Investigating the technical considerations for analog components used in motion control drives
Industrial drives are a crucial component of any industrial automation deployment. Three-phase brushless DC (BLDC), permanent magnet synchronous (PMSMS) and induction motors are extremely popular and reliable methods of achieving motion, whether for a conveyor belt or a robotic arm. Motion control drives provide the essential power supply and mechanical drive functions. However, motors and servos experience wear like any mechanical part, which ultimately might lead to failure. Predictive maintenance regimes assist in keeping the plant operating reliably while reducing the costs associated with disruptive and unplanned downtime. One technique used to monitor motor wear is a condition monitoring approach that uses ultrasound to detect changes in a motor's noise signature.
This article explores the importance of the motor drive power supply, current sensing of the drive circuits, and using ultrasound for condition-based monitoring.
Motor drives, an essential part of any motion control application
Brushless DC motors utilise an electronics-based method of commutation instead of using carbon brushes pressed against a rotor. A brushed motor has fixed magnets in the stator (casing) and rotating electromagnetic field windings - the armature - around the rotor. The popular three-phase brushless method reverses this approach by rotating the magnetic field using three fixed field windings and a magnetic rotor. This removes the need to regularly replace the brushes and is also far more energy efficient. Each winding is driven by a high-side and low-side power MOSFET with a pulse width modulation (PWM) signal. There are several ways the PWM signals are derived and shaft rotation achieved, but all offer a high degree of motor speed control across a wide range of load conditions.
The architecture of a motion control drive
Figure 1 highlights the main functional blocks of an industrial motion control drive.
Figure 1 - the primary building blocks of a discrete-based motion control drive (source ST)
Typically, a motion drive controller fits inside a control cabinet placed on the factory floor. Operational efficiency improvements such as Industry 4.0 emphasise using electronics-based control systems not only to power and control machinery but also to optimise and monitor its functional status. With factory floor space already at a premium, machinery manufacturers need to keep control systems as compact as possible to maximise cabinet space. Minimising excess heat using highly efficient and low power components is crucial.
A motion control drive architecture highlights the diversity of electronics in relatively close proximity. For example, from the power perspective, you have high power Si/SiC MOSFETs or IGBTs driving motor field windings down to signal conditioning functions working with small signals. As highlighted in Figure 1, a drive system comprises the power supply, processing and connectivity, motor drive, and sensing and signal conditioning functions. Also, monitoring the condition of the motor is increasingly an integrated function of a motion drive. Like any mechanical component, motors are prone to wear, with bearings, seals, and gaskets the most common. Excessive bearing wear may be detected using vibration analysis with a special MEMS vibration sensor or acoustic and ultrasonic sensing methods.
Achieving low noise DC-DC conversion
The power supply is common to all the circuit functions within a motion drive. Typically, multiple voltage rails are required from a single supply voltage, and for the sensing circuits, in particular, keeping the supply rails free of electrical noise is crucial. Noise can contribute to an unstable and unpredictable process in any feedback loop. Noise spikes or ripple induced on the power supply rail, for example, may become transferred through to the controlled process, resulting in a slight speed variation of the motor.
A motion drive controller may convert an AC line supply to a fixed DC output voltage for distribution within the unit. Each circuit function can then utilise a DC-DC converter to step the voltage up or down according to the circuit's need. There are several methods of achieving DC to DC conversion. Although the process is commonly referred to as conversion, in most cases, it also includes the regulation of the output voltage within defined limits. For example, a converter's datasheet may state a regulation of ±0.5 % of the nominal output across specific load conditions.
Common conversion methods termed topologies include linear regulators, and buck and boost switching converters. Linear regulators can only provide an output voltage lower than the input voltage, for example, an input of +12 VDC and an output of +3.3 VDC. However, a switching converter can either increase (boost) or lower (buck) the input voltage.
An example of a step down 'buck' converter IC is the 38 V 1.5 A ST L6981. Specifically designed for use in 12 V and 24 V industrial power systems, the non-isolated L6981 can accommodate input voltages from 3.5 V to 38 V and deliver an output from 0.85 V to Vin. Figure 2 illustrates the device's internal architecture.
Click to enlarge image
Figure 2 - The internal architecture of the ST L6981 38 V 1.5 A synchronous step-down converter (source ST)
Two FETs provide a synchronous topology driving an external inductor and capacitor, and the duty cycle of the PWM drive signal determines the output voltage. When the load current is below the threshold of 350 mA, the L6981C (Low Consumption) variant enters pulse skipping mode to boost light load efficiency. However, the pulse skipping can be observed as a change in frequency which may be perceived as noise on the supply rail. Therefore, there's also a Low Noise version (L6981N), which does not exhibit power saving operation at light loads and therefore has a noise figure more suitable for sensitive loads. Further, the L6981N can be synchronised to external clock signals that may be shared with multiple devices across the board. The L6981 LNM has a typical conversion efficiency of 90 % when delivering 1 A to the load (Vin 24 V, Vout 5 V, f 400 kHz).
Achieving accurate current sensing in motor drives
Safely driving and controlling a 3-phase BDLC motor across varying load conditions and at different speeds requires carefully monitoring the current through each field winding. Current measurement typically involves measuring the voltage drop across a shunt resistor. An extremely low-value resistor is used to ensure that the voltage drop at the load is insignificant, and also with high currents, to keep heat dissipation to a minimum. However, the voltage presented across the shunt resistor is small, requiring amplification before converting into a digital representation. An op-amp is ideal for amplifying the signal; however, attention to the device's offset voltage (Vio), thermal stability and gain bandwidth product (GBP), is crucial when selecting a suitable op-amp. One must also keep in mind that lower value shunts require higher amplification gain leading to lower precision and lower resulting signal bandwidth.
The offset voltage is a key op-amp datasheet parameter. It refers to the imbalance between the op-amp's differential inputs resulting in an output voltage. For an ideal op-amp, the output should be zero when both inputs are zero. However, the differential inputs will not exhibit the same characteristics due to slight manufacturing and chemical differences during fabrication. The offset voltage is the voltage difference across the inputs required to achieve a zero-output voltage. For some applications, the offset voltage may not be a criterion of concern; however, consider the current measurement example highlighted in Figure 3.
Figure 3 - The impact of Vio on a motor control current sensing application (source ST)
Figure 3 illustrates the current measurement across a 0.1 Ohm shunt resistor in a motor drive bridge circuit. The op-amp has a gain of 1,000, and at the correct speed, 100 µV appears across the shunt. In a perfect op-amp, with a 1,000 gain, a 100 mV output occurs. An ST TSZ121 high accuracy low offset voltage op-amp is used in the example. The TSZ121 has a Vio characteristic of only 5 µV at 25 °C and 8 µV across its whole temperature range from -40 °C to + 125 °C. With Vio added to the detected 100 µV, the differential input is 105 µV, giving a 5 % error in the sensed motor speed. However, consider an op-amp with a 100 µV offset voltage, resulting in a 100 % error.
Figure 4 - A chopper-stabilised op-amp achieves a high precision low offset voltage output by swapping the inputs and averaging out the offset (source ST)
The TSZ121 family of precision op-amps achieve low offset drift characteristics across temperature and frequency using a chopper architecture - see Figure 4. A chopper-stabilised op-amp constantly corrects the errors encountered across the inputs by synchronising swapping of them. A low pass filter then averages the output value, resulting in the offset voltage and temperature drift cancellation.
Condition monitoring a BDLC motor using ultrasound
Monitoring a motor's vibration or audible signatures has proved to be a reliable method of detecting mechanical bearing wear. Early indications of wear give plant management sufficient time to schedule maintenance to limit disruption and avoid costly unplanned downtime. Also, pre-trained machine learning algorithms may assist maintenance teams by inferring from the noise signatures which bearing requires replacement. Listening in the ultrasonic spectrum of 20 kHz to 80 kHz is proving to be particularly effective for the early indication of impending failures.
Figure 5 - The ST IMP23ABSU high-performance MEMS microphone with a flat frequency response up to 80 kHz for ultrasound applications (source ST)
An example of a MEMS microphone suitable for ultrasound-based condition monitoring applications is the ST IMP23ABSU - see Figure 5. This analog output microphone has a flat frequency response up to 80 kHz and has an omnidirectional sensitivity pattern. Its surface mount package measures 3.5 mm x 2.65 mm x 0.98 mm.
Figure 6 illustrates a recommended application circuit for the IMP23ABSU using an op-amp to amplify the output signal. The ST TS522 is an ideal op-amp for this application. This precision, low noise dual operational amplifier has a low input offset voltage of 850 µV and a low noise parameter of 4.5 nV/√Hz.
Figure 6 - A recommended application example using the IMP23ABSU ultrasound MEMS microphone and the TS522 low noise op-amp. (source ST)
Analog components crucial for motion control drives
This short article highlights some of the analog components used in industrial motion control drives. Choosing precision, low noise analog devices ensures that closed motion control loops and condition monitoring applications are reliable and stable. Selecting an op-amp with a low offset voltage, for example, will ensure that a motor's speed is kept tightly within specification.
The ST components showcased in this article are available from EBV, an authorised ST distributor.
About Author
Karl Lehnhoff joined EBV Elektronik in 2008 as a regional application manager. From 2011 till 2019 he headed the renewable energies segment, today called city and infrastructure, as a director. He has served in his current role since March 2019. Previously he held multiple different roles in R&D, field application engineering and field sales. He has a degree as Dipl.-Ing. (FH) in electronics from University of Applied Sciences in Dortmund.
About Author
Milan Ivkovic
Milan Ivkovic received his Dipl-ing. diploma in 2000 and MSc diploma in 2007 at the University of Be...
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Investigating the technical considerations | EBV Elektronik
