Power shift transmissions are commonly employed on agricultural and construction vehicles to allow the selection of gear ratios by selectably engaging clutches internal to the transmission. These clutches are typically fluid clutches actuated by air or a hydraulic fluid. To engage the transmission in a particular gear ratio, these clutches are selectively energized to lock gears to their respective shafts and allow the transmission of power from an engine coupled to an input shaft of the transmission, to the output shaft of the transmission. In contrast to this, a typical automobile with manual transmission engages and disengages gears mechanically, by sliding the gears or linkages between the gears to mechanically fix the gears to their respective shafts. With this kind of mechanical gear engagement, the gears are typically disengaged and spin freely, or they are completely engaged and spin at the same speed as the shaft on which they are mounted. Given this sudden engagement, it is necessary to disengage the transmission from the engine prior to changing gear ratios. This is typically performed by a clutch external to the transmission. In a power shift transmission, in which a plurality of gears are engaged by fluid clutches, a certain gradual engagement can be provided by gradually filling the clutches.
The engagement of all the clutches necessary to engage the transmission in a particular gear ratio must be carefully timed. As a power shift transmission changes from one gear ratio to another, there is a risk that clutch engagement may overlap, in which case the transmission may be simultaneously engaged in two incompatible gear ratio, resulting in severe damage to the transmission. Alternatively, if the first clutches necessary to engage the transmission in the first gear ratio are disengaged significantly prior to the time that the second combination of clutches to engage the transmission in the second gear ratio are engaged, the drive wheels of the vehicle will be momentarily disengaged from the engine. The result is that the vehicle may suddenly stop. Given the high loads on an agricultural vehicle, any significant delay between disengagement and reengagement will cause a noticeable and undesirable "jerk."
To provide for the accurate timing of transmission shifts, the clutches must be periodically calibrated. The calibration may compensate for such factors as changes in hydraulic fluid supply pressure, wear in the clutches themselves, changes in electrical supply voltage, manufacturing tolerances and general aging of the vehicle. To accurately time transmission shifts, the microcontroller regulating the clutch engagements and disengagements must be able to determine the precise moment of clutch engagement. This is particularly a problem for clutches that are controlled by proportional control valves. Proportional control valves are so named because an input signal may be applied to them (typically an electrical signal) that is theoretically proportional to the output signal (typically hydraulic pressure). The arrangement and connection of such valves to hydraulic clutches is known to those skilled in the art. Generally speaking, to engage a clutch connected to a proportional control valve, a gradually increasing (or decreasing, depending upon the valve arrangement) electrical signal is applied to a coil on the valve. This signal applied to the coil causes the coil to generate a magnetic field proportionate to the signal. This proportionate magnetic field, in turn, causes a proportional movement of a valve spool in the valve body, which in turn causes a proportional increase in fluid pressure provided at the outlet of the valve. The valve outlet is hydraulically coupled to the fluid clutch, which in turn experiences a proportional change in its internal hydraulic pressure. The change in clutch hydraulic pressure causes a corresponding proportional change in clutch engagement. Thus, by varying the electrical signal applied to the proportional control valve, one can vary the degree of engagement of the fluid clutch. As mentioned above, wear and age can affect the calibration of the clutch. In general terms, the clutch becomes uncalibrated when a particular electric signal is applied to the valve, the same degree of clutch engagement is no longer provided by the hydraulic clutch. To calibrate a proportional valve, therefore, means re-establishing a known relationship between the electrical signal applied to the valve coil, and the degree of clutch engagement.
Several solutions have been proposed to the problem of clutch calibration. In one prior art embodiment, the output clutches are the final drive clutches in the transmission, the last clutch in the transmission's internal drive chain. In general, the method employed involves fixing the output shaft of the transmission by applying the vehicle brakes, engaging all the clutches necessary to place the transmission in one of its gear ratios (except the output clutch to be is calibrated), measuring the speed of the engine shaft, then gradually incrementing the pressure in the hydraulic clutch. Since the engine is running, and since all of the clutches necessary to engage the transmission in a particular gear ratio are engaged, the input side of the output clutch being calibrated is spinning. Since the vehicle brakes have been applied preventing the output shaft from rotating, and since the clutch is an output clutch, meaning the output of the clutch always rotates at the speed of the output shaft, the output of the clutch is not rotating. Since the input of the clutch is rotating and the output of the clutch is not rotating, when the clutch reaches the point of incipient engagement, and therefore the clutch begins to carry a torque, a torque will be applied to the engine and the engine speed will drop. A torque will be transmitted from the vehicle brakes through the output shaft of the transmission through the output clutch being calibrated through the other gears in the transmission's drive train and finally to the engine. Once the engine speed drops a predetermined amount, the clutch is deemed to be calibrated, and the microprocessor controlling the clutches and performing the calibration will then store a value corresponding to the hydraulic pressure in the clutch. An alternative method of determining the point of incipient engagement is not to lock the output shaft preventing it from rotating by applying the vehicle's brakes, but to release the brakes. The result is that the engine's power at the point of incipient engagement will be transmitted through the transmission and to the vehicle's drive wheels, at which point the vehicle will move or lurch. This movement is sensed by a rotation sensor disposed on the output shaft of the transmission. Alternatively, brakes internal to the transmission may be applied to place a load on the engine. In either case, a value corresponding to hydraulic pressure is recorded and saved for future reference by the microprocessor.
There are several drawbacks to these and similar calibration methods, however. First, the operator must either manually apply the vehicle brakes or locate the vehicle to prevent an unexpected lurch during calibration. Second, since the engine speed is monitored, an engine speed sensor must be provided. Third, the calibration method relies on a predetermined change in engine speed, and predetermined changes in engine speed are not proportional to a predetermined change in load. Therefore, the calibration point determined by the procedure may vary depending upon the base or reference speed of the engine during the calibration process.
Accordingly, it would be desirable to provide a method of calibration that is fully automated and more accurate than those provided in the above references.