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发表于 2016-4-1 03:57:25
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Hydraulic systems are like a mass between springs. The mass is the piston and connected load. The two springs are the volumes of oil between the piston and valve on both sides of the piston. Therefore hydraulic systems tend to act like a second order under damped system. It is necessary to increase the damping factor electronically to reduce over shoot or oscillation.
For high performance hydraulic motion control the positions must be sampled at consistent time intervals and the motion controller must know exactly when these time samples are taken. Sampling at consistent time intervals allow the motion controller to compute accurate velocities and accelerations of the actuator. When accurate velocities and accelerations are known then it is possible to use the derivative and second derivative gain for closed loop control. The derivative and second derivative gain provide damping that reduce or eliminate oscillations. The derivative and second derivative gains are necessary when controlling systems with heavy loads and low natural frequency.
When positions are sampled asynchronously the motion controller does not know the time between two positions. Therefore it can’t calculate velocity by using
Velocity(n)=(Position(n)-Position(n-1))/(Time(n)-Time(n-1). The RMC motion controllers can calculate acceleration if the MDT ( Magnetostrictive Displacement Transducer ) rod samples are consistent increments. Then
Acceleration(n)=(Position(n)-2*Position(n-1)+Position(n-2))/Δt^2. If Δt is not known then the calculated acceleration will not be accurate.
MDT determine position by sending an electric pulse down a wave guide in the middle of the rod. A start pulse is returned that starts a high speed counter/timer. When the electric pulse meets the magnetic field of a magnet mounted on the back of the piston it interaction causes the wave guide to twist. The twisting motion takes time to get back to the twist detector in where the electronics are. A stop pulse is generated that stops the high/speed timer counter. The transmitter and twist detector are in the small case at the end of the rod. The time between when the pulse is sent down the rod ( the start pulse ) and the time the twist is detected ( the stop pulse ) determines how far the magnet on the back of the piston is away from the transmitter. What is important is that the pulse goes down the rod at almost the speed of light. The time the twist takes to get back from the magnet to the twist detector is what is measured. What is important is that the positon that is measure is the position at the time the pulse is sent down the rod. Therefore these pulses must be generated at constant time intervals to get positions at constant time intervals.
So what constant time interval should be used? The RMC motion controllers can be set to update every 250 micro seconds. If the MDT rod updates every 1 millisecond then the motion controller will see the same position 3 times in a row and then a big change on the fourth sample. This is unacceptable. It defeats the purpose of controller closing the loop every 250 microseconds. The MDT rod must provide positions to the motion controller when the motion controller asks for position and the pulses must be sent down at the same rate and time as the motion controller ask for them so that the positions are always synchronous the motion controllers closed loop update rate.
The need for synchronous positions was made clear to me in 1990 when I was at a trade show. I talked to the head of engineering at Balluff in the US. Back then there wasn’t SSI and the ability to count the time between the start pulse and stop pulse with precision didn’t exist or was just becoming available.
About 20 years ago Temposonics introduced the first SSI rods at a trade show. There was a lot of pressure for Delta to support the new SSI rods but we resisted because the new SSI rods would not sample synchronously with the motion controller. We could not see the advantage of being able to get back a position with a resolution of 10 microns unless the actuator was stopped. When moving at 1 meter per second the actuator move 1 millimeter per millisecond. The motion controller did not know when within the 1 millisecond period the 10 micron position was valid so it was impossible to compute accurate velocities. The simple method of counting the time between start and stop pulses provided less resolution but was actually more accurate when moving because the motion controller had control of when the start pulse was generated. About 2 years of heated discussion occurred before Temposonics decided to make their SSI rods synchronous. I am pretty sure that all the Temposonics rod meant for motion control are synchronous since the late 1990s. Since then we have had no problem with customers buying asynchronous SSI rods.
Balluff provided Delta with several SSI rods for testing. The first ones were asynchronous. Some could only update position every 2 millisecond. Later Balluff caught up with Temposonics. What bothers me is that Balluff still sells asynchronous SSI rods. If the customer should buy an asynchronous SSI and use it with our controller the results will not be good. Using proportional only control is not good for reason I have stated in previous threads.
This is how we test MDT rods.
http://v.youku.com/v_show/id_XMTI2NDk4OTc0OA==.html
Notice that the velocity is smooth.
Delta makes over 50%, a Rockwell marketing persons said over 80%, of the industrial hydraulic motion controllers used in the US and Canada. Delta makes the 1746-QS, 1756-HYD02 and 1756-M02AS that Rockwell ( Allen Bradley ) sells as their product. Both Balluff and Temposonic cooperate with Delta because the MDT rods must work with the hydraulic motion controllers used in the US and Canada. We have the means to test valves and MDT rods.
We get new MDT rods to test for compatibility before they are sold to the public.
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