We can broadly divide robots into industrial robots and service robots. Unlike industrial robots used in factories and other work sites, service robots refer to robots used to support everyday life.
It is expected that service robots will be used in nursing care and various other situations in the future. Accordingly, even a slight malfunction due to a noise problem in a robot may lead to a risk to life.
As we enter the era in which robots exist in the sphere of life of people, noise suppression is becoming important to ensure service robots do not fail or malfunction.
- Difference with General Home Appliances
The main difference between robots and general consumer products (e.g., home appliances) is that robots are equipped with motors.
The other technology used in robots is the same as in consumer products. Therefore, it is only the motor part that is technology specific to robots.
- Noise Problems in Service Robots
There are two possible noise problems in service robots.
First, there is the emission problem in which radiation noise is generated when the motor is driven, adversely affecting the peripheral equipment. Service robots are also used in homes and elsewhere. Therefore, the required noise suppression requirements are strict.
Next, there is the immunity problem in which service robots malfunction due to noise flowing in from outside, potentially causing harm to humans. LTE, Wi-Fi and other wireless communication radio waves crisscross in homes. If those radio waves become external noise and penetrate the electronic circuits inside robots, the robots may malfunction and cause harm to humans. Accordingly, we anticipate a high level of immunity resistance will be required.
We now introduce respective noise suppression methods for these noise problems.
- Noise Situation Confirmation and Noise Suppression Method Examination
We confirmed the current situation for both emission and immunity problems and then examined noise suppression methods to address these problems.
- Noise Suppression for the Emission Problem (Radiation Noise)
First, we confirmed the situation of emission noise. We measured the radiation noise in a commercially available service robot.
- We referred to CISPR16-2-3 for the measurement system
- We referred to the IEC61000-6-3 (housing and commercial environments) common standard for the noise tolerance value
- Radiation Noise Situation before Noise Suppression (Initial State)
We confirmed that the noise exceeds the standard value for IEC61000-6-3 in the 30 MHz to 200 MHz band. Noise in this band is often a problem for most service robots.
- Emission (Radiation Noise) Problem Generating Mechanism
We examined the noise source and transmission path.
The equipment we evaluated this time employs a brushless motor. Driver circuits are used to drive brushless motors. Three-phase PWM control circuits are commonly used as the driver circuits of brushless motors. Two switching elements per phase – six in total – are used in this control circuit. These elements generate switching noise. This switching noise is transmitted to the cable. However, the cables used in robots are often unshielded insulation-coated cables. Therefore, noise is radiated from the cable to the outside. Noise is also transmitted to the coil inside the motor. Accordingly, noise is radiated from the body of the motor.
d Mechanism of Noise Generation
- Switching noise is generated by the switching elements of the driver circuit
- Switching noise is transmitted to the cable and noise is then radiated from the cable (radiation noise A)
- Switching noise is transmitted to the winding of the brushless motor and noise is then radiated from the motor (radiation noise B)
- Emission (Radiation Noise) Noise Suppression Method
Switching noise generated in the motor driver circuit is transmitted to the cable. It is then radiated from the cable or the body of the motor. We can thus see that it is best to prevent transmission of switching noise to the cable.
To that end, we insert a noise filter near the connector of the cable. The noise frequency is in the 30 MHz to 200 MHz (300 MHz in some cases) band. Accordingly, we select a part that is effective in this frequency band. The NFZ series (e.g., NFZ2HBM, NFZ32BW and NFZ5BBW) and the BLT5BPT series are suitable. If the motor locks for some reason, a momentary spike current may flow. This means it is best to select a part with a rated current three to five times that of the regular current. We used the NFZ2HBM this time.
Noise Filter Insertion
Filters Introduced This Time
Noise Filter NFZ Series
These are small noise filters compatible with large currents. We select a constant to match the noise frequency.
NFZ2HBM (2.5×2.0mm)
NFZ32BW (3.2×2.5mm)
NFZ5BBW (5.0×5.0mm)
Ferrite Beads BLT5BPT Series
This series is compatible with large currents up to 11 A. These filters can be used at any point with a maximum operating temperature of 150°C.
BLT5BPT (5.0×5.0mm)
- Emission (Radiation Noise) Suppression Effect
We were able to suppress the noise to the standard value or below and to secure a margin of at least 5 dB when we installed a NFZ2HBM4R4SN10 near the connector to which the cable is mounted.
Horizontally
Vertically Polarized
Filter Used This Time
Ferrite Beads NFZ Series
These are small noise filters compatible with large currents. We select a constant to match the noise frequency.
NFZ2HBM (2.5×2.0mm)
- About Immunity (Proximity Irradiation Immunity)
Next, we conducted a wireless communications radio wave proximity irradiation immunity test. There are no official noise standards for this test. Therefore, we built our own measurement system to conduct the test.
We radiated radio waves in 10 MHz steps for every 100 MHz in the 700 MHz to 2,600 MHz frequency band on the assumption of LTE and Wi-Fi communications radio waves to confirm whether the behavior of the motor changes according to the motor rotational speed and consumption current.
Proximity irradiation immunity measurement system
■Test Conditions
Irradiated radio wave frequency band:
We conducted an immunity evaluation in 10 MHz steps for every 100 MHz from 700 MHz to 2,600 MHz (assuming LTE and wireless LAN communications radio waves)
Irradiated electric field strength:
10 V/m or more, 5-second irradiation at every frequency, pulse modulation of 0.2 kHz and a duty of 50% Proximity distance: 5 cm We used a wideband sleeve antenna (model number: NKU07M32G made by NoiseKen).
We continuously operated the motor for one cycle (positive rotation: 1.5 seconds / negative rotation: 1.5 seconds).
We monitored the counter in the main IC to measure the motor rotational speed and motor consumption current.
The left side of the figure below shows the motor rotational speed and current waveform when the motor is operating normally. Looking at the rotational speed, we can see that the positive rotation and reverse rotation repeat every 1.5 seconds. On the other hand, looking at the current waveform, there are characteristic changes. A large current flows (605 mA in this example) at the moment when the rotation direction of the motor switches. It then settles to a small current (100 mA) after that. The maximum value of the current at the time of this inversion increased when we conducted the immunity test. We can see that it is affected by external noise. The degree of impact of this amount of change differed depending on the frequency of the irradiated radio waves.
No Radio Wave Irradiation (initial: 0 V/m)
Radio Wave Irradiation
※Motor Rotational Speed and Consumption Current in the Initial State
- Immunity Problem Generating Mechanism and Noise Suppression Method
It is assumed that this problem is generated by the following mechanism.
When external radio waves are irradiated as noise, the noise combines with the board wiring and other parts in common mode. The operation of the motor is controlled by the main IC mounted on the board. A feedback circuit is configured between the motor and the main IC. Accordingly, the noise combined in common mode is superimposed on the feedback signals. When the feedback signals mixed with noise are input into the main IC, some of the noise in common mode in the IC is converted to differential mode. The main IC, which has received feedback at an incorrect value as a result of it being mixed with noise, then outputs an incorrect control signal. Current greater than the amount necessary to rotate the motor flows in this case. This means the excess current is consumed as waste energy.
It is effective in such cases to remove the noise of the common mode component from the feedback signals input into the main IC. The feedback signals of a three-phase motor are configured in three lines. Therefore, we use a common mode choke coil (CMCC) with three lines combined.
Assumed Noise Generating Mechanism and Suppression: Motor Control Circuit (Schematic Diagram)
- External noise is irradiated
- It is superimposed in common mode (noise)
- Some of the common mode is converted to differential mode in the main IC (noise is superimposed on the feedback signals)
- The main IC misrecognizes the actual operating state of the motor and excess current flows
- Current unnecessary for the motor rotation is consumed as heat (it becomes waste energy)
- Effect of Immunity Noise Suppression
The increase in the maximum current flowing to the motor significantly decreased when we conducted the same evaluation using a CMCC.
This prevents the control of the motor from being disturbed even if external noise penetrates it. In addition, excess current does not flow. Therefore, it is also possible to prevent the capacity of the battery from being unnecessarily reduced.
Changes in the Maximum Current after Noise Suppression
The maximum current drops by adding a three-line CMCC. It is possible to suppress main IC failures
The life of the battery is extended, which saves energy
- Summary
Service robots are used in home environments and similar settings. Accordingly, noise suppression is important. The issue caused by switching noise generated by the motor driver being radiated is attracting attention as an emission problem. In terms of immunity problems, external noise sometimes penetrates the feedback signal line of the driver, and incorrect control signals are then transmitted to the motor, causing a failure.
- Using a noise filter near the connector that is the affixation part of the cable connected to the driver IC suppresses emission noise.
- Using a CMCC in the feedback signal line suppresses immunity noise. The feedback signal line is configured in three lines. Therefore, we also select a three-line CMCC.
Filters Introduced This Time (For Emission Noise Suppression)
Noise Filter NFZ Series
These are small noise filters compatible with large currents. We select a constant to match the noise frequency.
NFZ2HBM (2.5×2.0mm)
NFZ32BW (3.2×2.5mm)
NFZ5BBW (5.0×5.0mm)
Ferrite Beads BLT5BPT Series
This series is compatible with large currents up to 11 A. These filters can be used at any point with a maximum operating temperature of 150°C.
BLT5BPT (5.0×5.0mm)
Filter Introduced This Time (For Immunity Noise Suppression)
Common Mode Choke Coil: Three-line CMCC
We select a product to match the noise frequency
Related Products:
https://www.murata.com/en-sg/products/emc/emifil/overview/lineup/audio
About This Article:
This article is provided by Murata Manufacturing Co., Ltd.
