DE202004020764U1 - Brushless DC motor controlling unit, has comparing unit comparing actual rotational speed with lower and upper threshold valves, where maximum duty cycle is reduced and increased when speed exceeds upper and lower valves, respectively - Google Patents

Abstract

The unit has a measuring unit for measuring an actual rotational speed of a brushless direct current (DC) motor (11) and a comparing unit for comparing the actual rotational speed with a lower threshold valve and an upper threshold valve. The maximum duty cycle is reduced and increased by a specific step when the actual rotational speed of the motor exceeds the upper threshold valve and the lower threshold valve, respectively. An independent claim is also included for a process for controlling a brushless direct current (DC) motor.

Description

Territory of invention

The The invention relates to the control of a brushless DC motor (BrushLess direct current motor: BLDC motor), and in particular a device for controlling a BLDC motor using an open Control circuit with pulse width modulation (PWM) and controllable duty cycle D.

State of technology

in principle can the performance of a BLDC motor or a BLDC fan Engine controlled using an open loop or a closed loop can be controlled by the speed n or the rotational speed ω as a function of certain parameters, e.g. a maximum power consumption of the engine, controlled or is regulated. The task of controlling or regulating BLDC Engines become common taken over by a microcontroller.

at an open loop becomes the desired rotation speed (Speed) set as input size and preferably using a pulse width modulation control circuit directly into a corresponding duty cycle of the control voltage to Activation of the power transistors of the motor transmitted. The nominal operating point of the motor, the defined speed corresponds, is only achieved if the specified nominal voltage is applied to the motor and the motor or the fan with its nominal load is working. This means that at a lower supply voltage (compared to the rated voltage) the speed of the motor decreases. A higher supply voltage leads to a higher one Speed n or rotational speed ω and therefore to a higher load or with fans a higher air flow.

In The scope of the disclosure will be the magnitudes of speed n, orbit frequency f and rotation speed ω = 2π f = 2π n is synonymous used and are interchangeable.

The maximum power P _max of the motor is achieved when the duty ratio D and the height of the supply voltage U reach _to their permitted maximum. In an open loop, for example, the motor of a fan must be designed for operation at maximum power consumption P _max , which is reached at the maximum allowable supply voltage U _max .

For example, in an application where the rated voltage U _{nom of} the motor is 48V DC, but the maximum supply voltage U _max may be up to 60V DC, for example, the fan must be rated at a maximum power P _max that is approximately 25% higher than rated power P _nom lies, as can be seen from equation (1): P Max ≈ (U Max / U nom ) · P nom = (60V / 48V) · P nom = 1.25 P nom (1)

A second method for driving a BLDC motor is the use of a closed loop for controlling the rotational speed ω. For example, a Hall sensor of the motor is used to determine the actual rotational speed ω of the motor. A microcontroller-internal control loop controls the duty cycle of the PWM such that a desired nominal rotational speed ω is achieved. Due to the closed control loop, the maximum power consumption P _{m a x can be} specified and limited (monitored). In a closed control loop, the rotational speed ω is independent of the supply voltage U _in .

To avoid the disadvantages of an open loop in terms of maximum power consumption or the maximum power loss, a closed loop can be used. As explained above, the maximum power P _max absorbed by the fan is thereby limited by the speed control of the control loop. A disadvantage of a closed loop is the additional computational effort in the microcontroller and the need for additional storage space. For low cost motors, however, the control program and electronics should be kept as simple as possible.

Further has e.g. a closed-loop fan motor at low supply voltages more power dissipation than when using an open loop and this can be a disadvantage depending on the application be.

epiphany the invention

Of the Invention is based on the object, a motor control for driving of a BLDC motor indicating the advantages of open and combined closed loop and still with a relative small additional Expense concerns electronics and programming.

These The object is achieved by a Motor control solved with the features of claim 1.

Preferred embodiments and vorteilhaf te developments of the invention are specified in the dependent claims.

According to the invention is a Motor control with an open loop and a fast-acting Algorithm for limiting the rotational speed of a brushless DC motor proposed.

The invention combines the advantages of an open loop at operating voltages up to about the rated voltage U _nom and the benefits of a closed loop at voltages above the rated voltage.

The desired operating conditions of the engine are determined by establishing a nominal rotational speed ω _nom . The nominal rotation speed ω _nom is then used to determine a lower threshold ω _hys-low for the rotation speed which is slightly higher than the nominal rotation speed ω _nom . Furthermore, an upper threshold value ω _{hys-high is} determined which is greater than or equal to the lower threshold value ω _hys-low . If the upper threshold is chosen to be greater than the lower threshold, sufficient hysteresis must be ensured.

The current rotational speed ω is determined in the microcontroller, for example on the basis of signals from Hall sensors, with which the rotor position for the exact commutation of the motor is determined. As soon as the current rotational speed ω exceeds the upper threshold value ω _hys-high , for example due to an increase in the operating voltage, the maximum permissible duty cycle D _max is reduced by a defined step ΔD.

As a result, the rotation speed ω decreases until it is equal to or smaller than the upper threshold value ω _hys-high . However, as soon as the current rotational speed ω _{falls below} the lower threshold value ω _hys-low , the maximum duty cycle D _{max is} increased by a defined step ΔD. As a result, the rotational speed ω increases until it is equal to or greater than the lower threshold value ω _hys-low . In this way, the rotational speed ω for operating voltages which are greater than the nominal voltage, between the values ω _hys-low and ω _{hys-high is} maintained. In some applications, no switching hysteresis is necessary, so that ω _hys-low and ω _hys-high can be set to the same value, or only one threshold is necessary.

The control method performed by the engine control system makes it possible to limit the power consumption of the engine (fan) to a maximum value P _max even when using an open control loop. The power consumption is limited to a maximum value P _max , which is reached at the upper speed threshold ω _hys-high .

The method is performed only at rotational speeds ω that are higher than the nominal value w _nom . Below the speed ω _nom , the engine control behaves like an open loop. The method allows a reduction of the rated power and a weaker design of the engine electronics compared to an open-loop engine. This saves costs in the production of the engine.

In addition, the rotational speed can be kept substantially constant as a function of the hysteresis Δω _hys = ω _hys-high -ω _hys-low for operating voltages higher than the nominal voltage. This leads to a reduced engine noise at high operating voltages.

The control method does not affect the function of the open loop. A reduction of the maximum duty ratio D _max is performed only after the determination of the currently selected duty cycle D.

That is, as long as the current duty ratio D does not exceed the predetermined maximum duty ratio D _max , the method according to the invention is not performed. This reduces the complexity of the equations for the open loop duty cycle calculations, and thus the programming overhead and hardware requirements for the microcontroller.

Of the proposed additional Algorithm for controlling a BLDC motor, also called the limiting algorithm can be called to limit the rotational speed ω, can be very easy to implement in the firmware of a microcontroller program, because he only a few extra Calculations and disk space required. Thus, the suitable described engine control for the application in "low-cost" engine applications, where a closed loop is not necessary and too expensive would.

The Invention leaves Apply to all types of BLDC motors by a microcontroller are controlled. The load operated on the motor can be a fan or another passive load, e.g. a pump wheel, be.

in the The invention will now be described by way of example with reference to FIG closer to the drawings explained.

Summary the drawings

1 shows an example of a procedure performed by the engine control according to the invention;

2 shows an example of rotational speed of a BLDC motor depending on the applied PWM duty cycle and the operating voltage.

description a preferred embodiment the invention

In 1 is a performed by the engine control process of the invention carried out with reference to a flowchart. The method is based on a motor control based on an open loop. The engine controller includes a PWM generator 10 which generates a variable duty cycle signal D in response to an input signal. The input signal is a control variable, e.g. B. a control voltage with which the Ausgangsstastverhältnis D can be controlled. In general, the duty cycle D is limited to a maximum value D _max to z. B. to protect the motor to be controlled from overloading. This is the procedure in step 1 the output signal of the PWM generator, that is, the output duty ratio D with the one maximum predetermined duty cycle D _{m a x} compared. If D is not greater than D _max , then the set duty cycle via a power electronics (not shown) to the BLCD motor 11 created. A commutation electronics (not shown) ensures that, depending on the rotational position of the motor 11 the individual windings are controlled in a corresponding order, so that the motor rotates at a given duty ratio D with a certain rotational speed ω.

In 2 the rotational speed ω of the motor is shown as a function of the applied duty cycle D and the operating voltage. If the motor is operated with its rated voltage U _nom , it reaches at a maximum duty cycle of, for example, 100%, a speed ω z. B. the rated speed ω _nom corresponds. This case represents the lowest line of 2 In practice, however, motors are often operated under conditions in which the supply voltage U may differ significantly from the rated voltage U _nom . Therefore, even under these conditions, a safe operation of the engine must be guaranteed. Is z. B. the operating voltage U1 greater than U _nom , the engine reaches its rated speed ω even at a lower duty cycle of the input voltage. The same applies to even higher supply voltages U2 and U3. Depending on the input voltage of the motor, it is inventively provided to limit the duty cycle to a maximum value D _max in order to prevent overloading of the motor. A check as to whether the duty ratio D exceeds the maximum duty ratio D _max is found in step 1 the procedure.

If the maximum duty ratio is exceeded, the current duty cycle is set to the maximum value D _max and the motor is driven with the maximum value D _max (step 2 ).

According to the present invention, the current speed ω of the engine 11 measured. This can be z. B. by evaluating the signals from Hall sensors 12 serve, which are used to control the commutation. The current speed ω is in step 3 of the method with a predetermined maximum speed ω _{m a x} subtracted. If the absolute value of this difference is greater than the value of a hysteresis, which is determined by a lower threshold value ω _hys-low and an upper threshold value ω _{hys-high of} the rotational speed, then step 4 continued. If the difference is smaller than the hysteresis Δω _hys , then step 1 continued.

In step 4 the current speed value ω is compared with the upper threshold value ω _hys-high . If the current speed _{value is} not greater than ω _hys-high , then step 6 continued. In step 6 the current speed _value ω is compared with the lower speed _{threshold value} ω _hys-low . If the current speed _value ω is not smaller than ω _hys-high , so will also step 1 continued.

Results in step 4 However, that the current speed _value ω is greater than ω _hys-high , so in step 5 the maximum duty ratio D is reduced by a certain value ΔD. The value .DELTA.D can z. B. between 1% and 2% of the maximum duty ratio D _max . With this reduced duty cycle Dmax will now step with 1 continued.

However, it is found that the current speed ω is smaller than the minimum threshold ω _hys-low , as in the step 6 given, the maximum duty cycle D _max must be increased by a corresponding value ΔD, so that the motor again reaches its rated speed ω _nom (see step 7 ). With this increased duty cycle D _max will then step 1 the procedure continued.

Coming back to 2 you need z. B. at an input voltage of U2, only a duty cycle of less than 80% in order to achieve the desired speed ω. With the help of the engine control of the invention can then z. B. for an input voltage of U2, the maximum duty cycle D _{max be set} to 80%. Now increases the input voltage continues to z. B. U3, so recognizes in the drawing, that at the set maximum duty cycle D _max the speed would exceed the upper threshold ω _hys-low . This would be detected by the motor controller according to the invention and the maximum duty cycle D _max z. B. be limited to a value of 65%.

1

step

2

step

3

step

4

step

5

step

6

step

7

step

8th

step

9

step

10

PWM generator

11

BLDC engine

12

Hall sensor

D _max

maximum duty cycle

D

Current duty cycle

.DELTA.D

increment the change of the duty cycle

ω

current rotation speed

ω _max

maximum rotation speed

ω _hys-high

upper threshold

ω _hys-low

lower threshold

Δω _hys

Hysteresis, difference between ω _hys-high and ω _hys-low

Claims (6)

Motor control for a BLDC motor for realizing an open loop with pulse width modulation and controllable duty cycle D, wherein the motor control a maximum duty D _max , a lower ω _hys-low and an upper threshold ω _{hys-low are given} for the rotational speed, characterized by: means for detecting the current rotational speed ω of the motor, means for comparing the current rotational speed with the two threshold values ω _hys-low and ω _hys-high , and means for reducing the maximum duty cycle D _max by a certain step ΔD when the current rotational speed ω of the motor exceeds the upper threshold value ω _hys-high , or to increase the maximum duty ratio D _max by a certain step ΔD when the current rotational speed ω of the motor _{falls below} the lower threshold value ω _hys-low . Motor control according to Claim 1, characterized that means for detecting the current rotation speed Hall sensors are. Motor controller according to claim 1 or 2, characterized in that the lower threshold value ω _hys-high the upper threshold value ω _hys-high corresponds. Method according to one of Claims 1 to 3, characterized in that ω _hys-high > ω is _hys-low , and the threshold values define a hysteresis Δω _hys = ω _{hys-high -ω hys-low} . Motor control according to one of claims 1 to 4, characterized in that it is part of a fan motor. Motor control according to one of claims 1 to 5, characterized in that it has a programmable microcontroller includes.