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Closed loop - what is it, and why is it necessary?

The term 'closed-loop' refers to a control system that uses an electronic control unit in conjunction with feedback sensors to modify its own operation in response to the sensor inputs - in real-time. In the context of semi-automatic gear selection, the closed-loop system monitors the gear position sensor to determine if and when a shift has been successfully completed. This information is fed back to the gearbox control unit (GCU) so that the pneumatic actuator, engine torque reduction or throttle blip can be turned on and off as necessary to effect the fastest and most reliable gear shifts.

By contrast, an 'open-loop' system does not have any feedback mechanism and simply relies on fixed time delays for engine cut, throttle blip and shift actuator operation. An open-loop system applies the same engine cut or throttle blip duration for every shift, regardless of whether it is the correct duration or not...

Closed-loop gearshift technology has been around for many years and the correct strategies have been well understood for some time by ourselves and a few others that are operating at the higher end of the motorsport market. These highly developed control strategies are certainly not slow or out-dated, as claimed by one of our rivals, and they provide 100% shift reliability and optimum shift speed.

To get a better understanding of why closed loop control is necessary, we must first examine the basic mechanics of a gear shift using dog engagement. However, I'm not going to go into exact details here, as it will give the game away to some of our rivals who have yet to gain a full understanding of the complexities of sequential gear shifting.

Any gear shift, whether it's an up-shift or a down-shift, is a 2 stage event. First we must successfully disengage the current gear, then we must engage the next gear. Both these events take a varying length of time to execute depending upon various dynamic factors acting upon the vehicle at the time. Disengaging the current gear is the most challenging aspect of the shift. This is due to a phenomenon known as transmission wind-up. On up-shifts, this wind-up has a tendency to keep the dogs locked together for a period of time after the engine power has been cut. A similar thing happens (but in reverse) on down-shifts during the throttle blip period. The time it takes for the dogs to unlock depends upon several complex factors which are impossible to model or predict, suffice to say that the strategy to successfully get out of the current gear and engage the next one as quickly as possible is not just a case of cutting the engine or blipping the throttle for a few milliseconds and hoping for the best!

So, we now know that the time it takes for the shift to complete can vary, depending upon various factors. What you might not appreciate is how dramatic that variation can be. The results of our datalogging show that in the ideal situation of a clean engagement with no transmission wind-up, the shift time is basically as fast as the pneumatic actuator can move the gear lever. On most gearboxes, this is in the order of 15-25ms. This is the typical time that would be measured 'on the bench' with no load. However, in the real world with the gearbox transmitting torque, the shift time can increase dramatically. Typically we would see upshift times ranging from about 35ms to as long as 150ms or more at the upper end.

Now, if you use a timer based (open-loop) shift system, how long do you set the cut time? Consider for yourself what the implications are if the timer expires and the engine power is resumed before the next gear has engaged... You can see some of our customer data logs using our closed-loop system here and here, which clearly demonstrate the necessity for variable cut times.

While on the subject of shift times, we have read claims on one of our rivals websites (and rather amusingly, blatantly copied on another) of performance gains of 1/5 of a second per gear shift. Compared with what, exactly? 1/5 of a second is 200ms, which is actually longer than a decent driver will take to change gear using a stick! So where does the saving come from? Of course there are no such like for like savings. Even if the shift time was reduced from 200ms to zero, there would not be a corresponding track time saving of 200ms because the vehicle still has forward velocity during the shift. The only saving comes from the extra time that the vehicle would be accelerating, which isn't going to add up to much over a period of 200ms unless you have a hyper-performance car! That's not to say that there aren't worthwhile savings to be made over a race distance, but we don't sell our system on that basis. We tell you the truth, even if sometimes it's not necessarily what you want to hear, or have been led to believe by others.

Anyway, back to the plot. Next we must consider what happens with an open-loop system if the shift completes before the end of the engine cut. In other words, the engine remains cut while the vehicle is in gear. When this happens, the forward momentum of the car drives the engine, and the gearbox is transmitting reverse torque. Remember that the throttle will be wide open at this point, and under these conditions the engine becomes nothing more than a compressor providing a very effective brake. So, instead of increasing performance, a timer based open-loop system could actually reduce performance and potentially cause vehicle instability because of the large dip in the acceleration curve. Furthermore, when the engine power is eventually resumed, the sudden torque reversal causes the transmission to shunt aggressively, leading to possible transmission breakages as well as the aforementioned chassis instability.

If the shift strategy is not optimised by use of closed-loop control methods, time-consuming and potentially damaging miss-shifts can occur as described above. Optimising the shift strategy is a very complicated affair and is governed by numerous factors. Even variations in track conditions can have a significant impact on the shift time. Hopefully you can appreciate that a fixed engine cut time rarely coincides with the actual shift time, and the cut will almost always be either too long or too short when using open-loop methods. So, what's the solution?

The only way to determine the required engine cut duration (and throttle blip duration on downshifts) is to monitor the gearbox barrel position sensor and alter the engine torque reduction dynamically. When the Geartronics system makes an upshift, the engine torque reduction is maintained until a pre-defined barrel angle has been achieved. The engine power is then resumed without any time wasting excess cut period, and there is no transmission shunting that can cause instability in the chassis.

No doubt you will be told (usually by companies that are selling the open-loop stuff) that it's not necessary to use a gearbox sensor, even on motorcycle gearboxes. They will do their utmost to convince you that closed-loop control is complicated and unnecessary. Well, if that were the case, then why have we (along with every other professional paddleshift designer up to and including F1) invested so much time and money in developing such a system? In the case of Geartronics, it's certainly not so that we can charge a lot more for our system.

We're not going to name the companies that are using open-loop controllers, that's up to you to do the research, but it doesn't take much investigation to find out. Simply ask the vendor if their system connects to a gear position sensor. If the answer is "yes", then the system is probably running some form of closed-loop control. If the answer is "no" then we suggest that you satisfy yourself that the system is suitable before handing over any money.

In summary: for any semi-auto shift system to work consistently & reliably under all conditions, the GCU needs to measure as an absolute minimum the gearbox barrel position, throttle position and engine speed.

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