DC motor mechanical shock protection system

Abstract

A DC motor mechanical shock protection system monitors the motor current to determine the torque the motor is applying to the gearbox, and takes mitigating action if the torque approaches a level that can damage the driven device and/or the drive system. This protects the system from mechanical shock without limiting the available torque during normal operation. The change in motor current is monitored to detect the inertial torque before the elasticity between the motor and the driven device is fully compressed and the motor mounting or gearbox begin to be damaged.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a flow chart of an algorithm carried out in one implementation of the shock protection system of the present invention;

FIG. 2 is a plot of current supplied to a DC motor with a gearbox that has run into an end stop at full operational speed without the shock protection system of the present invention; and

FIG. 3 is a plot of current supplied to a DC motor having the shock protection system of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to include all alternatives, modifications and equivalent arrangements as can be included within the spirit and scope of the invention as defined by the appended claims.

A DC motor and gearbox comprise a rotational system where a mechanical shock, linear or rotational, is translated into a change in the torque the motor is applying to a driven device via the gearbox, and to the motor mount. The mechanical shock is typically due to a sudden change in the motion of the driven device, which in turn is due to a change in the force opposing the motion.

The torque r applied by the motor to the gearbox and the motor mount is the torque from the motor electromagnetic field τ_emf and the torque from the rotor inertia τ_inertia.

τ_emf+τ_ineria

The torque τ_emf from the motor electromagnetic field is proportional to the motor current i.

τ_emf=K₁i

The torque τ_inertia from the rotor τ_inertia is proportional to the change in rotor angular velocity ω.

${\tau }_{\mathrm{inertia}}=-J\frac{\omega }{t}$

The current i for a DC motor is related to the terminal voltage V, the motor electrical resistance R, and the back-emf voltage αω.

$i=\frac{V-\mathrm{\alpha \omega }}{R}$

The change in rotor angular velocity

$\frac{\omega }{t}$

for a DC motor with constant terminal voltage is proportional to the change in motor current.

$\frac{\omega }{t}=-\frac{R}{\alpha }\frac{i}{t}$

The motor torque from the rotor τ_inertia is proportional to the change in motor current.

${\tau }_{\mathrm{inertia}}=K_2\frac{i}{t}$

The total torque ρ from the motor electromagnetic field and the rotor inertia can be described in terms of the motor current.

$\tau =K_1i+K_2\frac{i}{t}$

If the motion of the driven device is stopped, the electromagnetic field torque τ_emf and the inertial torque τ_inertia will continue to turn the motor shaft, compressing the elasticity in the motor mounting, the gearbox and the driven device up to the point where the motion was stopped. The mechanical shock is from the inertial torque and can be significantly above the device tolerances even though the current from the electromagnetic torque is well below the locked rotor level.

In one embodiment of the present invention, a DC motor mechanical shock protection system monitors the motor current to determine the torque the motor is applying to the gearbox, and takes mitigating action if the torque approaches a level that can damage the driven device and/or the drive system. This protects the system from mechanical shock without limiting the available torque during normal operation. The change in motor current is monitored to detect the inertial torque before the elasticity between the motor and the driven device is fully compressed and the motor mounting or gearbox begin to be damaged.

FIG. 1 is a flow chart of an algorithm carried out in one embodiment of the invention. The DC motor current is sampled at step 10 and then filtered at step 11 to remove the commutation ripples caused by the brushes crossing the commutator segments. The system then calculates the change Δi that occurs in the magnitude of the motor current i during each successive time interval At, at step 12. Step 13 determines whether the change Δi calculated at step 12 is a positive change and, if the answer is affirmative, the system proceeds to step 14. A negative answer at step 13 causes the system to return to step 12 to calculate the magnitude of the change in current magnitude during the next time interval Δt.

Step 14 determines whether a positive change in the motor current rises above a threshold that represents inertial torque that can damage the system, i.e., to detect an incipient mechanical shock condition. The threshold is preferably significantly above the maximum normal operating level. If step 14 yields an affirmative answer, step 15 adjusts the motor current to stop or reverse the motor, to counteract the incipient shock before any damage is done to the motor or gearbox. A negative answer at step 14 causes the system to return to step 12 to calculate the the magnitude of the change in current magnitude during the next time interval Δt.

The system continues to successively calculate the change Δi, if any, in the motor current i in each successive time interval Δt. Each time a calculated change Δi is positive, that change is compared with the preselected threshold value Δi_th and, if the measured change Δi exceeds the threshold value Δi_th, the current i is immediately adjusted to stop or reverse the motor. The effect of this system is to monitor the first derivative (the instantaneous rate of change) of the motor current i with respect to time, to monitor the magnitude and polarity of that first derivative, and to detect positive changes of magnitude that exceed a preselected threshold.

The algorithm shown in FIG. 1 can be implemented with a pair of integrated circuits, such as a MC33887 H-Bridge Power IC (available from Freescale Semiconductor, Inc. in Chandler, Ariz.) and a programmable microcontroller such as a PIC18F2420 (available from Microchip Technology Inc. in Chandler, Ariz.). The bridge circuit provides DC current to drive a DC motor, and also produces a feedback signal that is proportional to the motor drive current and can be “read” by a microcontroller having an analog-to-digital converter. The microcontroller can be programmed to filter this feedback signal to remove the commutation ripples, and then calculate any change that occurs in the magnitude of the filtered current in successive time intervals Δt. The microcontroller can also be programmed to determine whether each calculated change is a positive change, whether each positive change exceeds a prescribed threshold value and, if so, to produce a control signal that causes the bridge circuit to zero, or reverse the polarity of, the motor terminal voltage.

FIG. 2 is a plot of the current from a DC motor with a gearbox that has run into an end stop at full operational speed. Trace A shows the raw motor current, and trace B shows the motor current that has been low-pass filtered to remove the commutation ripple, digitized at 5V/1 A, and limited to 1 A. Both traces A and B show the uncontrolled current rise from the full operational speed level to the locked rotor level. The rise time of 43 ms is the time it takes to compress the elastic portion of the motor coupling to the device.

FIG. 3 is a plot of the current from the same DC motor equipped with the mechanical shock prevetion system. The vertical scale in FIG. 3 is twice that of FIG. 2. Trace C in FIG. 3 shows the controlled current rise using the shock protection system. The change in current exceeds the allowable threshold 12 ms after the rise begins, and the motor is stopped long before the elastic portion of the motor coupling to the device is fully compressed.

One example of an application in which the mechanical shock protection system of this invention is useful is in transfer switches, which are used to connect an electrical load to one of typically two separate power sources. A transfer switch connects a load to only one of the sources at a time using one switch per source, and a DC-motor-driven actuator opens and closes the switches, as described in U.S. Patent Application Publication No. 2006/0131146, which is incorporated herein by reference.

While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method of controlling the torque produced by a DC motor having a rotor, comprising monitoring the motor torque produced by rotor inertia, andadjusting electrical current supplied to said motor in response to an increase in said motor torque that exceeds a preselected threshold.

2. The method of claim 1 wherein said monitoring includes monitoring said electrical current supplied to said motor, and detecting increases in said electrical current in selected time intervals,

3. The method of claim 1 wherein said monitoring includes sampling said electrical current supplied to said motor, and filtering said sample to substantially remove commutation ripples.

4. The method of claim 1 wherein said threshold is significantly above the increases that occur in said torque during normal operation of the motor.

5. The method of claim 1 wherein said monitoring includes calculating any change that occurs in the magnitude of said electrical current supplied to said motor in selected time intervals, and determining whether each calculated change is positive.

6. The method of claim 1 wherein said adjusting of said electrical current stops or reverses said motor.

7. A method of controlling the torque produced by a DC motor, comprising monitoring the electrical current supplied to said motor,detecting increases in said electrical current in selected time intervals,comparing each of said increases with a preselected threshold, andadjusting the electrical current supplied to said motor in response to an increase that exceeds said preselected threshold.

8. The method of claim 7 wherein said selected time intervals are successive intervals of equal length.

9. The method of claim 7 wherein said monitoring includes sampling the electrical current supplied to said motor, and filtering said sample to substantially remove commutation ripples.

10. The method of claim 7 wherein said threshold is significantly above the increases that occur in said electrical current in said selected time intervals during normal operation of the motor.

11. The method of claim 7 wherein said detecting includes calculating any change that occurs in the magnitude of said current in each of said selected time intervals, and determining whether each calculated change is positive.

12. The method of claim 7 wherein said adjusting of said electrical current stops or reverses said motor.

13. The method of claim 7 in which said motor actuates a transfer switch.

14. A method of controlling the torque produced by a DC motor, comprising monitoring the first derivative of the electrical current supplied to said motor, andadjusting the electrical current supplied to said motor in response to an increase in said first derivative that exceeds a preselected threshold.

15. The method of claim 14 that includes detecting increases in said first derivative, and comparing said increases with said preselected threshold.

16. The method of claim 14 wherein said monitoring includes monitoring said electrical current supplied to said motor, and detecting increases in said electrical current in selected time intervals.

17. The method of claim 14 wherein said monitoring includes sampling said electrical current supplied to said motor, and filtering said sample to substantially remove commutation ripples.

18. The method of claim 14 wherein said threshold is significantly above the increases that occur in said torque during normal operation of the motor.

19. The method of claim 14 wherein said monitoring includes calculating any change that occurs in the magnitude of said electrical current supplied to said motor in selected time intervals, and determining whether each calculated change is positive.

20. The method of claim 14 wherein said adjusting of said electrical current stops or reverses said motor.