O+P Ölhydraulik und Pneumatik 10/2016

O+P Ölhydraulik und Pneumatik 10/20161





































































70 FORSCHUNG UND ENTWICKLUNG STEUERUNGEN UND REGELUNGEN FAIL OPERATIONAL CONTROLS FOR AN INDEPENDENT METERING VALVE Michael Rannow As intelligent hydraulic systems with embedded sensors become more ubiquitous, the real or perceived reliability challenge associated with sensors must be addressed to encourage their adoption. In this paper, a fault-tolerant control strategy for an intelligent independent metering valve that allows continued operation if a sensor fails is described. 70 O+P Fluidtechnik 10/2016


72 FORSCHUNG UND ENTWICKLUNG STEUERUNGEN UND REGELUNGEN this approach, the beginning of the movement will typically be slower than desired, but the controller will adjust at a rate governed by the controller gains. This controller will work for both a passive and an overrunning load. For an overrunning load, the cross-port pressure control will work as described, the meter-out valve will start by assuming a heavy load and then adapt until the meter-in pressure is held to be a constant. With a passive load, the meter-in pressure will be determined by the load pressure, which will likely be higher than the desired pressure set point. In this condition, the meter-out valve will not be able to control the meter-in pressure down to the desired value, but it will try to do so anyway. The consequence of a meter-in pressure that is too high is that the meter-out valve will try to open more to lower the pressure. Thus, the meter-out valve will open until it is completely out of the way, which is typically the desired condition for a passive load. 2.2 FAILED METER-IN PRESSURE SENSOR The fail-operational controller for a failed pressure sensor on the meter-in side is similar to the one for the failed meter-out pressure sensor, although the fact that the meter-in pressure sensor is used to send an electronic load sense demand to the pump adds a small change. The control structure is similar to the previous case, with the meter-out valve commanded to give the desired meter-in flow times a ratio of the actuator areas. The meter-in valve is then used to maintain a low constant pressure on the meter-out side. x = f (P ?PQ % ) (5) in,des meter?in supply in, indes , = K (P ?P )+K (P ?P )dt+ in p out,des out i out,des out d dP ( out,des ? Pout 72 O+P Fluidtechnik 10/2016 As in (3) and (4), the desired spool position is determined using the normal flow control function for a meter-in spool, f meter-in ,but an estimate of the missing pressure is used in place of the measured value. The pressure can be estimated using many different adaption functions, such as the PID example used here, which are driven by the error between the measured meter-out pressure and a constant set point. In some cases, a simple integrator could provide the required adaptation. The pressure estimate derived in (6) can also be sent as a load sense pressure demand to the pump, allowing the valve to maintain its electronic load sense functionality. Alternatively, the pump could be set to be continuously at max pressure to ensure that there is always be enough pressure to move the load, but this would waste a significant amount of energy. The initial value for the meter-in pressure can be set in a number of ways, but, if the estimate is being used to set the pump pressure, using an initial estimate of the maximum possible load pressure would prevent any back flow from a heavy load to the supply line. This control approach can work for both passive and overrunning loads. If the load is passive, the meter-in pressure estimate will adjust until the pressure on the meter-out side is at its desired value and the proper meter-in flow is achieved. For an overrunning load, which should be higher than the low, constant set point, the meterin valve will try (unsuccessfully) to lower the pressure by lowering the pump load sense demand and closing to the pump. In this case, if an anti-cavitation valve is available, it will open to supply the unloaded side of the actuator from the tank line. The controller can also detect when the estimate is driven to a low value, and either set a minimum value on the meter-in pressure estimate to hold the valve partially open, or command the valve to open to tank. 2.3 FAILED METER-OUT POSITION SENSOR For a failed position sensor, the challenges of not being able to detect a passive/overrunning load and not knowing how to set the pump pressure are alleviated because the pressure sensors are still functioning. However, the fact that there is no steady-state relationship between the input current and the spool position presents a significant challenge. For the CMA valve, the current into the pilot stage is related to the flow out of the pilot stage. This gives a relationship between the input current and the mainstage velocity, meaning that the control input (current) is one integrator removed from the desired output (spool position). This forces the controller to be less aggressive and much more damped than the position controller on a non-faulty valve. The cross-port pressure control can again be used for a failed position controller, but rather than the error term being used to determine the desired position, the difference between the meter-out pressure and a desired set point is used to determine the current sent to the meter-in pilot spool: =? ? p[ indes , in)+ i indes , i K P P K P P dt K dP ( in,des ? Pin +K x +K g(P P )]+ ff des damp in, out deadband In this example, the function ? is a mapping from the input current to the spool velocity which removes some of the nonlinearity in the system. For a CMA valve, a training routine is used to learn the relationship between the current and velocity to remove nonlinearity and part-to-part variation from the controller, but this could also be generated from a known model of the system. In addition to the PID terms, a feedforward term is added based on the desired spool velocity which improves the response when starting or adjusting a command. A damping function, g, based on the two pressures is also added to help damp out oscillations. As in the failed pressure case, the control tries to drive the meter-in pressure to a constant value. As seen in (2), if the pressure is a constant, then the flow in and flow out of the actuator are matched. Thus, if the meter-in flow is controlled and the meter-in pressure is constant, then the meter-in flow is also controlled to the correct value. Controlling the flow out of an actuator with an overrunning load without position feedback is the most challenging condition, since any error or instability in the spool movement can result in a falling load. For a passive load, the same controller can be used; the meterin pressure will likely be higher than the constant set point, which will drive the meter-out spool fully open and out of the way. This is the desired behavior in a passive condition. In this case, the integrator in (7) must be disabled to avoid integrator wind-up. 2.4 FAILED METER-IN POSITION SENSOR In the case of a failed position sensor on the meter-in side, the controller structure can be a bit different. In many cases, the meter-in spool is not needed to control the speed of the load, so the spool can be fully opened to supply pressure (passive loads) or tank (overrunning loads). The speed of the actuator is then controlled by the meter-out spool. This makes the control of the spool simple, but it does create a challenge when setting the load sense demand. Typically, the load sense demand from a service is set to be some margin above the measured meter-in pressure. The pressure difference between supply and the actuator then occurs across the meter-in spool. However, if the meter-in spool is fully open, there will be only minimal pressure drop, meaning that the load sense demand will increase up to its maximum value as it tries to maintain

73 STEUERUNGEN UND REGELUNGEN 01 Traces of a boom lowering with a failed meter-out pressure sensor 02 Traces of a boom raising with a failed meter-in pressure sensor the desired pressure margin. However, as the supply pressure increases, the pressure on the meter-out side, which is controlling the flow out of the actuator, will increase. In a passive case, the back pressure is typically desired to be low, so a rise in the meter-out pressure can be used to reduce the load sense demand: P = P +P ? P ?max(P ? P 0) (8) is,des in margin out out out,limit, Aout In (8), the load sense demand will increase until it is limited by the meter-out pressure increasing above the specified meter-out pressure limit. This equation is used for a passive load. For an overrunning load, the meter-in spool can be connected to tank, so the pump pressure setting is irrelevant. In an alternative method for the passive case, an error term can be generated that is used to control the meter-in spool to a partially open position. This is useful for shielding the service from high pump pressures that are requested by another service operating simultaneously. As in the failed meter-out position sensor case, this requires a controller that sets a desired current to the pilot stage. Using this approach the load sense demand can be set as normal. =? ? [ out,des ? )+ i out,des ? i K P P K P P dt K dP ( out,des ? Pout +K ff des]+ideadband O+P Fluidtechnik 10/2016 73

74 FORSCHUNG UND ENTWICKLUNG STEUERUNGEN UND REGELUNGEN Notice that the damping term is not as critical for this case since the meter-in case is less sensitive to dropping a heavy load. This control is only necessary for a passive load. For an overrunning load, the meter-in spool can be fully opened to tank and the actuator speed controlled with the meter-out valve, as in the conventional case. 3 EXPERIMENTAL RESULTS The fail operational controller was implemented on the actuators of a backhoe loader. While the boom, arm, and bucket were all successfully tested, the boom provided the heaviest load, with the most 74 O+P Fluidtechnik 10/2016 potential to fall if there were any controller errors. Thus, the boom was used for demonstration. Note that the sensors did not actually fail, so their readings are included in the plots. Internally to the valve controller, the feedback from the faulty sensors was disconnected. 3.1 TRACES OF A BOOM LOWERING WITH A FAILED METER-OUT PRESSURE SENSOR Figure 1 shows an example of a system working with a failed meterin pressure sensor. In this example, the load is overrunning, which 03 Traces of a boom lowering with a failed meter-out position sensor 04 Traces of a boom raising with a failed meter-in position sensor




78 FORSCHUNG UND ENTWICKLUNG DICHTUNGEN side 2 test seal acceleration sensor pressure side 1 test seal working cylinder 78 O+P Fluidtechnik 10/2016







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