The output impedance of the voltage divider is very low, and the output voltage range is not ideal for the ADC, so this method is not efficient and the measurement result is not accurate.
Figure 3 uses an operational amplifier as a buffer to adjust the temperature output signal, provides a high-impedance node for the voltage divider and low-impedance nodes to drive the ADC, and adjusts the output range to the best ADC resolution. The influencing parameters of the operational amplifier include DC accuracy (offset voltage, voltage drift) and stability.
Figure 3: NIC thermistor sensing circuit
Design challenge 4: The accuracy of the positioning and navigation system is low due to the inaccurate odometer measurement.
When the sweeping robot constructs a map of the environment, the odometer should provide an accurate travel distance for drawing. Inaccurate odometer measurement will result in lower robot positioning and navigation accuracy.
Solution 4: Use a robust odometer signal enhancement circuit.
The common method of measuring mileage is to use photoelectric decoder or Hall-effect sensor and count the pulses to obtain mileage information. Generally speaking, the odometer is installed inside the wheel, so the printed circuit board has a long trace and is more susceptible to switching noise, which causes the output signal to be distorted at the input port of the MCU. The buffer circuit shown in Figure 4 can generate standard logic signals without jitter and malfunction.
Figure 4: Buffer for robust logic output circuit
Design challenge 5: Noisy/distorted motor drive signals can cause the motor to run unexpectedly.
The system controller is usually located in the center of the control board, and the motor is mounted on the edge of the circuit board. Therefore, the drive signal directly connected to the MCU port is more prone to noise or distortion, which may cause the motor to run unexpectedly.
Solution 5: Pulse width modulation (PWM) booster circuit in the motor drive path.
The solution here is to install an operational amplifier used as a booster instead of a circuit that connects the drive signal to the MCU pin. Figure 5 shows a discrete motor drive solution for brushed DC motors. The controller generates a PWM signal through the totem pole field effect transistor driver to drive the H-bridge power transistor. The PWM enhancer circuit helps minimize delay and enhance the PWM signal while reducing noise and distortion.
Figure 5: Enhanced PWM circuit
Design challenge 6: The distance detection error of the sweeping robot caused a collision or fall accident.
The anti-fall sensor is used to detect the height of the stairs, and the collision sensor is used to detect obstacles around the sweeping robot. When the distance detection is wrong, the performance of the sensor will be inaccurate, resulting in a collision or falling event, and damage to the robot.
Solution 6: High-precision infrared output signal adjustment.
As shown in Figure 6, infrared LEDs and phototransistors are widely used as low-cost solutions for detecting distances. The distance information is related to the amplitude of the echo carried by the fixed frequency modulated wave.
Figure 6: Signal conditioning circuit of infrared LED receiver
Transimpedance operational amplifier circuits with low input bias current are widely used here. The reference circuit is shown in SBOA268A.
TI’s TLV906x, TLV905x and TLV900x general-purpose amplifiers are very suitable for the above six situations, and designers can use them to shorten the time to market and overcome common design challenges.
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