MOTOR CONTROL CIRCUIT FOR A SIMULATED WEIGHT STACK

Abstract: A control for a DC motor coupled to a user interface to provide a counterforce which simulates a weight stack in an exercise machine. The control includes a position transducer coupled to the motor to provide a signal proportional to the displacement of the user interface. A circuit is provided to generate a control voltage which follows the displacement signal when it is less than a maximum torque signal, the con­ trol voltage following the maximum torque signal when it is less than the displacement signal. The control voltage is used to soft start the motor such that the counterforce provided by the motor simulates a weight stack supported by a spring over small displacements of the user interface.

Description: MOTOR CONTROL CIRCUIT FOF A SIK1LATED WEIGHS STACK Technical Field The present invention relates to a control for a DC motor coupled to a user interface to simulate a weight stack in an exercise machine and more particularly to a control for a DC motor simulating a weight stack initially supported by a spring over small displacements of the user interface. Background of the Invention Exercise chines are known in which various devices are used to simulate a weight stack to provide a counterforce to a user of the machine. One known exercise machine employs a hydraulic cylinder to apply a counterforce to an exercise bar engaged by the user wherein a computer controls the force applied by the hydraulic cylinder. Another knouts type of exercise machine employs an alternator or generator coupled to an electrical load resistor to apply a counterforce to the user. In this type of machine, as the rotational velocity of the alternator or generator increases, so does the counterforce felt by the user. These knows types of exercise machines are very costly and do not adequately simulate ç weight stack. To provide an economical exercise machine, it is desirable to use a DC motor to provide a counterforce which simulates a weight stack. Attempts have been made to control a DC motor to apply maximum force against the user as soon as the user interface or exercise bar is displaced. Such attempts have included applying a maximum control voltage to the DC motor immediately upon sensing displacement of the user interface. Bowever, because the control voltage goes from zero to a maximum value almost instantaneously, such controls are very unstable. Suirimary of the Invention In accordance with the present invention, the disadvantages of prior exercise machines as discussed above have been overcome The exercise machine of the present invention includes a DC motor for providing a counterforce to a user interstate suc as an exercise bar and a control for soft starting the DC motor so that the motor simulates G weight stack initially supported by a spring over small displacements of the user interface, The control includes a position transducer for sensing the displacement of the rotor or armature of the DC motor and thus the displacement of the user interface to provide a voltage proportional thereto. The control also includes a potentiometer for providing a maximum torgue voltage which represents the maximum counterforce to be provided by the DC motor te simulate the total weight of the weight stack. k circuit is provided which forms a control voltage froir. the minimum of the displacement voltage and the maximum torquf voltage such that the control voltage increases in proportion with the displacement voltage to the level of the maxirur torgue voltage. The control voltage is used to generate an armature current for the DC motor, the armature current being proportional to the control voltage in order to soft start the DC motor and provide a counterforce which gradually increases to a maximum value when the user interface is initially displaced. These and other objects and advantages of the invention, as well as details of an illustrative embodiment, will be more fully understooa from the following description and from the drawings Brief Description of the Drawings FIG. 1 is a schematic diagram of the DC motor control circuit for an exercise machine according to the present invention; and FIG. 2 is a graph illustrating how the control voltage generated for the DC motor of FIG. 1 varies as a function of displacement of the user interface. Best Mode For Carrying Out The Invention The exercise machine of the present invention, as shown in FIG. 1, includes G user interface 10 such as an exercise bar or the like which is engaged by a user of the machine to exert a force. The user interface 10 is coupled to G DC motor 12 which exerts a counterforce against the user. The DC motor 12 is controlled by the remaining portion of the circuit shown in FIG. 1 to provide a counterforce which simulates a weight stack initially supported by a spring over very small displacements of the user interface. A position transducer 14 is provided to sense the position of the armature or rotor 16 of the DC motor 12. The position transducer 14 provides a voltage, VD, as shown in FIG. 2, which is proportional to the displacement of the rotor 16 and thus proportional to the displacement of the user interface 10. The displacement voltage VD is coupled to the non-inverting input terminal 1E of an operational amplifier 20, which is configured as a voltage follower. The output of the operational amplifier 20 is coupled to the cathode of a diode 22, the anode of which is coupled to +15 volts through a 1k D resistor 24. A potentiometer 26, which is coupled to +15 volts, provides a maximum torque voltage, Vmax, which represents the maximun counterforce to be provided by the DC motor 12 to simulate the total weight of the weight stack. By varying the setting of the potentiometer 26, the maximum counterforce or total weight of the stack simulated by the DC motor 12 can be changed. The maximum torquf voltage, Vmax, is applied to the non-inverting input terinal 28 of an operational amplifier 30 which is also configured as a voltage follower. The output of the operational amplifier 30 is coupled to the cathode of a diode 32, the anode of which is coupled to +15 volts through the resistor 24 to control the miximum value of a control voltage, VC, generated at node 34. The control voltage VC follows the lessor of the displacement voltage VD and the maximum torque voltage Vmax so that it increases gradually in proportion with VD to the level of max The control voltage VC is coupled to a plurality of power blocks through a voltage divider 40 comprised of a g.1k # resistor 42 and ç 910 n resistor 44. For simplicity only two power blocks 36 and 38 out of a plurality of power blocks, numbering on the order of ten for example, are shown in FIG. 1. The power blocks are connected in parallel and coupled to the DC motor 12 to control the armature current, ia, of the motor. Each power block, as shown for block 36, includes an operational amplifier 46 having non-inverting input terminal to which is applied the reduced control voltage from the voltage divider 40. The output of each operational amplifier is coupled to the base of a transistor 48, the emitter of which is connected to ground through a .1 n resistor 50. The transistor 48 may be a 710 amp, 760 watt Darlington transistor or a power FET. When current from the operational amplifier 46 is applied to the base of the transistor 48, a collector current ic is provided which is proportional tc the control voltage Vo The armature current i of the DC motor 12 is equal to the suJr of the collector currents ic from each of the power blocks 36 through 38. That is, ia = nic where n is equal to the number of power blocks. The armature current i is proportional to the control voltage VC so that it increases gradually over small displacements of the user interface 10 to soft start the DC motor 12. When ° user initially engages the interface 10 to exert e force against the force provided by the DC motor 12, the position transducer generates a voltage SD, as shown in FIG. 2 which gradually increases as ç function of the dis- placement of the user interface. When VD is less than the maximun torgue voltage Vmax, the control voltage VC applied to the voltage divider 40 follows VD. The power blocks 36 through 3E, in turn generate an armature current which gradually increases in proportion to VD when VD < Vmax to soft start the DC motor 12. During this period the DC motor 12 provides fi counterforce which simulates ç weight stack initially supported by a spring. When the user interface 10 is displaced about one-half of an inc (1.2cm) as shown in FIG. 2, the displacement voltage, VD, eguals the maximum torgue voltage, Vmax. As the user interface 10 is displaced more, VD rises above Vmax Because V@ follows the minimur of VD and Vmax, when the user interface is displaced onehalf of an inch (1.2cm) or more, VC remains egual to The constant maximum torgue voltage, Vmax. The power blokes respond to the maximum control voltage to provide the maximum armature current to the DC motor 12 to cause the motor to provide a counterforce simulating the total weight of the stack without the support of a spring. Because the DC motor is soft started, the control circuit shown in FIG. 1 is very stable as well as economical.

Claims: CLAIMS: 1. An exercise machine having a user interface (10) engaged by the user to exert a force, and a DC motor (12) coupled to the user interface (10) for exerting against the user a counterforce simulating a weight stack, CHARACTERISED IN THAT the exercise machine further comprises a control means for the DC motor comprising: means (14) for sensing the displacement of the user interface (10) to provide a displacement signal (VD) proportional thereto; and means (20,30) responsive to the displacement signal (VD) for soft starting the DC motor (12) to cause the motor to provide a counterforce which simulates a weight stack that is initially supported by a spring. 2. An exercise machine according to claim 1, wherein the soft start means (20,30) includes means for providing to the motor G control signal (VD) which increases to a maximun value in proportion to the displacement signal t when the user interface (10) is initially displaced to cause the DC motor to provide a counterforce which increases gradually to a maximum value over short displacements of the user interface. 3. An exercise machine having a user interface (10) engaged by the user to exert a force, a DC motor (12) coupled to the user interface (10) for exerting a counterforce against the user, and control means for controlling the DC motor, CHARACTERISED IN THAT The control means comprises: means (14) for sensing the displacement of the user interface (10) to provide a displacement signal (VD) proportional thereto; means (20,30) responsive to the displacement signal (VD) for providing a control voltage (VD) which increases to a maximum value (V max) in proportion to the displacement signal; and means coupling the control voltage (vac) to the DC motor (12) to control the motor to exert a counterforce which increases progressively to a maximum value over short displacements of the user interface. 4. An exercise machine according to claim 3, wherein the control voltage providing means (20,30) includes: a first voltage follower (20), to an input (18) of which is applied the displacement signal (VD) to provide voltage which is proportional to the displacement of the user interface; a second voltage follower (30), to an input (28) of which is applied a signal (V max) representing the maximum counterforce to be exerted by said DC motor; and means (40) coupled to the outputs of the first and second voltage followers for selecting the minimum voltage output from the followers as said control voltage. 5. An exercise machine having a user interface (10) engagable by the user to exert a force, and means (12) for exerting a counterforce against the user, CHARACTERISED IN THAT the counterforce exerting means (12) comprises: a DC motor (12) coupled to the user interface (12) to exert a counterforce against the user, which motor has an armature (16) the displacement of which represents the displacement of the user interface; means (14) for sensing the displacement of the armature (16) to provide a displacement signal (VD) proportional thereto; ; and means (20,30) responsive to the displacement signal and coupled to the DC motor (12) for providing an armature current (ia) which increases to a maximum value in proportion to the displacement signal (VD) and coupled to the DC motor (12) for providing an armature current (ia) which increases to a maximum value in proportion to the displacement signal (VD) to control the motor to provide a counter force which is proportional to the armature current.

Inventors: KEANE,MARTIN A.
Applicants: BALLY MANUFACTURING CORPORATION
Application year: 1986
Main classification: A63B21
IPC codes: A63B24, G05D15, A63B21