Temperature compensation for DC motor PWM applications

Motor control systems and methods. A motor control system includes a temperature compensated power supply configured to receive a supply voltage and output a temperature compensated reference voltage. The reference voltage varies according to an ambient temperature of the motor control system. The motor control system includes a microcontroller configured to receive the temperature compensated reference voltage and a sensed motor current and to produce a corresponding pulse-width-modulated (PWM) motor control signal. The microcontroller is configured to increase a duty cycle of the PWM motor control signal as a function of the temperature compensated reference voltage. The motor control system includes a motor configured to be controlled according to the duty cycle of the motor control signal.

TECHNICAL FIELD

The present disclosure is directed, in general, to motor control systems, devices, and methods.

BACKGROUND OF THE DISCLOSURE

It can be important to correctly control motor operations even in increased or extreme temperature conditions to compensate for motor torque reduction at extreme temperatures.

SUMMARY OF THE DISCLOSURE

Various disclosed embodiments include motor control systems and methods. A motor control system includes a temperature compensated power supply configured to receive a supply voltage and output a temperature compensated reference voltage. The reference voltage varies according to an ambient temperature of the motor control system. The motor control system includes a microcontroller configured to receive the temperature compensated reference voltage and a sensed motor current and to produce a corresponding pulse-width-modulated (PWM) motor control signal. The microcontroller is configured to increase a duty cycle of the PWM motor control signal as a function of the temperature compensated reference voltage. The motor control system includes a motor configured to be controlled according to the duty cycle of the motor control signal.

In various embodiments, the sensed motor current determines the PWM. This technique effectively temperature compensates the sensed current by changing the reference voltage of an analog-to-digital convertor.

In specific embodiments, the temperature compensated power supply includes a resistor and a Zener diode connected between the supply voltage Vs and a ground, wherein a cathode of Zener diode is connected to the resistor. The temperature compensated power supply also includes an NPN transistor, wherein a collector of the NPN transistor is connected to the supply voltage and an emitter of the NPN transistor is connected to produce the control voltage. The temperature compensated power supply also includes a diode, wherein a cathode of the diode is connected to a base of the NPN transistor and an anode of the diode is connected to the connection between the cathode of the Zener diode and the resistor. The circuit may be described as an emitter follower in that the output is taken from the emitter, and the emitter follows the reference voltage created by the Zener diode and the additional diode.

DETAILED DESCRIPTION

In certain smoke control applications, it is necessary for the actuator to meet certain UL requirements. These requirements call for the actuator to operate at an extended temperature of 250° F. for up to 45 minutes as an initial qualification test. This test is not performed on every unit.

One actuator design employs a DC brush with pulse width modulation (PWM) control provided by a microcontroller.

The torque of a DC brush motor decreases substantially at elevated temperatures. This implies that the motor current, or PWM duty cycle, needs to increase as temperature rises to provide adequate torque. Another function of the PWM control is to sense the increased motor current once the actuator reaches the end stop, and reduce the motor current to minimize stress on the gear train. The PWM duty cycle will be reduced to a level that provides enough motor current so the actuator is held in the open position. Due to the motor torque reduction at high temperature this hold current must increase above the level used at normal temperatures.

Disclosed embodiments include alternate ways of adding temperature sensors to a design. For example, a sensing element can supply a signal to an analog-to-digital converter (ADC). The microcontroller can then calculate the required change needed to the PWM duty cycle. This such a solution, however, requires extra hardware components and additional software overhead in the microcontroller. This circuitry would also need to operate at 250° F., which may be difficult or expensive to achieve.

Another approach is to operate the motor at higher currents at normal operating temperatures. This approach, however, will degrade the motor and gear train, reducing actuator life.

Disclosed embodiments include low cost, simple approach to adjusting the motor current as a function of temperature. This allows the motor to operate at low currents under normal operation, thereby extending actuator life, with automatic adjustment under increased temperatures.

FIG. 1illustrates a motor-control system100in accordance with disclosed embodiments. In this example, temperature-compensated power supply110provides a reference voltage122(VCC) to a microcontroller120. Reference voltage122also is used as a reference voltage to ADC124. Microcontroller120controls motor130. Microcontroller120can be can be implemented as any controller, ASIC, or other control circuitry.

According this embodiment, microcontroller120is used to pulse width modulate the voltage to a DC brush motor130at the PWM OUT connection to transistor132. The speed and torque of motor130is varied according to the voltage and duty cycle of the PWM OUT pulse signal controlling transistor132. Control voltage122therefore determines the “volts per count” of the A/D and the PWM OUT signal.

The motor current Imis sensed by the voltage drop across current sense resistor134, and fed back to an ADC124, which can be internal or external to the microcontroller120, so the motor speed and torque can be controlled by the microcontroller120by varying the duty cycle of motor control signal PWM OUT based on a control algorithm of microcontroller120. The voltage across134is read by the A/D. The pulse width of motor control signal PWM OUT varies according to the sensed motor current (Voltage drop across resistor134), and the A/D reference voltage122. When the motor reaches its end of travel, a voltage across resistor134, corresponding to a high motor current Imis sensed and the pulse width is decreased to a “hold current” to prevent damage to the gear train.

The temperature compensated power supply110varies the reference voltage122according to the temperature by using the temperature coefficients of Zener diode114, the base-emitter (BE) junction of transistor118, and diode116. As the temperature increases, these effects produce a higher voltage at the emitter of NPN transistor118(reference voltage122).

In the temperature compensated power supply110, a resistor112and Zener diode114are connected between a supply voltage Vs(in this example 24V) and a ground. The cathode of Zener diode114is connected to the resistor112. Temperature compensated power supply110includes an NPN transistor118with its collector connected to the supply voltage Vsand its emitter connected to provide reference voltage122to microcontroller120. The base of NPN transistor118is connected to be controlled by the cathode of diode116. The anode of diode116is connected to the cathode of Zener diode114and resistor112. The temperature coefficients of114,116, and118sum to produce the temperature-compensated power supply at reference voltage122.

Disclosed embodiments address issues of motor control under increased temperatures by adjusting the ADC reference voltage122. Disclosed embodiments exploit the temperature coefficient of the Zener diode114, the BE junction of a bipolar junction NPN transistor115, and an additional silicon diode116to provide a power supply control voltage122. The ADC124samples the current sense voltage generated by current Imacross transistor134and converts it to a digital feedback signal for use by the microcontroller. The ADC reference voltage determines the volts for each A/D count. The motor control algorithm then controls the pulse width of the motor control signal PWM OUT.

As the reference voltage control voltage122increases, the volts per count of the ADC124increases, hence the motor current increases based on the same PWM control algorithm count values. The microcontroller120is configured to adjust the motor current Imaccording to the counts measured in the digital feedback signal. Using this technique, there is no need for any modifications to the microcontroller programming to account for temperature increases. The increased motor current at high temperature is a result of the ADC reference voltage122increasing as temperature increases.

Such an embodiment avoids the need for such components as a temperature sensing element, conditioning circuitry, ADC readings, and software overhead.

FIG. 2illustrates a simulation of temperature compensated power supply110, showing that the power supply control voltage122, shown on the Y axis, increases as the temperature increases, as shown on the X axis.

FIG. 3depicts a flowchart of a process in accordance with disclosed embodiments that may be performed, for example, by a motor control system as disclosed herein.

The motor control system receives a supply voltage by a temperature compensated power supply (305). This can be the supply voltage Vs(in this example 24V).

The motor control system generates an A/D reference voltage from the supply voltage by the temperature compensated power supply (310). The reference voltage directly varies with the ambient temperature of the motor control system.

The motor control system modifies a pulse-width-modulated motor control signal using a microcontroller according to the reference voltage (315). The temperature-compensated reference voltage effectively changes the “gain” or scaling of the ADC as described herein.

The motor control system controls a motor using a pulse-width-modulated motor control signal by a microcontroller (320). The pulse-width of the modulated signal directly varies with the A/D reference voltage and the sensed motor current (according to the voltage drop across resistor134). The microcontroller receives the A/D reference voltage from the temperature compensated power supply.

Of course, those of skill in the art will recognize that, unless specifically indicated or required by the sequence of operations, certain steps in the processes described above may be omitted, performed concurrently or sequentially, or performed in a different order.

Those skilled in the art will recognize that, for simplicity and clarity, the full structure and operation of all circuits or devices suitable for use with the present disclosure is not being depicted or described herein. Instead, only so much of a circuit as is unique to the present disclosure or necessary for an understanding of the present disclosure is depicted and described. The remainder of the construction and operation of motor control system100may conform to any of the various current implementations and practices known in the art. Note that the specific logical divisions described herein between the power supply, the microcontroller, and other elements is exemplary and not intended to be limiting. The various elements and components can be grouped, associated, or separated as may be useful so long as the claimed interrelations and operations are satisfied.

None of the description in the present application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope: the scope of patented subject matter is defined only by the allowed claims. Moreover, none of these claims are intended to invoke 35 USC § 112(f) unless the exact words “means for” are followed by a participle. The use of terms such as (but not limited to) “mechanism,” “module,” “device,” “unit,” “component,” “element,” “member,” “apparatus,” “machine,” “system,” “processor,” or “controller,” within a claim is understood and intended to refer to structures known to those skilled in the relevant art, as further modified or enhanced by the features of the claims themselves, and is not intended to invoke 35 U.S.C. § 112(f).