Figure 4 shows the second step toward motion control, which uses a proportional valve instead of the discrete directional valve. As with Figure 2, the schematic diagram is not complete, rather, it is intended only to point out the obvious advantages of using proportional rather than on-off control. Note that the cylinder is outfitted with a position sensor that measures the position of the piston relative to the cylinder tube. The sensor works full-time — it is always sending an analog output signal to the controller.
The controller is depicted as a digital device into which a deceleration point can be entered, probably by means of a conventional keyboard. Being a digital device, it can give its attention to only one item or task at any given instant. That is, when it "looks at" the deceleration set point, it cannot be looking at the position sensor output. There must necessarily be a time lag between these two events. Additionally, the PLC probably has many other tasks that will divide its attention, such as monitoring and controlling temperature, reservoir level, and more. The resulting delay is the scan time of the controller, and it dictates how well the system performs.
Thus, we see that each of the tasks is actually serviced at regular intervals. The total time lag between the instant when one task — say monitoring the position sensor output and the instant when it does so again - is the total scan time. Furthermore, the instant when an event occurs will not be synchronous with the instant that the controller actually looks and sees the event. This results in random variation in the stopping point.
Continuing around the loop of Figure 4, note that the output of the controller consists of the start-stop signal, which is generated based on a comparison between the deceleration setpoint and the actual position from the position sensor. Also note that the start-stop signal from the controller is not connected directly to the valve solenoids. Rather, it is connected through the ramp generator.
The ramp generator is a common option on many proportional valve amplifiers. It ramps up the output until it equals the command voltage if the command input goes to a high level. Without the ramp, the amplifier output would jump up almost as quickly as the command signal from the controller. But with the ramp generator, even if the command is a step (as shown in the figure), the output is a signal to the valve coil that changes slowly. This prevents the spool from suddenly jumping to a proportional position. Instead, the spool slowly opens or closes, giving a considerable degree of acceleration control.
The ramp circuit is outfitted with trim pots to allow the user to adjust the rate at which the output changes, thus controlling the rate at which the valve spool shifts. This constitutes a vital improvement over the on-off control offered in the discrete directional system because the shocks are significantly reduced. Furthermore, the adjustability of the ramps gives the user immediate feedback as to the effectiveness of the adjustments.
A case in point
In a typical operational scenario, imagine that the start command has been given, the valve spool has ramped to a proportional position, and the cylinder is extending at some desired speed. Now suppose that the position sensor puts out a signal of, say, 1 V for every inch of travel away from the cap end of the cylinder, and that the deceleration setpoint is set to 6 V. We then expect that the deceleration will be initiated when the cylinder extends to 6 in. This is the progression of events:
In the early parts of the cycle, the cylinder will be at a position less than the 6 in. — 6 V from the position sensor. Next, the controller obtains the position signal and compares it to the deceleration setpoint. Because the setpoint is greater than the feedback signal, the controller issue a high output, commanding the valve to be open.
The piston continues extending and eventually will reach a position of 6.0000 in. Assuming perfect calibration, the position sensor will be generating 6.0000 V. Unfortunately, the controller will not be looking at the position sensor at that instant - the likelihood is very nearly zero. Instead, it will look only after the cylinder has over-traveled slightly. The amount of overtravel depends on the PLC scan time and the piston velocity at the instant the scan takes place.
At any rate, the position feedback sensor output eventually will exceed the deceleration setpoint value, and the controller will get around to scanning the sensor. The comparison will be made and judged to have resulted in over-travel, and the controller will issue a stop command.
The stop command merely means that the command will go to zero within a few milliseconds, perhaps even within a few microseconds of detection. But because of the ramp generators, the valve will not center quickly. Instead, it brings the cylinder to smooth and gentle stop. This will have been an effective means for controlling shock and vibration. And to some extent, it will have overcome the challenges from having a low hydromechanical resonant frequency.