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There are a number of different methods for driving brushless motors which alter the way in which the motor can perform.
Each method has its pros and cons, both technically speaking and in some cases financially.
Trapezoidal Control -
Trapezoidal commutation can be split into two variations. The first is two quadrant control and four quadrant control. Figure 1 (below) shows the fundamental concept of a trapezoidal drive control for one phase of the motor. Power is turned on, held, and then turned off in a way that creates a trapezoid shape when measured on a graph.
FIGURE 1 -
Two quadrant basically means that the PWM in each phase is only on or off at any one particular point in time. By controlling the switching of these phases one can then get the motor to turn (for more on basic brushless motor theory please click here)
Four quadrant control is then be split into two again, the simplest is by using a standard PWM (Uni-
Unipolar four quadrant control is the simplest form of four quadrant control.
You then have full bipolar four quadrant control that shifts the adjacent mosfets asynchronously with the driven phase that essentially drives current in the opposite direction of the motor phase in order to brake the motor much faster than it otherwise would in normal two quadrant control.
Two quadrant is the lowest cost option however it cannot create a fully controlled acceleration/deceleration of the motor because it is unable to actively brake the motor in the way that four quadrant control can.
Bipolar four quadrant control offers the the most comprehensive control of a brushless motor however it is much less efficient as current is being driven on the opposite direction.
The Unipolar four quadrant control is in the middle of these two as it helps with the active braking (via the back EMF) and is also much more efficient that the bipolar version.
Ultimately, the best option for your application really depends on what you need as where there is considerable load on the motor shaft it is often unnecessary to have four quadrant control.
Sinusoidal control is generally regarded as the next step up from the trapezoidal control. Figure 2 shows the basic drive pattern for one phase of the motor.
FIGURE 2 -
In this case, the controller measures two commutation points and then estimates and implements a sinusoidal wave form between these points using normal PWM (see figure 2 above).
This has huge benefit when it comes to reducing the audible noise created from the motor/load as there is minimal cogging (current jumps between two commutation steps) as the current is smoothed out (the comparison between figure 1 and 2 shows this clearly).
It is worth noting that this is much newer technology than standard trapezoidal drive and much more difficult to perfect. However, as there is less cogging in the motor and the current is smoothly transitioned between each commutation step, much lower speeds can be achieved.
Sinusoidal control can also be implemented using high CCR encoders to more accurately predict the sinusoidal wave form.
FOC or Field Orientated control – the most intelligent method currently available
Field Orientated Control (FOC) was actually developed for AC motors a long time ago but technological developments made in recent years have meant that this technology can now be used with lower voltage BLDC motors.
In the past the actual magnetic field was measured within the motor using sensors whilst it was running. However now due to the reduction in cost of powerful microcontrollers, advanced formula and calculations can be completed fast enough to very accurately measure the current within two motor phases (the third can be calculated) to dictate the exact orientation of the motor rotor.
This enables the microcontroller to very rapidly complete the calculations and send optimised PWM’s to all three motor phases (around 10 000 times per second) to ensure the motor produces optimal torque at all times. FOC is sometimes confused with sinusoidal control as the current waveforms are almost identical, the difference being with sinusoidal commutation the current produced in each position is normally approximated and does not take into account manufacturing tolerances or the motor (or encoder if used) and any rotor/load inertia of the motor or application.
As the FOC driver is continually checking and performing positioning/loading information, it is always optimised for any application and always runs at MAXIMIUM efficiency as the electromagnetic field is always originated at exactly 90 degrees to each motor pole. As a result of this the motor speed can be taken down to almost 0 RPM.
Frequently Asked Questions
There are a huge range of applications which can be effectively dealt with using sensorless bldc motor controllers.
Why not explore some of our stock range of BLDC motor controllers and if you have a particular project that you would like to discuss you can get in touch and our engineers will be more than happy to help.
Zikodrive Motor Controllers is a trading name of Round Bank Engineering ltd. Registed Company Number 08288866. VAT Number: GB226504428
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