CA2351201A1 - An improved motor driver circuit - Google Patents

Abstract

An improved motor driver circuit comprises a firmware adapted to provide digital control signals, a digital-to-analog converter coupled to said firmware to receive the digital control signals for generating a reference voltage at an output, a compare circuit having a first input coupled to the output of said digital-to-analog converter, a second input coupled to a sense circuit for receiving a sense voltage and an output, and a switch circuit coupled in series to said sense circuit wherein said compare circuit compares the reference voltage and the sense to generate a compare voltage at the output to be fed into said switch circuit for directing the switch circuit to conduct or not to conduct a drive current flowing through said switch circuit and said sense circuit.

Description

AN IMPROVED MOTOR DRIVER CIRCUIT
Field of the Invention The present invention relates generally to a driver circuit and, more particularly, to a motor driver adapted to provide selectable current levels of a drive current for driving a motor externally coupled to the motor driver.
Background of the Invention A conventional motor driver, such as a chopper motor driver, is generally designed to limit the amount of current that a motor can draw from a power supply. As a result, the motor will not draw more current than necessary for operation, thereby increasing the power efficiency of the motor and the motor driver. More importantly, by limiting the current supplied to the motor, the conventional motor driver prevents too much current from flowing through the motor than the motor could handle so that the motor will not burn up.
A conventional arrangement of the motor driver 10 and the motor 30 is shown in Figure 1. The conventional motor driver 10 includes a driver circuit 12 coupled to a reference circuit 14. The reference circuit 14 typically includes a voltage divider 22 for generating a predetermined reference voltage V, to the motor driver 10. Ordinarily, the voltage divider 22 has first R1 and second R2 resistors 24, 26 connected in series between power supply V~ 32 and ground node 38. The reference voltage V, is defined as the voltage at node 28, where the first R1 and second R2 resistors 24, 26 join.
As a result, the reference voltage V, is defined as Vf = V~ x R2/(R1+R2) where the V~ is a predetermined stable voltage. By selecting the first R1 and second R2 resistors 24, 26, a user of the motor driver 10 can provide a fixed reference voltage V, for determining how much current could flow through the motor 30, as will be further explained in the following paragraphs.
The drive circuit 12 of the conventional motor driver 10 includes a compare circuit 20, as shown in Figure 1. Normally, the node 28 is coupled to one terminal 42 of the compare circuit 20 for receiving the reference voltage V,. In addition, the other terminal 44 of the compare circuit 20 receives a sense voltage from an output 40 of a sense circuit 18. Therefore, the compare circuit 20 compares the reference voltage V, with the sense voltage VS to generate a compare voltage Vop at its output. As shown in Figure 1, the compare circuit 20 often includes an op-amp 36 to compare the reference voltage Vf with the sense voltage Vg for generating the compare voltage Vop at the output of the op-amp 36 based on the relative difference of the Vf and Vs.
As shown in Figure 1, the drive circuit 12 also includes the switch circuit 16 coupled in series between the motor 30 and the sense circuit 18 of the drive circuit 12. The sense circuit 18 generally includes a sense resistor RS 34 coupled in series between the switch circuit 18 and the ground node 38.
As a result, a current I~ flowing through the motor 30 will also flow through the sense resistor R, 34 by way of the sense circuit 18.
Figure 1 shows the sense resistor Rs 34 being coupled to the compare circuit 20 at the output node 40. Thus, the sense voltage VS, which is the voltage at the output node 40, is defined as Va = Ic X Rs where the current I~ is the same current flowing through the motor 30, the switch circuit 16, and the sense resistor R, 34.
As mentioned, the sense voltage Vs is fed into the other terminal 44 of the comparator 36 to be compared with the reference voltage Vf fed into one input terminal 42 of the op-amp 36. If the VS is larger than the V,, the output voltage VoP of the op-amp 36 will force the switch circuit 16 to open, i.e., to stop conducting the current I~ from the motor 30. As a result, the motor 30 will not draw any current from power supply Vp$ 46. Simultaneously, the sense voltage Vs will start to drop since no current is now flowing through the sense resistor Rg 34 due to the open circuit of the switch circuit 16. After the sense voltage VS has dropped to a level lower than the reference voltage V,, the output voltage Vop of the op-amp 36 will then cause the switch circuit 16 to close, i.e., to start conducting the current I~ from the motor 30, and the sense voltage Vs will then start to rise. In short, the configuration of the conventional motor driver 10 provides a feedback loop between the drive circuit 12 and the reference circuit 14, whereby a feedback voltage, i.e., the sense voltage Vs, is compared to the reference voltage V, for determining the status (open or close) of the switch circuit 16. Therefore, the sense voltage Vs will be confined within a range approximately equal to the reference voltage V, and the current flowing through the motor 30 will be defined approximately as I~ = Vt / RS.
Comparing the sense voltage V, with the predetermined reference voltage V, effectively allows the conventional motor driver 10 to control how much current current I~ will flow through the motor 30. Therefore, the motor 30 will be protected from burning up due to the excessive heat generated by the excessive current. Also, a specific torque characteristic can be achieved.
The conventional motor driver 10, however, has several disadvantages.
Generally, the motor 30 needs a relatively large current to start the motor's rotor during an initial operating stage. In contrast, the motor 30 needs only a much lower current level to maintain rotation once it passes the initial operating stage. For example, a larger initial torque ~; is needed to overcome the static friction fs so as to initially start the motor 30 from rest.
When the motor's rotor starts to rotate, a smaller torque f~ is required to overcome a dynamic friction fd for maintaining rotation. The dynamic friction Fd usually is smaller than the static friction fs. As a result, the initial torque ~;
has to be larger than the rotating torque f~. Since only one reference voltage V, is provided to the conventional motor driver 10, the reference voltage V, has to be set at a value that is sufficient to drive the motor 30 during the initial stage. As a result, the current I~, which is determined by the Vf, is defined sufficiently large to drive the conventional motor 30 during the initial stage but is often larger than necessary after the motor 30 starts running.
Consequently, the conventional motor driver 10 consumes more electrical power than necessary for operation and is less efficient.
Moreover, since a larger than necessary current I~ flows through the motor 30, the motor driver 30 often generates excessive heat during operation. The excessive heat will have an adverse effect on the reliability of the motor 30, or conventional motor driver 10. A heat sink may be added to the conventional motor driver 10 to eliminate the excessive heat problem.
However, adding the heat sink will inevitably increase the complexity and manufacturing costs and size of the conventional motor driver 10. An improved motordriver solution is thus needed to resolve the above-mentioned difficulties.
Summar~of the Invention The present invention is directed to an improved motor driver circuit having a select circuit for controlling a current level flowing through a motor coupled to the improved motor driver. In a preferred embodiment of the present invention, the select circuit includes a DAC (Digital-to-Analog Converter) for determining the current level of the motor. The DAC is coupled to a drive circuit of the improved motor driver circuit to provide the drive circuit a reference voltage that is selectable from a plurality of voltage levels. The drive circuit then, in response to the selected reference voltage supplied by the DAC, controls the current level of the motor to its appropriate level during different stages of operation. In another embodiment, the present invention further comprises a firmware program for controlling the DAC to output the reference voltage. The firmware may be pre-programmed according to the needs of the motor during various stages of operation.

The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
Brief Description of the Drawings Figure 1 shows a conventional motor drive circuit coupled to a motor and a power supply.
Figure 2 shows a preferred embodiment of an improved motor drive circuit according to the present invention.
Detailed Description of the Invention The present invention is directed to an improved motor drive circuit 100 which provides a selectable current level for a motor 102 during different stages of motor operation. The motor 102 is coupled to the motor driver 100 and is not part of the present invention. The motor driver 100 includes a control circuit 104 coupled to a first input of a drive circuit 106. In the preferred embodiment, the control circuit 104 includes a DAC (Digital-to Analog Converter) 108 having an output coupled to the first input of the drive circuit 106. The output of the DAC 108 supplies a selectable reference voltage Vf to be fed into the first input of the drive circuit 106.
The drive circuit 106 includes a compare circuit 112 coupled to a switch circuit 114 at the output. The compare circuit 112 has a first and a second input respectively coupled to the output of the DAC 108 through the first input of the drive circuit 106 and to a sense circuit 116, as shown in Figure 2. The switch circuit 114 is coupled to the sense circuit 116 in series, whereby the node 122 is positioned directly between the switch circuit 114 and the sense circuit 116. Moreover, the switch circuit 114 is further coupled to the motor 102 in series, as shown in Figure 2.

The compare circuit 112 also has an output terminal coupled to the switch circuit 114. Thus, an output voltage Vop of the op-amp 118 is provided to the switch circuit 114 for controlling the status (open or close) of the switch circuit 114. For instance, when the output voltage Vop is at one level, the switch circuit 114 will be closed (i.e., conducting) and is adapted to conduct an electric current I~ flowing through the motor 102. When the output voltage Vop is the opposite, the switch circuit 114 will be open and will not conduct the electric current I~ from the motor 102.
The sense circuit 116 defines a sense voltage VS at the node 122 to be fed back to the comparator or op-amp 118. Basically, the sense circuit 116 uses a resistive load for determining the sense voltage VS at the node 122.
In the preferred embodiment, the sense circuit 116 includes a sense resistor R$ coupled in series between the node 122 and the ground node 124, as shown in Figure 2, so that the I~ will flow from the switch circuit 114 through the sense resistor Rs into the ground node 124. The sense voltage VS at the node 122 is, therefore, defined as Vs=I~XRs where I~ is the electric current flowing through the motor 102 and the sense resistor RS and the Ra is the resistance of the sense resistor RS. In an alternative embodiment, the sense resistor Rg may be replaced by a load circuit or by other electrical components adapted to provide the suitable means for determining the sense voltage Vs.
As mentioned, the output of the DAC 108 generates the selectable reference voltage V, which is fed into the compare circuit 112, as shown in Figure 2. The reference voltage V, functions to control the switch circuit 114 for determining the current level of the I~ flowing through the motor 102.
Moreover, when the Vop is at one level, the switch circuit 114 will be closed to conduct the I~ from the motor 102. When the Vop is at the other level, the switch circuit 114 will open to disconnect the sense circuit 116 from the motor 102. Therefore, the sense voltage Vg will start to drop due to the decreasing current flow of I~. Once the sense voltage V$ has dropped to a level lower than the reference voltage Vf, the VoP of the op-amp 118 will switch to the first level again and the switch circuit 114 will start conducting the I~ and start raising the sense voltage Vg. By providing this dynamic feedback loop of the drive circuit 106, the present invention can, thus, control the I~ in the motor 102 approximately at a level corresponding to a selected voltage level of the reference voltage V,.
As noted, the reference voltage V, provided by the DAC 108 is selectable, i.e., the DAC 108 may choose the voltage level of the reference voltage V, within a predetermined range. The voltage range of the V, should be sufficiently large to provide enough voltage for rotating the motor rotor during an initial operation stage of the motor 102. Moreover, the reference voltage Vf should provide sufficient sustaining voltage for continuing to rotate the motor rotor after the initial operation state. The DAC 108 works to convert pre-selected digital bit information into analog voltage levels for outputing the reference voltage Vt. In the preferred embodiment, the DAC 108 has an eight-bit configuration. As a result, the DAC 108 is capable of providing 256 voltage levels for the reference voltage Vf. In other alternative embodiments, the DAC may have any suitable numbers of digital bit configuration adapted for various applications.
In the preferred embodiment, the control circuit 104 of the present invention further comprises a firmware 110 coupled to the DAC 108 for controlling the same. The firmware 110 is pre-programmed to generate digital bit information, 8 bits in the case of the preferred embodiment, according to the operation stage of the motor 102. The digital bit information of the firmware 110 is then fed into the DAC 108 for generating reference voltage V, corresponding to the digital bit information.
By having the firmware 110 and the DAC 108 of the control circuit 104, the present invention is adapted to provide different voltage levels of the reference voltage V, to provide adequate current levels of the motor 102 during different stages of the motor operation. For example, during the initial operation stage, the reference voltage V, is set to a high value in order to allow sufficiently large I~ in the motor 102 to provide a large initial torque f; for rotating the motor rotor. Once the motor 102 starts rotating, the firmware 110 causes the DAC 108 to generate a lower reference voltage V,, and thus lower Ic, sufficient to maintain rotation of the motor 102. Because the motor 102 runs on a higher I~ only during the short period of the initial operation stage, the present invention has a better power efficiency as compared the conventional motor driver. Moreover, since the DAC 108 has an 8-bit configuration to provide 256 levels of reference voltage V,, the present invention may dynamically adjust the reference voltage V, according to various stages of the motor operation. This may further save substantial power consumption, and improve power efficiency, of the motor driver 100.
As a result, the present invention needs no heat sink to dissipate heat generated by the motor 102, as many conventional motor drivers do to prevent thermal damages of the motor and/or the motor driver.
From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made by persons skilled in the art without deviating from the spirit and/or scope of the invention. Specifically, the switch circuit can be any standard switch circuit in the industry that is suitable to control the h at the current levels within the range of the present invention. Also, a digital analog converter is not required. It could be a series of resistors that form a low current digital analog converter function. The improved motor driver can also be utilized to regulate current levels of other electrical elements.
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Claims (6)

1. An improved motor driver circuit coupled to an external motor, comprising:
a digital-to-analog converter for generating selectable voltage levels of a reference voltage at an output; and a drive circuit coupled to the output of said digital-to-analog converter, said drive circuit being adapted to receive the reference voltage from the digital-to-analog converter for regulating a drive current flowing through said drive circuit and the motor.

2. The improved motor driver circuit of claim 1, wherein said drive circuit comprises:
a sense circuit, said sense circuit being adapted to provide a sense voltage;
a compare circuit coupled to said digital-to-analog converter and said sense circuit, said compare circuit being adapted to compare the reference voltage and the sense voltage to generate a compare voltage at an output; and a switch circuit coupled to said sense circuit in series, said switch circuit being further coupled to the output of said compare circuit wherein the compare voltage direct the switch circuit to conduct or not to conduct the drive current flowing through said switch circuit and said sense circuit.

3. The improved motor driver circuit of claim 2, wherein said sense circuit comprises a sense resistor coupled in series between said switch circuit and a ground terminal, said sense resistor having the drive current flowing through therein to provide the sense voltage to be fed into the compare circuit.

4. The improved motor driver circuit of claim 2, wherein said switch circuit is coupled in series to the motor allowing the drive current to flow from the motor through said switch circuit into said sense circuit, said switch circuit being controlled by the compare voltage for opening or closing said switch circuit.

5. The improved motor driver circuit of claim 1, further comprising a firmware coupled to said digital-to-analog converter, said firmware being configured to provide digital control signals to said digital-to-analog converter for generating the reference voltage.

6. The improved motor driver circuit of claim 1, wherein said digital-to-analog converter is an eight-bit digital-to-analog converter adapted to provide 256 voltage levels of the reference voltage.