Sensibility - Questions about the deadband & propo

Hello,

Please correct me if I say anything wrong and excuse my average english, not my native language. I have a question about the sensibility that you can configure for the various sensors that you can put with the controller. In the user manual for the moba matic, compatible with version 4.28-2044 and above, there is this graph that I attached (sensor.png). It shows the values of the db and dp for the sonic, ski sensor, big-ski and LS 3000 with proportionnal valves (I could have shown any, it's just for the example).

My local furnisher told me that the dead band was centered around 0 meaning that, for a value of X, any gap between [- X/2 ; X/2 ] will be ignored by the device. He also told me that this wasn't the case for the proportionnal band. For a value of dp going from X to Y, the command of the valve will be proportionnal for any gap between ]- Y ; - X [U] X ; Y [. This is illustrated in the second picture I attached (db dp.png).

Okay, so here is my question :

Since the sensors is accurate at ± 1 mm (information found on your site) and since the deadband is centered, how can the deadband be sometimes under 2 mm ? 2 mm means it's at ± 1 mm around 0, equals to the precision of the sensor.

I'm not saying it doesn't work, I just want to know what is the logic behind. If I misunderstood anything, please correct me.

Thanks.

I may not fully understand your question. Perhaps this can be helpful.

Deadband is a function of (and calculated in) the MOBA-Matic controller (not the grade sensor). The deadband is defined as a distance from the desired set-point where a deviation is calculated but no corrections are made.

The Sonic Ski sensor has an accuracy (or repeatability) of ± 1 mm. The sensor reports a measurement to the controller and the controller then takes that measurement and then determines if there is a deviation and if that deviation is greater than the deadband a command for movement will be given. As you point out the Sonic Ski sensor can have an error of ± 1 mm but the controller does not consider this. It simply takes the measured value reported by the sensor and determines if there is a deviation to set-point. Our system has several sensor options for measuring distance. Other contacting sensors will have higher accuracy than an ultrasonic sensor if that higher accuracy is really needed.

Yeah it does answer my question, thank you.

By the way, so it is correct that the db is centered but not the dp ? It can be confusing for some people I'm interacting with so I'd like to clear that.

Another question that arises :

Sensor has a ± 1 mm accuracy and the controller displays the values in cm, so you have a mm accuracy on the display (eg. 0,X).

I wonder how the system, in the example of the sensibility 10, can detect a gap of 0,025 mm ( 0,5 mm / 2 ) ?

Thanks.

When you say ""dp"" is that Proportional Band? I see your attachment but I should have studied French more in school

""I wonder how the system, in the example of the sensibility 10, can detect a gap of 0,025 mm ( 0,5 mm / 2 ) ?"" In this case the controller is looking for deviations (up or down) of >,5 mm before driving the up or down valve. If you can be +,5 mm or -,5 mm from set-point this would equal 1 mm and not ,025 mm. Of course if the sensor has an accuracy of +/-1 mm you can imagine how difficult it would be to control the system to your desired set-point. Which is why this sensitivity has a default setting of 6.

Proportional Band is a function of the valve drivers. For the sake of this question I will assume the system is connected to a proportional valve. There are many different types of proportional valves with many different hydraulic flow ratings and electrical coils. This is why we need to program the minimum current and maximum current to drive whatever valve is installed on that machine. Proportional Band can be viewed as the amount of distance in deviation from set-point between minimum valve output to maximum valve output that is programmed into the controller. Remember that we program this valve coil in both the up and down directions. For example if you programmed your minimum valve current to .50 and maximum valve current to 1.00 in both the up and down direction as soon as the controller determines a correction is needed (in either the up or down direction) it will output current between the .50 (minimum) and 1.00 (maximum) based on the distance between deviation and set-point. In other words when there is only a small correction needed we only want to provide the minimum current to the valve for a slow movement back to set-point. But if there is a large correction needed we then want to drive the valve with more current to make the hydraulic cylinder move faster.

With the controller sensitivity set to 10 the set-point deviation distance between minimum valve current and maximum valve current is very small and the deadband is also very small. On most machines this will be very difficult to control and the system will be making corrections very frequently. I would not recommend using a sensitivity value of 10. Especially on an asphalt paving machine where there are other influences on the screed that can cause a slight raising or lowering like changes in paving speed or variances in the amount of asphalt placed in front of the screed (or what we call ""head of material""). Remember it is not just the grade control that causes changes to the set-point distance. And as you pointed out, if the ultrasonic sensor is only repeatable to +/- 1 mm this will only make it harder to control at a setting of 10.

When you say "dp" is that Proportional Band? I see your attachment but I should have studied French more in school

Indeed it is. Sorry I haven't explained the french diagram ; it shows how the valve works in the dead and proportionnal bands, I'm sure you are familiar with this. "I wonder how the system, in the example of the sensibility 10, can detect a gap of 0,025 mm ( 0,5 mm / 2 ) ?" In this case the controller is looking for deviations (up or down) of >,5 mm before driving the up or down valve. If you can be +,5 mm or -,5 mm from set-point this would equal 1 mm and not ,025 mm.

I'm confused. You say the controller is looking for deviations ( = distance between the mesure and the 0, right ?) of 0,5 mm but in the MOBA diagram provided by my retailer we can see the dead band is centered around 0 (I mean, the value of the dead band is split in half around 0), so it should be looking for deviations (up or down) of 0,025 mm, isn't it (so that 2 * 0,025 = 0, 5 = the value shown in the table).

Proportional Band is a function of the valve drivers. For the sake of this question I will assume the system is connected to a proportional valve. There are many different types of proportional valves with many different hydraulic flow ratings and electrical coils. This is why we need to program the minimum current and maximum current to drive whatever valve is installed on that machine. Proportional Band can be viewed as the amount of distance in deviation from set-point between minimum valve output to maximum valve output that is programmed into the controller. Remember that we program this valve coil in both the up and down directions. For example if you programmed your minimum valve current to .50 and maximum valve current to 1.00 in both the up and down direction as soon as the controller determines a correction is needed (in either the up or down direction) it will output current between the .50 (minimum) and 1.00 (maximum) based on the distance between deviation and set-point. In other words when there is only a small correction needed we only want to provide the minimum current to the valve for a slow movement back to set-point. But if there is a large correction needed we then want to drive the valve with more current to make the hydraulic cylinder move faster.

I understood this was how it worked ; thanks for confirming that to me. Does the maximum valve output equals a continuous output? I mean that for a variation equals to the end of the proportionnal band or above, is the controller sending a continuous output? Or does it send the max valve output every xx seconds?

With the controller sensitivity set to 10 the set-point deviation distance between minimum valve current and maximum valve current is very small and the deadband is also very small. On most machines this will be very difficult to control and the system will be making corrections very frequently. I would not recommend using a sensitivity value of 10. Especially on an asphalt paving machine where there are other influences on the screed that can cause a slight raising or lowering like changes in paving speed or variances in the amount of asphalt placed in front of the screed (or what we call ""head of material""). Remember it is not just the grade control that causes changes to the set-point distance. And as you pointed out, if the ultrasonic sensor is only repeatable to +/- 1 mm this will only make it harder to control at a setting of 10.

Thanks for the clarification, indeed it seems a bit extreme for an asphalt paver. This makes me wonder why there is these extreme sensitivities in the first place? The various furnisher of MOBA system I talked with told me that SE = 8, 9, 10 are too sensitivie and SE = 1, 2, 3 maybe 4 are too slow. Why there isn't 10 ""relevant"" sensitivies then for us to work with ? Once again I do not mean to be rude or something like that, I just want to understand the better I can this system to be able to teach my crews =)