CLIAMS:I claim:
1. A device to control a Continuously Variable Transmission (CVT), said device comprising:
an actuator operatively coupled to a movable sheave of a secondary pulley of said CVT;
a memory element adapted to store desired engine speed corresponding to Brake Specific Fuel Consumption (BSFC) for at least one operating condition, said memory element further adapted to store belt efficiency corresponding to at least one said operating condition;
a control unit adapted to control said actuator to adjust the position of said movable sheave of said CVT, said control unit adapted to measure an actual engine speed from at least one engine speed sensor, said control unit adapted to retrieve a desired engine speed from said memory element and determine a difference between said desired engine speed and said actual engine speed, said control unit further adapted to compute a required clamping force of said belt based on said difference in desired and actual engine speed and said belt efficiency, and control the movable sheave of said secondary pulley based on said required clamping force.

2. The device as claimed in claim 1, wherein said actuator is an electromechanical actuator.

3. The device as claimed in claim 1, wherein said actuator is adapted to pull and/or push said movable sheave of secondary pulley to achieve an optimal gear ratio.

4. The device as claimed in claim 1, wherein said at least one operating condition is selected from a group comprising a speed ratio, a belt tension, a rotational speed, an external load, a diameter of the pulley, a throttle position, a surface gradient and the like.

5. The device as claimed in claim 1, wherein said at least one engine speed sensor is selected from a group comprising a flywheel tooth sensor, an alternator based sensor, an inductive sensor and the like.

6. The device as claimed in claim 1, wherein said required clamping force is adapted to determine an actuation force to control said actuator.

7. A method of controlling a Continuously Variable Transmission (CVT), said method comprising:
measuring an actual speed of an engine from at least one engine speed sensor;
retrieving a desired speed of said engine from a memory element based on BSFC for at least one operating condition;
determining a difference of said desired engine speed and said actual engine speed;
retrieving a belt efficiency from said memory element for said operating condition;
computing a required clamping force of said belt from said difference in desired and actual engine speed and said belt efficiency; and
controlling an actuator based on said required clamping force of said belt to achieve a required gear ratio.

8. The method as claimed in claim 7, wherein said required clamping force is adapted to determine an actuation force to control said actuator.

9. The method as claimed in claim 7, wherein said required clamping force is applied to said actuator for maintaining an optimal engine speed for said at least one operating condition ,TagSPECI:Complete specification: The following specification particularly describes the invention and the manner in which it is to be performed.

[001] Field of the invention:
[002] The present disclosure relates to a device to control a continuously variable transmission (CVT).
[003] Background of the invention:
[004] A Variable Diameter Pulley (VDP)-CVT comprises a primary pulley, a secondary pulley and a V-belt. The primary pulley comprises a fixed and a movable sheave coupled to the engine crankshaft and a variator (which is a circular disc with rollers having pre-defined weight arranged around the centre of the shaft). The rollers start moving outwards in the variator with increasing engine speed which moves the movable sheave towards the fixed pulley sheave centrifugally. The secondary pulley comprises a fixed and a movable sheave coupled to the driven side shaft to the wheels, the compression spring normally called contra spring (with pre defined stiffness) holds the movable sheave from moving away from the secondary pulley fixed sheave, there is a centrifugal clutch which also opens and connects the drive to the wheels at a predefined speed. A V-Belt or a wedge shaped belt is looped around the two sheaves between primary pulleys and connects the two sheaves of a secondary pulley. The V-Belt is the main source of transmission.
[005] Mechanical Variable diameter Pulley (VDP) for a CVT is designed to maintain the engine speed constant at optimum zone by driving the driven element at all possible effective CVT ratio. But, the engine speed in the VDP-CVT drops from the optimum zone because the mechanical CVT does not have any feedback on the operating condition. Design of CVT components and its mechanical ability (e.g. roller weights, stiffness of the spring) are best suited and tuned for flat road condition and the CVT becomes in-efficient at other operating conditions like cruising at higher vehicle speed and suddenly approaching uphill condition, during which load on the wheels is not sensed properly and the engine speed drops because no transition to lower gears. Another in-efficient operating condition comprises but not limited to braking to lower vehicle speed and sudden opening of throttle yield to poor acceleration because the CVT ratio is still in higher gear ratio and lack of torque.
[006] Brief description of the accompanying drawings:
[007] An embodiment of the disclosure is described with reference to the following accompanying drawings,
[008] Fig. 1 illustrates a schematic view of a device to control a CVT, according to an embodiment of the present disclosure, and
[009] Fig. 2 illustrates a flow diagram of a method for controlling the CVT, according to an embodiment of the present disclosure.
[0010] Detailed description of the embodiments:
[0011] Fig. 1 illustrates a schematic view of a device to control a CVT, according to an embodiment of the present disclosure. The device comprises an actuator 112 operatively coupled to a movable sheave 110 of a secondary pulley of the CVT. A fixed sheave 108 of the secondary pulley is mounted to a suitable stationary part of a vehicle. The actuator 112 is an electromechanical, electromagnetic actuator such as but not limited to an electromagnetic clutch. The device is associated with a memory element 120 adapted to store desired engine speed corresponding to Brake Specific Fuel Consumption (BSFC) for at least one operating condition. The memory element 120 is further adapted to store belt efficiency corresponding to the at least one operating condition. The memory element 120 is either an internal (built-in) or external. The device further comprises a control unit 118 adapted to control the actuator 112 to adjust the position of the movable sheave 110 of the CVT. Before controlling the actuator 112, the control unit 118 is adapted to measure an actual engine speed from at least one engine speed sensor (not shown in the Fig. 1), and is further adapted to retrieve a desired engine speed from the memory element 120 and determine a difference between the desired engine speed and the actual engine speed. Furthermore, the control unit 118 is adapted to compute a required clamping force of the belt 106 based on the difference in desired and actual engine speed and the belt efficiency. The computed required clamping force is adapted by the control unit 118 to control the movement of the movable sheave 110 of the secondary pulley. The required clamping force is adapted to determine an actuation force to control the actuator 112. A clutch 116 such as but not limited to a centrifugal clutch is shown to impart the torque to the wheels of the vehicle.
[0012] In accordance to an embodiment of the present disclosure, the actuator 112 is operatively coupled to a movable sheave 104 of a primary pulley and controls the movable sheave 104 to maintain optimal engine speed based on BSFC for at least one operating condition. The fixed sheave 102 of the primary pulley is mounted to a stationary part of the vehicle.
[0013] In accordance to an embodiment of the present disclosure, the belt 106 between the primary pulley and the secondary pulley is a metallic or a non-metallic belt 106 such as a rubber.
[0014] In accordance to an embodiment of the present disclosure, the at least one operating condition is selected from a group comprising but not limited to a speed ratio, a belt tension, a rotational speed, an external load, a diameter of the pulley, a throttle position, a surface gradient and the like.
[0015] In accordance to an embodiment of the present disclosure, the at least one engine speed sensor is selected from a group comprising but not limited to a flywheel tooth sensor, an alternator based sensor, an inductance based sensor and the like.
[0016] In accordance to an embodiment of the present disclosure, the actuator 112 is adapted to pull and/or push the movable sheave 110 of the secondary pulley to achieve an optimal gear ratio. The belt 106 connecting primary pulley and secondary pulley does not slip as the force on the primary pulley is balanced by controlling the movable sheave 110 of the secondary pulley. Instead of individual control of primary and secondary pulleys, a suitable additional correction force is provided by the control unit 118 for maintaining the secondary pulley at efficient gear ratio. The clamping force on the secondary pulley by electro mechanical actuator 112 is corrected.
[0017] In accordance to an embodiment of the present disclosure, the actuator 112 provides an assisting force at the secondary pulley. By connecting the actuator 112 at the secondary pulley less assisting force is provided to change gear ratios towards overdrive at part load conditions, i.e. provides gain over belt efficiency. Also, the rate of acceleration at part load conditions is improved, since the assisting force is directly on the secondary side.
[0018] In accordance to an embodiment of the present disclosure, the device is retrofit to secondary side of existing mechanical VDP-CVT. The electromagnetic actuator 112 is operated only when the pulley position is required to be corrected. Thus the belt 106 position on the pulley is corrected, thereby enabling optimum CVT ratio to maintain engine speed. The device is housed inside a spring 114 such as contra spring and the actuator 112 is controlled based on the clamping force required. Alternately, the device is housed outside the spring 114.
[0019] In accordance to an embodiment of the present disclosure, the disclosed device is able to extract full efficiency of CVT at part load conditions by controlling the belt slip. The part load conditions refers to low torque conditions. The device holds the engine speed lower to achieve best BSFC points for a corresponding at least one operating condition such as a throttle position. The device ensures optimum or required belt 106 tension for transfer of part throttle power. The device improves fuel consumption by controlling engine speed and CVT gear ratio and rate of change of gear ratio. Thus, a belt 106 such as but not limited to a rubber belt, which is very inefficient at part load conditions are optimally controlled.
[0020] In accordance to an embodiment of the present disclosure, the disclosed device is able to control the drivability and improve fuel efficiency of the vehicle in full load conditions. Further, the device provides improved pick-up/ acceleration and enough traction force on the wheels by delivering high values of engine torque. The device also maintains speed in an optimum zone where the engine power balances the resistance of the scooter motion on the driving surface.
[0021] Fig. 2 illustrates a flow diagram of a method for controlling the CVT, according to an embodiment of the present disclosure. The method comprises the steps of measuring an actual speed of the engine from at least one engine speed sensor. The existing engine speed sensor or an external engine speed sensor is used (202). The method further comprises retrieving a desired speed of the engine from a memory element based on BSFC for at least one operating condition (204). A control unit is adapted to determine a difference of the desired engine speed and the actual engine speed (206). Further, belt efficiency from the memory element is retrieved for at least one operating condition (208). The control unit is again adapted to compute a required clamping force of the belt from the determined difference between the desired and actual engine seed along with the belt efficiency (210). After computing the required clamping force, the same is adapted for controlling an actuator connected to the movable sheave of the CVT to achieve the required optimal gear ratio for the at least one operating condition (212).
[0022] In accordance to an embodiment of the present disclosure, the required clamping force is adapted to determine an actuating force to control the actuator. The actuation force is obtained by controlling the electric current to the electromagnetic clutch actuator.
[0023] In accordance to an embodiment of the present disclosure, the required clamping force is applied to the actuator for maintaining an optimal engine speed for the at least one operating condition.
[0024] According to an embodiment of the present disclosure, the device is able to maintain the engine at a defined engine speed zone for better performance and fuel consumption across the transmission ratio of the CVT. The device overcomes a drop in efficiency at part load conditions by just correcting the gear ratio to improve belt efficiency. The control unit controls the CVT for reaching the optimum gear ratio where the belt efficiency is highest for at least one operating condition without any compromise on the drivability. The electro mechanical linear actuator is controlled in a manner to provide force required to shift the sheaves in the secondary pulley at reasonable speed and accuracy. Also, at least one limit switch is provided for the actuator to not overdrive the sheaves. The control mechanism employed is the force required to shift the sheaves in the secondary pulley versus percentage of duty cycle to be realized, up-shifting and down-shifting is done with desired engine speed as input.
[0025] It should be understood that embodiments explained in the description above are only illustrative and do not limit the scope of this invention. Many such embodiments and other modifications and changes in the embodiment explained in the description are envisaged. The scope of the invention is only limited by the scope of the claims.