By NORWOOD BROWN,
 Staff Field Application Engineer,
 ams,

 www.ams.com

 

 

In some categories of motor drive applications, users have
little or no tolerance for unpredictable, uneven or irregular motor behavior. While
this certainly cannot be said of all motor-driven products — electric
toothbrushes, for instance, or of battery-powered toys maintain a tight focus
on bill-of-materials (BoM) cost, and will almost always accept a small amount
of erratic motor behavior as a reasonable trade-off for minimizing the cost of
the motor – other motor drive applications demand a superior level of operation.

 

Power tools are an example of a product type in which
reliable and predictable motor performance is an absolutely essential feature. Consider the potential for injury
and/or upset of a user of a power saw which jumped backward or produced
a forward ‘hiccup’ motion on start-up – particularly if they had just stalled the
tool in the middle of a cut. Equally, the market would quickly reject an
electric drill or similar power tool which started up with different torque
and/or acceleration during each use.

 

Manufacturers of such performance-critical motor systems are
keenly aware of their users’ requirements, and have typically sought to meet
them through the use of brushed DC motors, which offer a proven ability to
maintain commutation and full torque at start-up and while managing varying
loads. Counting against the
brushed DC motor, however, are its relatively low efficiency and the inherent tendency
for the brushes to fail before other components, due to mechanical wear or
chemical contamination.

 

By contrast, brushless DC (BLDC) motors are greatly
superior to brushed DC motors in many respects:

 

  • efficiency
  • zero electrical wear
  • clean operation

 

The main challenge
for the designer of a BLDC motor control system is that the motor suffers from
hiccupping, and inconsistent torque and acceleration, when the commutator is forced to operate in the
absence of accurate and real-time absolute rotary position data. Absolute
position sensing has, in the past, only been available from extremely expensive
sensors: the lower-cost sensing solutions suitable for the BoM budgets of most motor
system manufacturers have not met this requirement adequately.

 

In power
tools and other performance-critical end-products, then, efficient and reliable
BLDC motor technology has generally not found favor. This article
suggests, however, that power tool manufacturers and others with similar
requirements could adopt the BLDC motor by taking advantage of a semiconductor
product type – the magnetic position sensor IC – which, along with a simple
magnet, provides absolute position data, comes at a low system cost, is easy to
assemble into a motor system, and which enables a BLDC motor to maintain
optimal commutation at all times.

 

AMS01_BLDCmotorcontrol_feb2017

 

Fig. 1: to maintain
maximum torque, the commutator has to maintain a magnetic field through the
stators which is orthogonal to the rotor’s magnetic field as the rotor spins

 

Position sensor
choices for BLDC motor control

 

A BLDC motor control system has to provide clean start-up
operation, maintain continuous commutation, achieve the highest possible efficiency
and extract maximum torque from the available electrical power. The key to
achieving all of these goals is knowledge of the position of the rotor relative
to the stator, information which enables the motor control system designer to
implement a robust electrical drive management solution (see Figure 1).

 

In particular, the availability of absolute position data
enables the motor to start up smoothly from any position. By contrast, a system using discrete sensors
or other control technology might perform a jump or ‘hiccup’ at start-up
in order to calculate its starting position relative to the stators before beginning normal operation. The reduction in
torque attributable to inaccurate position data is demonstrated in Figure 2.

 

Unfortunately, the simplest and cheapest position-sensing systems
available to BLDC motor designers to date have not enabled accurate absolute
positioning.

 

AMS02_BLDCmotorcontrol_feb2017

 

Fig. 2: reduction in
torque attributable to zero-point shift in a four pole-pair motor

 

Back-EMF (Electro-Motive
Force) position sensing
for commutation requires the motor to be in motion
in order to induce a magnetic field for sensing. This means that a back-EMF
system has no positional data for a static motor unless it has previously been
hard-driven to an alignment point – an operation which will result in forward
or backward movement of the motor to such an alignment point, independently of
the user. And after stalling or jamming, this process must be repeated to enable
an orderly re-start. In all cases, until the motor reaches a commutation lock
point, it will suffer from reduced torque and delivered power in the absence of
data on the absolute rotor/stator positioning.

 

 

AMS03_BLDCmotorcontrol_feb2017

 

 

Fig. 3: Hall
switches, optical encoders and resolvers have all been widely used in BLDC
motor control systems, but are now being superseded by magnetic position
sensing technology

 

Discrete Hall switch
systems
typically consist of three, five or more Hall sensors fixed in
position during the production of the motor (see Figure 3). Errors in placement
produce loss of efficiency or power, so extremely precise assembly is required
for a discrete Hall sensor system to work effectively. Each Hall sensor also requires
its own signal wires, further complicating the production process. Worse, even
though the sensors themselves are fixed in absolute positions they cannot generate
absolute position data over an entire 360° rotation, but are limited to measuring within the angular
switching response of the nearest Hall sensor in any given position. The
resulting position measurement error can be significant when angle-related
torque loss is considered.

 

An optical encoder
can produce absolute position information, but this requires either physical
alignment of the encoder to the motor components during assembly, or
system-level storage of zero-point information. The most damaging drawback of
this component type is its potential vulnerability to dust, dirt and other contaminants. Unless protected
by a sealed enclosure, contamination can impair the encoder’s
performance at any time.

 

A resolver is
capable of providing extremely precise and accurate postion measurements. But the
high cost of a typical resolver solution, which includes the resolver unit
itself plus additional analog and digital support circuitry, is prohibitive in most
consumer applications, and even in motor drive systems for end products in the industrial
and other market segments.

 

Each of these position sensor options, then, are
undermined by one or more of these characteristics:

 

  • unpredictable/unexpected
    motor movement at startup, outside the user’s control
  • lack of accurate absolute position information
    in all conditions
  • cost and difficulty of assembly into the motor
  • high unit cost
  • vulnerability to contamination

 

But what if multiple Hall sensors were integrated in a
single chip? This is the approach taken in a family of devices known as
absolute magnetic position sensors. By fabricating multiple, highly sensitive
Hall elements on a die, alongside analog, signal processing and digital
circuitry, a position sensor system can be implemented in a single chip paired with a simple magnet. The chip will
typically be fixed at the end of the motor’s shaft, parallel to a small,
low-cost circular two-pole
magnet mounted on or in the end
of the motor’s shaft (see Figure 4).

 

AMS04_BLDCmotorcontrol_feb2017

 

Fig. 4: a magnetic
position sensor-on-chip is paired with a small, low-cost magnet

 

A single-chip alternative to a discrete Hall sensor
solution benefits from the following characteristics:

 

  • accurate absolute position information at all
    times, through 360°
  • simple assembly: the chip is mounted on a simple
    PCB and requires only one set of wire-to-board connections (see Figure 5)
  • low unit cost, since an integrated circuit
    benefits from the economies of scale inherent in the semiconductor fabrication
    process
  • immune to contamination by chemicals, particles
    or other materials

 

AMS05_BLDCmotorcontrol_feb2017

 

Fig. 5: a position sensing
system-on-chip such as the AS5047 from ams requires only the IC (the circuit pictured
has additional items on the PCB not related to the position sensor) and the
magnet mounted at the end of the shaft in a non-ferrous carrier

 

An example of such a single-chip Hall sensing product is
the A5047 from ams, which has pioneered the magnetic position sensor product
category. In the AS5047, ams has included features which make the sensor system
easy to design in and manufacture:

 

  • the acceptable range for the air gap between the
    magnet and the IC – typically 1-2mm, depending on the strength of the magnet’s
    field – allows for a large amount of tolerance in the production process
  • likewise, permanent, absolute alignment of the rotor
    and stator is accomplished electronically after mechanical assembly through an
    on-chip OTP memory programming step
  • the differential sensing scheme implemented in
    ams position sensors provides for extremely high levels of stray field immunity,
    and so the special magnetic shielding arrangements necessary when using
    discrete Hall sensors are not required.

 

This simple
solution can provide absolute position sensing data from start-up to a high maximum
speed of 28,000rpm. DAEC™ (Dynamic Angle Error Correction) technology from ams internally
compensates for propagation delay at high speeds, bringing the dynamic angle
error down to no more than 0.36° at at a constant speed of 28,000rpm. A
typical absolute position data update time of 222ns means that absolute
position information is available for use in real time across a wide range of
RPM. Drawing on this position data, a motor control system can drive inconsistent
loads without lag or lost commutation.

 

Paired with
a low-cost circular, diametrically magnetized magnet the AS5047 converts
magnetic field strength measurements into position data. To ease system design,
it can provide this data in the form of UVW output signals for steady-state
commutation of motors with from one to seven pole pairs, as well as offering
absolute 14-bit position data, incremental position data, and other position
information formats if needed. This lightens the burden on the host processor,
and increases system efficiency.

 

By monitoring the absolute position at defined intervals,
the system can instantly switch to absolute angle and computed drive under
start-up conditions or for varying loads, when only commutation management
based on absolute stator/rotor position will offer optimal motor operation.
This absolute position sensing method can be implemented in all BLDC motor
management schemes: six-step commutation, 12-step commutation, field-oriented control,
and even PMSM-style sinusoidal drives.

 

The sensor
also provides useful diagnostic information, via its serial peripheral
interface, when the magnetic field strength is outside its specified range and in
the event of other operational faults, which enables the system to flag
possible concerns that might require attention.

 

In conclusion, then, a sensor IC containing multiple
on-chip Hall sensing elements is simpler to mount in motor assemblies than
discrete Hall sensors, is cheaper than a resolver, avoids the optical encoder’s
vulnerability to contamination and, unlike back-EMF sensing, provides accurate
absolute position data at all times. By providing absolute position
measurement, the AS5047 sensor enables a BLDC motor to be managed so as to
provide smooth and predictable motor performance, with optimal torque at all
times. By using an integrated magnetic position sensor IC, then, manufacturers
of power tools and similar end products can now replace brushed DC motors with
the more modern, efficient BLDC motor type while maintaining the high motor
performance that users expect.

 

Summary

 

Electric motor manufacturers have in the past met users’
requirement for smooth and predictable performance through the use of brushed
DC motors, which maintain commutation and full torque at start-up and while
managing varying loads. Counting
against the brushed DC motor, however, are its relatively low efficiency and
the inherent tendency for the brushes to fail before other components,
due to mechanical wear or chemical contamination.

 

By contrast, brushless DC (BLDC) motors are greatly
superior to brushed DC motors in many respects, including:

 

  • efficiency
  • zero electrical wear
  • clean operation

 

The main challenge
for the designer of a BLDC motor control system is that the motor suffers from
hiccupping, and inconsistent torque and acceleration, when the commutator is forced to operate in the
absence of accurate and real-time absolute rotary position data. Absolute
position sensing has, in the past, only been available from extremely expensive
sensors: the lower-cost sensing solutions suitable for the bill-of-materials
budgets of most manufacturers have not met this requirement adequately.

 

In power
tools and other performance-critical end-products, then, efficient and reliable
BLDC motor technology has generally not found favor. This article
suggests, however, that power tool manufacturers and others with similar
requirements could adopt the BLDC motor by taking advantage of a semiconductor
product type – the magnetic position sensor IC – which, along with a simple
magnet, provides absolute position data, comes at a low system cost, is easy to
assemble into a motor system, and which enables a BLDC motor to maintain
optimal commutation at all times.