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Components of the DCT Components of the DCT

As mentioned previously, a DCT mechanically consists of 2 manual transmissions that interact very smartly through the use of electricals and electronics. So, if you're not familiar with how manual transmissions work, it would be useful to read up on that first:



This gearbox combines standard gearbox mechanicals, electro-mechanical actuators and control electronics. We'll now take a run through schematics that help build up these components into the overall gearbox. These schematics are meant to illustrate the concept and are not accurate in terms of dimensions - so please excuse any oddities (e.g. 6th gears not touching!)



Dual Clutches and Dual Input Shafts

Firstly, we look at the torque input side, namely the dual clutches, and the shafts they are connected to.

There are 2 input shafts, one of which is hollow (blue) and the other is solid (yellow) and sits coaxially within the hollow shaft.

The inner shaft (yellow) has fixed gears for gears 1, 3 and 5; while the outer shaft (blue) has fixed gears for 2, 4, 6 and reverse. Note that this shaft has only 2 physical gears, each of which are used for 2 gear positions.

Each of these shafts are connected to a clutch via splines on the outside of the shaft.

This arrangement allows for compact packaging of both clutches.

Unlike other clutches seen in manual transmissions, in the normal resting state the clutch is held open by springs (i.e. it won't transmit torque), and needs to be actuated to close, and are held closed by a holding current applied to the actuator.

The transmission electronics ensures that only one clutch is closed at any time.

Output Shafts

The gearbox has two output shafts (shown in cyan/pale blue). Contrary to initial thoughts, these don't carry gears corresponding to the input shafts. Instead, the gears they carry are determined by the order of the selector forks.

The gears on the output shafts are not fixed, but are free-wheeling. Like a manual transmission, they are equipped with synchronizers to match speeds and lock the gears.

Gears 1, 3,4, 5, 6 and reverse are equipped with a single synchronizer, while gear 2 is double synchronized.

The second gear is coupled to the reverse gear on the same shaft (although both can free-wheel when not engaged, they do so together).

Note that the orange reverse gears on both output shafts are engaged directly with one-another. However, they do not engage with either the yellow or blue input shafts.

As a result, the output shafts and input shafts are not in the same plane - instead they're arranged in a triangular formation (see the end-on view on the right of the above image).

Differential

Both output shafts transfer torque via the output pinion to a common differential shaft (shown in green).

This differential shaft is not in the same plane as the output shafts, it is again offset - the 4 shafts are arranged in a parallelogram formation (see the end-on view in the above image).

The differential serves the same purpose as a MT-equipped car - it allows each of the driven wheels to rotate at different speeds (e.g. when turning).

Synchronizer Sleeves & Selector Forks

When discussing the output shafts, I mentioned that none of the gears are fixed to the shafts, but are instead free-wheeling.

There are 4 synchronizer sleeves (and associated assemblies) that allow these free-wheeling gears to match the speed of the output shaft, and to lock the gears. 3 of these sleeves are used to engage two gears (at separate times), while 1 sleeve is used for just a single gear.

Each of these synchronizer sleeves have a corresponding selector fork, that can move the sleeve to either side (to lock a gear) or to the middle (to unlock a gear).

Until this point, the components we've seen are all familiar, since they closely resemble manual gearboxes - rather, two gearboxes, since we have two clutches, two input shafts and two output shafts. It is only at the differential that both these units are unified into a single output. From this point onwards is where the 'magic' of a DCT happens.



Shift Actuators



Image copyright Ford Motor Company While we'll come to the TCM itself in more detail later.

For now we need to focus on the two electric motors present in the TCM, as they provide rotational output from the TCM, to actuate the selector forks.

The motors are of a brushless DC design. They have Hall sensors built in to determine the position of the rotor, and count the number of rotations it has gone through.

Via a system of spur gears, these rotate selector drums through a specific angle (the range of travel for these drums is 200 - 290 degrees).

The selector drums have a slot cut in them. The selector forks have a tongue that sits in this slot.

The slot is angled towards the ends of the travel, so that when the selector drum rotates, the tongue is forced perpendicular to the direction of rotation (i.e. parallel to the axis of the selector drum). If this is confusing to understand, imagine how a screw converts the rotational motion of the screwdriver into forward motion.

By this, a rotational motion produced by the electric motors can be converted into front-to-back movement of the selector forks. This allows the selector forks to move the synchronizer sleeves forward or backward to lock and unlock specific gears.

motion produced by the electric motors can be converted into of the selector forks. This allows the selector forks to move the synchronizer sleeves forward or backward to lock and unlock specific gears. In comparison, in a manual transmission the selector forks are manually controlled by the linkages to the gear lever.

Clutch Actuators

Similar to the shift actuator, the clutch actuator converts the motion of an electric motor into lateral motion.

Once again, a brushless DC motor is used.

As mentioned previously, the clutch is held open by spring pressure, by default, and doesn't transmit torque.

To close the clutch, the motor rotates a worm gear, that pushes the clutch actuator.

To keep the clutch closed, a holding current is applied to the motor.

The following 2 animated images is a representative view of how each of the clutches are actuated. For info, these are clutches in a VW DSG, but the principle is the same.



Image credit autohome.com.cn

Transmission Control Module (TCM)

Until now, we've seen the components that go into the plain gearbox, and we've seen the actuating components (i.e. the muscles) that move things in the gearbox. We now come to the TCM, which is the brain of the DCT.

In the image for the shift actuators, you've seen the pink unit described as the TCM. Here's a view of the other side of it, which has input connectors from the ECU. The side opposite to this has the output of the 2 motors that we'd seen earlier.

The TCM collects input signals from various sensors, assess the input, and control the actuators accordingly. The principle is simple, the execution is complex!

Input used by the TCM includes: Transmission range (P/R/N/D/S/L, etc.)

Vehicle speed

Engine speed and engine torque

Throttle position

Engine temperature

Ambient temperature (to decide how viscous the transmission oil is, for cold starts)

Steering angle (to avoid upshifts or downshifts during cornering)

Brake inputs

Input shaft speed (for both input shafts)

Vehicle attitude (tilt) from the body control module (BCM)

The TCM controls the actuator motors via an open-loop control, to allow adaptive control. This allows the TCM to identify and adapt to the following: Clutch engagement points (F1 fans will have heard of the 'clutch bite point')

Clutch friction coefficient

Position of each synchronizer assembly Information for the above are stored in non-volatile RAM in the TCM. This is what constitutes the learnt driving patterns for a particular gearbox.



In a later section, we will look at the various driving modes and how the TCM determines the appropriate course of action for them.



Sensors

There are multiple sensors that gather and provide information to the TCM, both from within the DCT, and elsewhere within the vehicle. Those associated with the DCT itself are: Input shaft speed sensor (ISS sensor) - a magneto-resistive sensor - one per input shaft

Output shaft speed sensor (OSS sensor) - again, a magneto-resistive sensor - one sensor, attached to the differential

Transmission range sensor (TR sensor) - to detect the position of the selector lever, and convert it to a PWM signal

Cutaway

Thus far, we've been looking mostly at simplified schematics. Here's a 3-D cutaway of the real thing:





Images copyright Ford Motor Company As mentioned previously, a DCT mechanically consists of 2 manual transmissions that interact very smartly through the use of electricals and electronics. So, if you're not familiar with how manual transmissions work, it would be useful to read up on that first: http://auto.howstuffworks.com/transmission2.htm This gearbox combines standard gearbox mechanicals, electro-mechanical actuators and control electronics. We'll now take a run through schematics that help build up these components into the overall gearbox. These schematics are meant to illustrate the concept and are not accurate in terms of dimensions - so please excuse any oddities (e.g. 6th gears not touching!)Firstly, we look at the torque input side, namely the dual clutches, and the shafts they are connected to.Until this point, the components we've seen are all familiar, since they closely resemble manual gearboxes - rather,gearboxes, since we have two clutches, two input shafts and two output shafts. It is only at the differential that both these units are unified into a single output. From this point onwards is where the 'magic' of a DCT happens.Until now, we've seen the components that go into the plain gearbox, and we've seen the actuating components (i.e. the muscles) that move things in the gearbox. We now come to the TCM, which is the brain of the DCT.In the image for the shift actuators, you've seen the pink unit described as the TCM. Here's a view of the other side of it, which has input connectors from the ECU. The side opposite to this has the output of the 2 motors that we'd seen earlier.Input used by the TCM includes:The TCM controls the actuator motors via an open-loop control, to allow adaptive control. This allows the TCM to identify and adapt to the following:Information for the above are stored in non-volatile RAM in the TCM. This is what constitutes the learnt driving patterns for a particular gearbox.In a later section, we will look at the various driving modes and how the TCM determines the appropriate course of action for them.There are multiple sensors that gather and provide information to the TCM, both from within the DCT, and elsewhere within the vehicle. Those associated with the DCT itself are:Thus far, we've been looking mostly at simplified schematics. Here's a 3-D cutaway of the real thing: Last edited by arunphilip : 28th September 2015 at 08:08 .