CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 U.S.C. § 119 to Japanese Application No. 2019-036568 filed Feb. 28, 2019, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a driving technology for a stepping motor.

Description of the Prior Art

Stepping motors are extensively applied in electronic machines, industrial machinery, and robots. Stepping motors are synchronous motors that rotate synchronously with an input clock generated by a host controller, and have excellent control properties in starting, stopping and positioning. Thus, stepping motors feature capabilities for positioning control in an open loop and suitability for digital signal processing.

In a normal state, a rotor of a stepping motor rotates synchronously in a unit of a step angle directly proportional to the number of input clocks. However, if a drastic change in the load or speed occurs, said synchronization is lost and such is referred to as “out-of-step”. Once out-of-step happens, special processing needs to be performed in order to subsequently drive the stepping motor normally. Thus, avoiding out-of-step is desired.

Hence, during acceleration and deceleration having higher probabilities of out-of-step, a target value of a driving current is set to a constant value, so as to obtain an output torque large enough yet without causing out-of-step with respect to speed change.

Patent document 5 proposes a technique as follows—power consumption is reduced to improve efficiency by avoiding out-of-step and optimizing an output torque (i.e., a current amount) by means of feedback.FIG. 1shows a block diagram of a conventional stepping motor and a motor system of a driving circuit thereof.

A host controller2supplies an input clock CLK to a driving circuit4. The driving circuit4changes the excitation position in synchronization with the input clock CLK.

FIG. 2shows a diagram illustrating excitation positions. The excitation position can be understood as a combination of coil currents (driving currents) IOUT1and IOUT2flowing through two coils L1and L2of a stepping motor6. InFIG. 2, eight excitation positions 1 to 8 are depicted. In 1-phase excitation, current alternately flows in the first coil L1and the second coil L2, thus changing the excitation position among 2, 4, 6 and 8. In 2-phase excitation, current flows in the first coil L1and the second coil L2, thus changing the excitation position among 1, 3, 5 and 7. 1-2-phase excitation is a combination of 1-phase excitation and 2-phase excitation, and causes the excitation position to change among 1 and 8. In micro-step driving, the excitation position is even more finely controlled.

FIG. 3shows a diagram of a driving sequence of a stepping motor. At startup, the frequency fINof the input clock CLK rises along with time, and the stepping motor accordingly accelerates. The frequency fINis kept constant upon reaching a predetermined target value, and accordingly the stepping motor rotates at a constant speed. To later stop the stepping motor, the frequency of the input clock CLK is reduced to accordingly decelerate the stepping motor. The control inFIG. 3is also referred to as trapezoidal wave drive.

In a normal state, a rotor of a stepping motor rotates synchronously in a unit of a step angle directly proportional to the number of input clocks. However, if a sudden change in the load or speed occurs, synchronization is lost and such is referred to as “out-of-step”. Once out-of-step happens, special processing needs to be performed in order to subsequently drive the stepping motor normally. Thus, avoiding out-of-step is desired.

Hence, during acceleration and deceleration having higher probabilities of out-of-step, a target value IREFof a driving current is set to a constant value IFULL(high-torque mode), so as to obtain a fixed output torque large enough, with the margin for out-of-step having been taken into consideration.

In a situation where the revolution count is stable and the probability of out-of-step is lower, efficiency (high-efficiency mode) is improved by reducing the target value IREFof the driving current. Patent document 5 proposes a technique as follows—power consumption is reduced to improve efficiency by avoiding out-of-step and optimizing an output torque (i.e., a current amount) by means of feedback. More specifically, a load angle ϕ is estimated according to a counter electromotive force (EMF) VBEMF, and the target value IREFof the driving current (coil current) is controlled by way having the load angle ϕ approach close to the target value ϕREF. The counter EMF VBEMFis represented by equation (1):
VBEFM=KE×ω×cos ϕ  (1)

In equation (1), ω is the angular speed (to be referred to as the revolution count or frequency) of the stepping motor, and KEis a counter EMF constant and is an inherent constant of the stepping motor.

In the technique disclosed by patent document 5, a feedback loop is formed by way of having the detection value cos ϕ approach close to the target value cos(ϕREF) thereof, thus optimizing the coil currents IOUT1and IOUT2in the high-efficiency mode.

PRIOR ART DOCUMENTS

Patent Publication

[Patent document 1] Japan Patent Publication No. JPH09103096[Patent document 2] Japan Patent Publication No. JP2004-120957[Patent document 3] Japan Patent Publication No. JP2000-184789[Patent document 4] Japan Patent Publication No. JP2004-180354[Patent document 5] Japan Patent Publication No. JP6258004

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

As disclosed in patent document 5, when feedback control for the current target value IREFbased on the load angle is incorporated, the load angle in a stable state is stabilized at the target value; however, if there is a change in the load, the load angle ϕ is deviated from the target angle ϕREF.

In the motor system using the driving circuit disclosed in patent document 5, a detection value cos ϕ corresponding to the load angle ϕ is generated within the driving circuit. The load angle ϕ includes information useful to the design or control of the system. Nonetheless, in current situations, there is no way of knowing from the outside of the driving circuit the value of the load angle currently being used for driving the stepping motor6.

The present invention is accomplished in view of this issue. One object of an exemplary embodiment of the present invention is to provide a driving circuit capable of outputting information useful to the design or control of a system to outside.

Technical Means for Solving the Problem

A driving circuit of a stepping motor is provided according to an implementation form of the present invention. The driving circuit includes: a counter electromotive force (EMF) detection circuit, detecting a counter EMF generated in a coil; a revolution count detection circuit, detecting the revolution count of the stepping motor; and a load angle estimating portion, calculating a load angle according to the counter EMF and the revolution count. The driving circuit is configured to be capable of outputting angle information associated with the load angle to the outside, or accessing angle information from the outside.

The load angle ϕ can be understood as a delay in a mechanical angle relative to an electrical angle. The load angle ϕ is associated with the margin before the occurrence of out-of-step or a drastic change in the load. Hence, by outputting information associated with the load angle, such information can be effectively used for determining the state of a stepping motor or optimization of control parameters. Further, the so-called “calculating a load angle” can also include, in addition to calculating the load angle ϕ, generating the detection value cos ϕ associated with the load angle ϕ.

The driving circuit can further include an interface circuit that outputs the angle information in form of a digital signal to the outside.

The driving circuit can further include a digital-to-analog converter (DAC) that converts the angle information to an analog signal, and a buffer circuit that outputs the analog signal to the outside.

The angle information can also be a margin before the occurrence of out-of-step. For example, the margin can be a difference between π/2 of out-of-step and the load angle ϕ, or can be a difference between an actual out-of-step limit set from the outside and the load angle ϕ.

The angle information can also be the load angle.

The driving circuit can further include: a current value setting circuit, generating a current setting value; a constant current chopper circuit, generating a pulse modulation signal that modulates by way of having a detection value of a coil current flowing through the coil approach close to a target value based on the current setting value; and a logic circuit, controlling a bridge circuit connected to the coil.

The current value setting circuit can generate the current setting value according to the counter EMF.

The current value setting circuit can further include a feedback controller, which generates the current setting value by way of having the load angle ϕ approach close to the target value ϕREFthereof. The feedback controller can also generate the current setting value by way of having the detection value cos ϕ corresponding to the load angle ϕ approach close to a target value cos(ϕREF) thereof.

The constant current chopper circuit can further include: a comparator, comparing the detection value of the coil current with a threshold value based on the current setting value; an oscillator, performing oscillation according to a predetermined frequency; and a flip-fop, outputting the pulse modulation signal, which transitions to an off level according to an output of the comparator and transitions to an on level according to an output of the oscillator.

The driving circuit can be integrated on a semiconductor substrate. The term “integrated” includes a situation where all constituent elements of the circuit are formed on the semiconductor substrate and a situation where main constituent elements of the circuit are formed on the semiconductor substrate; alternatively, a part of resistors or capacitors can be provided outside the semiconductor substrate so as to adjust circuit constants. By integrating the circuit in one chip, circuit area can be reduced while preserving balanced characteristics of electrical elements.

Further, any combination of the constituent elements above, and any invention derived from mutual substitutions made to the constituent elements of the invention and methods, devices and systems presented therefrom are also considered as effective embodiments of the present invention.

Effects of the Invention

According to an embodiment of the present invention, information useful to the design or control of a system can be provided to the outside.

