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How to Accurately Control a BLDC Motor’s Torque and Speed in Industrial Applications

Contributed By Digi-Key's North American Editors

Brushless DC (BLDC) motors are an integral part of industrial production floors, primarily for use in servo, actuation, positioning, and variable speed applications. In these applications, precise motion control and stable operation are critical. As BLDCs operate on the principal of a moving magnetic field to produce the motor’s torque, when designing an industrial BLDC system, the primary control challenge is to accurately measure the motor’s torque and speed.

To capture the BLDC motor’s torque, two of the three inductive phase currents need to be simultaneously measured with a multi-channel, simultaneous sampling, analog-to-digital converter (ADC). A microcontroller with suitable algorithms calculates the third instantaneous phase current. This process takes an accurate, instantaneous snapshot of the condition of the motor, a key step in the development of a high precision, robust, motor torque control system.

This article will briefly discuss the issues associated with achieving precise torque control, including a cost-effective means of implementing a required shunt resistor. It will then introduce the AD8479 precision difference amplifier and the AD7380 dual sampling successive approximation register ADC (SAR-ADC), both from Analog Devices, and show how they can be used to get accurate phase measurements for a robust system design.

How BLDC motors work

A BLDC motor is a permanent magnet synchronous motor with a back electromotive force (EMF) waveform. The observed terminal back EMF is not constant; it changes with both the torque and the speed of the rotor. While a DC voltage source does not directly drive the BLDC motor, the basic BLDC principle of operation is similar to a DC motor.

The BLDC motor has a rotor with permanent magnets and a stator with inductive windings. This motor type is essentially a DC motor turned inside out by eliminating the brushes and commutator, and then connecting the windings directly to the control electronics. The control electronics replace the commutator function and energize the windings in the correct sequence for the required motion. The energized windings rotate in a synchronized, balanced pattern around the stator.  The powered stator winding leads the rotor magnet and switches just as the rotor aligns with the stator.

The BLDC motor system requires a three-phase, sensorless BLDC motor driver that generates the currents in the motor’s three windings (Figure 1). The circuit is supplied via a digital power factor correction (PFC) stage with inrush current control that provides stable power for the three-phase sensorless driver.

Diagram of BLDC motor driver three windingsFigure 1: Motor control system comprises a PFC to stabilize the power, a three-phase sensorless driver for the BLDC motor windings, shunt resistors and current sensing amplifiers, a simultaneous amplifier ADC, and a microcontroller. (Image source: Digi-Key Electronics)

Three excitation currents drive the BLDC motor, each energizing and creating the phases in the windings, each with differing phases that add up to 360°. The differing phase values is significant: As the excitation of the three legs maintain a 360° total, they balance out evenly to 360°, for example, 90° + 150° + 120°.

While the current in all three windings of a system must be known at any given time, to accomplish this in a balanced system, the currents of only two of the three windings need to be measured. The third winding is calculated using a microcontroller. The two windings are simultaneously sensed using shunt resistors and current sensing amplifiers.

The end of the signal path requires a dual simultaneous sampling ADC that sends the digital measurement data to the microcontroller. The magnitude, phase, and timing of each excitation current provide the motor torque and speed information required for precise control.

Current sensing using pc board copper resistors

While there is much to be concerned about in such a precise measurement and data acquisition design, the process starts at the front-end with developing an effective, low-cost way to sense the BLDC motor winding phase signal. This can be done by placing a small value, inline pc board resistor (RSHUNT) and using a current sense amplifier to detect the voltage drop across this small resistor (Figure 2). Assuming the resistor value is low enough, the voltage drop will also be low, and the measurement strategy will have minimal effect on the motor circuit.

Diagram of Analog Devices’ AD8479 and a high-resolution AD7380 ADCFigure 2: Motor phase sensing system using a current shunt resistor (RSHUNT) to measure the instantaneous motor phase with a high-precision amplifier, such as Analog Devices’ AD8479 and a high-resolution ADC (AD7380). (Image source: Digi-Key Electronics)

In Figure 2, the current sense amplifier captures the instantaneous voltage drop of IPHASE x RSHUNT. The SAR-ADC then digitizes this signal. The shunt current resistor selection value involves interactions between RSHUNT, VSHUNT, ISHUNT, and amplifier input errors.

An increase in RSHUNT causes an increase in VSHUNT. The good news is that this lessens the significance of the amplifier’s voltage offset (VOS) and input bias current offset (IOS) errors. However, the ISHUNT x RSHUNT power loss with a large RSHUNT reduces the system’s power efficiency. Also, the power rating of RSHUNT impacts the system’s reliability as the ISHUNT x RSHUNT power dissipation can produce a self-heating condition which can lead to a change in the nominal RSHUNT resistance.

For RSHUNT, special purpose resistors are available from several vendors. However, a low-cost alternative is to use careful layout techniques for the fabrication of a pc board trace resistance for RSHUNT (Figure 3).

Diagram of pc board layout techniquesFigure 3: Careful pc board layout techniques are a cost-effective way to create the appropriate RSHUNT value. (Image source: Digi-Key Electronics)

Calculating the pc board trace for RSHUNT

As temperatures can be extreme in industrial applications, it’s important to factor temperature into a board shunt resistor design. In Figure 3, the temperature coefficient (α20) of a copper pc board trace shunt resistor at 20°C is approximately +0.39%/°C (the coefficient varies according to temperature). The length (L), thickness (t), width (W), and resistivity (rñ) determine the pc board trace resistance.

If a pc board has 1 ounce (oz) copper (Cu), the thickness (t) is equal to 1.37 thousandths of an inch, and the resistivity (r) equals 0.6787 microohms (µW) per inch. The pc board trace area is measured in terms of trace square (•), which is an area of L/W. For example, a 2 inch (in.) trace with a width of 0.25 in. is an 8 • structure.

With the variables above, the pc board 1 oz Cu trace resistance, R•, at room temperature, is calculated using (Equation 1):

Equation 1 Equation 1

Where T = temperature at the resistor.

For example, starting with a 1 ampere (A) (maximum) current per BLDC motor leg on a 1 oz. Cu pc board, an RSENSE length (L) of 1 in. and a trace width of 50 mil (0.05 in.), RSHUNT at 20°C can be calculated using Equations 2 and 3:

Equation 2 Equation 2

Equation 3 Equation 3

The power dissipation of this resistor with a 1 A shunt current is calculated using Equation 4:

Equation 4 Equation 4

Simultaneous sampling ADC conversion

The ADC in Figure 2 converts the voltage at a point in the phase cycle to a digital representation. It is critical that the simultaneous phase voltage of all three windings be part of this measurement. This is a balanced system, so as alluded to earlier, only two of the three windings need to be measured; an external microcontroller calculates the phase voltage of the third winding.

An appropriate ADC for this motor control system is the AD7380 dual simultaneous sampling SAR-ADC (Figure 4).

Diagram of Analog Devices AD7380 SAR-ADC (click to enlarge)Figure 4: A fast, low-noise dual simultaneous sampling SAR-ADC such as the AD7380 can capture an instantaneous picture of two of the motor windings. (Image source: Digi-Key Electronics)

In Figure 4, the AD8479, is a precision difference amplifier with a very high input common-mode voltage range (±600 volts) to survive wide motor current drive excursions from the three-phase sensorless driver. The AD8479’s characteristics are such that it can replace costly isolation amplifiers in applications that do not require galvanic isolation.

Key characteristics of the AD8479 also include low offset voltage, low offset voltage drift, low gain drift, low common-mode rejection drift, and an excellent common-mode rejection ratio (CMRR) to accommodate swift motor changes.

The AD7380/AD7381 are 16-bit/14-bit, respectively, dual simultaneous sampling, high speed, low power, SAR-ADCs that feature throughput rates up to 4 Msamples/s. The differential analog input accepts a wide common-mode input voltage. A buffered internal 2.5 volt reference (REF) is included.

To achieve precise torque and speed control, the dual simultaneous sampling SAR-ADC structure performs an instantaneous capture of the current sensing amplifier’s output. For this, the AD7380/AD7381 have two identical, internal ADCs that are clocked simultaneously. They each also have a capacitive input stage with a capacitive charge redistribution network (Figure 5).

Diagram of ADC conversion stage for one of the Analog Devices AD7380’s two channelsFigure 5: Shown is the ADC conversion stage for one of the AD7380’s two channels. Signal acquisition begins when SW3 is opened and SW1 and SW2 are closed. At that point, the voltage across CS changes to changes to AINx+ and AINx-, causing the comparator inputs to become unbalanced. (Image source: Analog Devices)

In Figure 5, VREF and ground are the initial voltages across the sampling capacitors, CS. Opening SW3 and closing SW1 and SW2 initiates signal acquisition. When SW1 and SW2 close, the voltage across the sampling capacitors, CS, changes per the voltage at AINx+ and AINx-, causing the comparator inputs to become unbalanced. SW1 and SW2 are then opened, and the voltage across CS is captured.

The CS voltage capture process involves the digital-to-analog converters (DACs). The DACs add and subtract fixed amounts of charge from CS to bring the comparator back into a balanced condition. At this point, the conversion is complete, SW1 and SW2 are opened, and SW3 is closed to remove residual charge and prepare for the next sampling cycle.

During the DAC conversion time, the control logic generates the ADC output code and the data is accessed from the device via a serial interface.

Conclusion

The accurate measurement of BLDC motor torque and speed begins with an accurate, low-cost shunt resistor. As shown, this can be cost-effectively implemented using a pc board trace.

By adding this to the combination of an AD8479 current sense amplifier and an AD7380 simultaneous sampling SAR-ADC, a designer can create a high-precision, robust, torque and velocity control system measurement front-end for environmentally hostile motor control applications.

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Digi-Key's North American Editors