METHOD OF OPERATING A STEPPER MOTOR IN A DENTAL TOOL MACHINE

Abstract

The present invention relates to a method of operating a stepper motor for use in a dental tool machine (1) for removing material from a dental blank (2), the method comprising: a step of adapting torque reserves of the stepper motor at operating points to net load moments (M_net) respectively, characterized by further comprising: a first step (S1) of predicting, through simulation, the net load moments (M_net) beforehand; a second step (S2) of predicting, through simulation, the supply current (I_tr) to be supplied to the stepper motor for setting up the torque reserves that correspond to the predicted net load moments (M_net) respectively; and a step (S3) of driving the stepper motor based on the predicted supply current (I_tr).

Claims

1. A method of operating a stepper motor for use in a dental tool machine for removing material from a dental blank, the method comprising: a step of adapting without use of any rotary encoder-based regulation torque reserves of the stepper motor at operating points to net load moments (M_net) about the rotational axis of the stepper motor respectively, wherein the adapting step comprises: a first step (S1) of predicting, through simulation, the net load moments (M_net) beforehand; a second step (S2) of predicting, through simulation, the supply current (I_tr) to be supplied to the stepper motor for setting rip the torque reserves that correspond to the predicted net load moments (M_net) respectively; and further comprising: a step (S3) of operating the stepper motor based on the predicted supply current (I_tr).

2. The method according to claim 1, wherein the net load moments (M_net) corresponds to a superposition of the load moments (M_df, M_mf) respectively due to drive forces arising through a drive train of the stepper motor and the machining forces arising through the material removal from the dental blank which are respectively predicted in the first predicting step based on a drive train simulation (S_dt) and a material removal simulation (S_mr) of the dynamic acceleration/deceleration along a drive train trajectory corresponding to the moveinent of the dental tool, and the dental tool trajectory.

3. The method according to claim 1 wherein the supply current (I_tr) is predicted in the second predicting step (S2) based on a torque reserve simulation (S_tr) of a dynamic current supply.

4. The method according to claim 1, wherein the first and second predicting steps (S1,S2) are performed in advance of the driving step (S3).

5. The method according to claim 1, further comprising: a step of generating an enhanced supply current (I_tr′) by adding to the predicted supply current (I_tr) a constant amount and/or by multiplying the predicted supply current (I_tr) through a constant factor greater than one, wherein the stepper motor is driven based on the enhanced supply, current (I_tr′).

6. The method according claim 1, further comprising: a step of generating a step shaped supply current (I_tr″) having two or more levels based on the predicted supply current (I_tr), wherein the stepper motor is driven based on the step shaped supply current (I_tr″).

7. The method according to claim 6, wherein the stepper motor is driven based on a smoothed step shaped supply current (I_tr″) which is obtained by at least one of interpolation, momhing and filtering thereof.

8. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for finishing the dental blank and a relatively higher level is used for roughing the dental blank.

9. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for making partial cut paths in the dental blank and a relatively higher level is used for making full cut paths in the dental blank.

10. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for machining with a first type of dental tool and a relatively higher level is used for machining with a second type of dental tool different than the first type of dental tool.

11. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for lubricated machining of the dental blank and a relatively higher level is used for dry machining of the dental blank.

12. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for a first revolution speed of the dental tool and a relatively higher level is used for a second revolution speed of the dental tool different than the first revolution speed.

13. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for a first type of material of the dental blank and a relatively higher level is used for a second type of material of the dental blank different than the first type of material.

14. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for a relatively low acceleration of a carriage of the dental tool and a relatively higher level is used for a relatively high acceleration of the carriage of the dental tool.

15. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for a first velocity of a carriage of the dental tool and a relatively, higher level is used for a second velocity of the carriage of the dental tool different than the first velocity.

16. The method according to claim 6, wherein the step shaped supply current (I_tr″) has at least two levels, wherein a relatively lower level is used for a low jerk in the trajectory of a carriage of the dental tool and a relatively higher level is used for a high jerk in the trajectory of the carriage of the dental tool.

17. A dental tool machining system comprising: a dental tool machine for removing material from at least one dental blank, wherein the dental tool machine has one or more stepper motors for driving a carriage that movably holds one or more dental tools and one or more stepper motors for driving a retainer which movably holds the dental blank; a control means for operating the stepper motors; wherein the control means is further adapted to operate the stepper motors in accordance with claim 1.

18. A program comprising computer readable codes for causing a computer-based dental tool machine to carry out the method steps according to claim 1.

19. A non-transitory computer readable storage medium which stores the program according to claim 18.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the subsequent description, further aspects and advantageous effects of the present invention will be described in more detail by using exemplary embodiments and referring to the drawings, wherein

[0018] FIG. 1—is a flow diagram showing a method of operating a stepper motor for use in a dental tool machine for removing material from a dental blank according to an embodiment of the present invention;

[0019] FIG. 2—shows a diagram of a predicted supply current, an enhanced supply current, and a step shaped supply current versus the dental tool trajectory according to embodiment of the present invention;

[0020] FIG. 3—is a schematic partial perspective view of a dental tool machine according to embodiment of the present invention.

[0021] The reference numbers shown in the drawings denote the elements as listed below and will be referred to in the subsequent description of the exemplary embodiments: [0022] 1. Dental tool machine [0023] 2. Dental blank [0024] 2a. Shaft [0025] 3. Dental tool [0026] 4. Carriage [0027] 4a. Arm [0028] 4b. Shaft

[0029] M_net: Net load moment

[0030] M_df: Load moment due to the drive force

[0031] M_mf: Load moment due to the machining force

[0032] I_tr: Supply current setting up the torque reserve

[0033] I_tr′: Enhanced supply current

[0034] I_tr″: Step shaped supply current

[0035] S_dt: Drive train simulation

[0036] S_mr: Material removal simulation

[0037] S_tr: Torque reserve simulation

[0038] An embodiment of a dental machining system is partly shown in FIG. 3. The dental machining system has a dental tool machine (1) for removing material from a dental blank (2). The dental tool machine (1) has two carriages (4) each movably holding a dental tool (3). The carriages (4) are arranged on opposite sides of the dental blank (2). The present invention is not limited to the use of a double carriage (4) and can be alternatively applied to a dental machining system with less or more carriages (4). The dental tools (3) are exchangeable. The user can selectively mount a dental tool (3) for milling, grinding, polishing or drilling and the like. The dental tool machine (1) has preferably two stepper motors (not shown) for driving each carriage (4). The dental machining system also has a control means (not shown) for individually operating the stepper motors, thereby, also allowing simultaneous machining of the dental blank (2). Each carriage (4) has a shaft (4b) and an arm (4a) fixed to the respective shaft (4b). The two stepper motors are linked to the respective shaft (4b) for rotating and translating the same respectively. Each shaft (4b) is rotatable around the y-direction through the respective stepper motor. Each shaft (4b) is translatable along the y-direction through the respective stepper motor. Each arm (4a) extends in the radial direction perpendicular to the y-direction. Each dental tool (3) is driven by a separate dental tool spindle motor (not shown) which is positioned on the respective arm (4a). Each arm (4a) may support one or more dental tools (3). The dental tool spindle motors can be individually controlled by the control means. The dental tools (3) are aligned parallel to the y-direction. The dental blank (2) is detachably attachable through a retainer (not shown) to a shaft (2a) rotatable about the x-direction through a stepper motor (not shown) which is also controlled by the control means. The shaft (2a) that holds the dental blank (2) is also translationally movable along the x-direction relative to the carriage (4) through a stepper motor (not shown) which is also controlled by the control means. The dental blank (2) can be moved into and out of the region between the two dental tools (3). The carriages (4) are translationally and rotationally movable relatively to each other along the y-direction and around the y-direction respectively via the stepper motors which are controlled by the control means.

[0039] The present invention provides a method of operating each of the stepper motors in the dental tool machine (1) for removing material from the dental blank (2). The control means is further adapted to individually operate the stepper motors in accordance with the method of the present invention. The present invention provides further a program which has computer readable codes for causing the computer-based dental machining system to carry out the method. The present invention further provides a computer readable storage which stores the program.

[0040] FIG. 1 shows a flow diagram of the method of operating a stepper motor in the dental tool machine (1) for removing material from the dental blank (2) according to an embodiment of the present invention. The torque reserves of the stepper motor at operating points are adapted to net load moments (M net) respectively. The net load moment is equal to the torque due to the net load acting about the rotational axis (x,y) of the stepper motor i.e., the torque vector is parallel to the rotational axis (x,y). In the present invention this is achieved without any rotary encoder-based regulation. For that reason, the method comprises a first step (S1) of predicting, through simulation, the net load moments (M_net) beforehand; a second step (S2) of predicting, through simulation, the supply current (I_tr) to be supplied to the stepper motor for setting up the torque reserves that correspond to the predicted net load moments (M_net) respectively; and a step (S3) of operating the stepper motor based on the predicted supply current (I_tr).

[0041] As shown in FIG. 1, the net load moment (M_net) corresponds to a superposition of the load moments (M_df, M_mf) which are respectively due to the drive forces arising through a drive train of the stepper motor and the machining forces arising through the material removal from the dental blank (2). In the first predicting step (S1), the load moments (M_df, M_mf) are predicted based on a drive train simulation (S_dt) and a material removal simulation (S_mr) of the dynamic acceleration/deceleration along the drive train trajectory corresponding to the movement of the dental tool (3), and the dental tool trajectory respectively. The dental tool trajectory and thus the drive train trajectory of the drive axes involved in the dental tool (3) movement are known in advance for the specific application of interest. The dental tool trajectory may also comprise one or more sections along which no material is removed. In the second predicting step (S2), the supply current (I_tr) is predicted based on a torque reserve simulation (S_tr) of the dynamic current supply. The first and second predicting steps (S1,S2) are performed in advance of the driving step (S3). For instance, the simulation to predict the supply current (I_tr) or related data is preferably performed in a computer that is external to the dental tool machine (1) to save resources. Such computer is linked to the dental tool machine (1) through a wired or wireless data communication line via a network. Alternatively, the simulation may be performed in the dental tool machine (1).

[0042] The method further comprises an optional step of generating an enhanced supply current (I_tr′) as shown in FIG. 2, by adding to the predicted supply current (I_tr) a constant amount and/or by multiplying the predicted supply current (I_tr) through a constant factor greater than one. Subsequently, the stepper motor is driven based on the enhanced supply current (I_tr″).

[0043] The method further comprises an optional step of generating, based on the predicted supply current (I_tr′), a step shaped supply current (I_tr″) as shown in FIG. 2. The step shaped supply current (I_tr″) has two or more levels. Thereafter, the stepper motor is driven based on the step shaped supply current (I_tr″). Optionally, the step shaped supply current (I_tr″) can be further smoothed by means of interpolation, morphing and/or filtering. And the stepper motor can be driven based on the smoothed step shaped supply current (I_tr″). The levels may be utilized in several different machining applications:

[0044] In an application, the relatively lower level is used for finishing the dental blank (2) and the relatively higher level is used for roughing the dental blank (2).

[0045] In another application, the relatively lower level is used for making partial cut paths in the dental blank (2) and the relatively higher level is used for making full cut paths in the dental blank (2).

[0046] In another application, the relatively lower level is used for machining with a first type of dental tool (3) and the relatively higher level is used for machining with a second type of dental tool (3) different than the first type of dental tool (3).

[0047] In another application, the relatively lower level is used for lubricated machining of the dental blank (2) and the relatively higher level is used for dry machining of the dental blank (2).

[0048] In another application, the relatively lower level is used for a first revolution speed of the dental tool (3) and the relatively higher level is used for a second revolution speed of the dental tool (3) different than the first revolution speed.

[0049] In another application, the relatively lower level is used for a first type of material of the dental blank (2) and the relatively higher level is used for a second type of material of the dental blank (2) different than the first type of material.

[0050] In another application, the relatively lower level is used for a relatively low acceleration of the carriage (4) of the dental tool (3) and the relatively higher level is used for a relatively high acceleration of the carriage (4) of the dental tool (3).

[0051] In another application, the relatively lower level is used for a first velocity of a carriage (4) of the dental tool (3) and a relatively higher level is used for a second velocity of the carriage (4) of the dental tool (3) different than the first velocity.

[0052] In another application, the relatively lower level is used for a low jerk in the trajectory of a carriage (4) of the dental tool (3) and a relatively higher level is used for a high jerk in the trajectory of the carriage (4) of the dental tool (3).