Electrohydraulic control apparatus and method

An apparatus, is adapted to controllably position a movable element in a hydraulic motor. A controller receives a request signal and velocity and/or position signals from sensors. The controller, in accordance with a control scheme using velocity and/or position feedback, determines the flow of hydraulic fluid to the cylinder needed to reduce velocity and/or position errors. A directional valve, actuated by an electrohydraulic pilot system, delivers hydraulic fluid to the hydraulic motor.

DESCRIPTION 
Technical Field 
This invention relates generally to an apparatus and method for controlling 
the velocity and/or position of a hydraulic motor and more particularly, 
to an apparatus and method for controllably modulating piston velocity in 
a hydraulic cylinder using velocity feedback. 
Background Art 
Hydraulic systems are particularly useful in applications requiring a 
significant power transfer and are extremely reliable in harsh 
environments, for example, in construction and industrial work places. 
Earthmoving vehicles, such as excavators, backhoe loaders, and wheel type 
loaders are a few examples where the large power output and reliability of 
hydraulic systems are desirable. 
Typically, a diesel or internal combustion engine drives the hydraulic 
system. The hydraulic system, in turn, delivers power to the vehicle's 
work implement. The hydraulic system typically includes a pump for 
supplying pressurized hydraulic fluid and a directional valve for 
controlling the flow of hydraulic fluid to a hydraulic motor which in turn 
delivers power to a work implement, i.e. a bucket. 
For example, a typical excavator has three basic implement circuits, 
consisting of: boom, stick, and bucket appendages. Each appendage is 
controlled by individual directional valves and hydraulic cylinders. An 
operator controls the flow of hydraulic fluid, and therefore the velocity 
of each appendage, through one or more control handles which may be 
mechanical, electrical or electrohydraulic devices. The control handles 
provide means for manual operation; in which the displacement of the 
control handles is indicative of the desired flow of hydraulic fluid. 
Fluctuations in pressure and flow of the hydraulic fluid supplied by the 
pump are inherent characteristics of hydraulic systems. These fluctuations 
present several problems which the control system must accommodate. 
Supply pressure fluctuations have several causes. First, hydraulic circuits 
are often connected in series and are driven by the same pump. Each 
hydraulic circuit, through its individual operations, affects the 
hydraulic supply pressure. Second, since the pump is driven by the engine, 
the engine RPM also affects supply pressure. 
Also, a varying load on the work implement affects the amount of flow 
request needed to produce the desired cylinder velocity (the work 
implement may be empty or may be filled and the load may vary while the 
implement is moving). 
In order to have consistent system response, it is necessary to have a 
fixed flow of hydraulic fluid to the cylinder for a fixed flow request. 
Supply pressure variations and varying loads affect the flow rate and 
therefore, cause the control system to produce undesirable behavior. 
U.S. Pat. No. 4,586,332, issued to Schexnayder, on May 6, 1986, discloses a 
two spool valve design for providing pressure compensation. As shown in 
FIG. 1, a directional control spool 104 has extend, retract and neutral 
positions for controlling the flow of hydraulic fluid to a hydraulic motor 
102 A flow control spool 106 maintains a predetermined pressure 
differential across the directional control spool 104. Excess fluid from 
the pump is bypassed by the flow control spool to tank. This two spool 
valve design attempts to give a fixed flow rate for the extend and retract 
positions of the direction control spool 106 regardless of the load. 
However, the valve design is complex and adds cost to the system. Further, 
the two spool valve design does not accommodate over-running cylinder 
loads. 
Some control systems may be responsive to position requests instead of flow 
requests. That is, rather than receiving signals indicative of a desired 
flow of hydraulic fluid (from control handles or other sources), the 
control system receives signals indicative of a desired cylinder position 
or displacement. 
Industrial robots are typical applications where the large power output and 
reliability of hydraulic systems prove beneficial and require a control 
system to respond to position signals. However, the tasks industrial 
robots are designed to perform require a control system with fast response 
times and high accuracy. Present hydraulic control systems are unable to 
provide acceptable response time and the accuracy necessary for this type 
of application. For these reasons, industrial robots are usually powered 
by expensive electric motors. 
The present invention is directed at overcoming one or more of the problems 
as set forth above. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention, an apparatus for controllably 
moving a movable element within a hydraulic motor in response to a flow 
request signal is provided. Hydraulic fluid is delivered to the hydraulic 
motor in response to pilot pressure signals. The actual velocity of the 
hydraulic motor is sensed and a desired velocity signal is produced based 
on the flow request signal. A velocity error signal is produced based on 
the desired velocity signal and the actual velocity. A flow command signal 
is produced based on the desired velocity signal and the velocity error 
signal. A pilot pressure signal, indicative of the desired flow of 
hydraulic fluid, is produced based on the flow command signal. 
In another aspect of the invention, a method for controllably moving a 
movable element within a hydraulic motor in response to a flow request 
signal is provided. The method includes the following steps: the actual 
velocity of the movable element is sensed and an actual velocity signal is 
produced. The velocity signal is processed and a flow command signal 
indicative of the desired flow of hydraulic fluid is produced.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to FIG. 2, the present invention 200 is adapted to 
controllably position a movable element 202 in a hydraulic motor 204. In 
the preferred embodiment, the hydraulic motor 204 is a hydraulic cylinder 
and the movable element 202 is a piston within the cylinder, as shown. 
A means 206 delivers hydraulic fluid to the hydraulic cylinder 204. In the 
preferred embodiment, the hydraulic fluid delivering means 206 includes an 
open center directional valve 208 (spool valve). The directional valve 208 
has three flow positions 210,212,214 for controlling the flow of hydraulic 
fluid to the hydraulic cylinder 204. 
As shown in FIG. 3A, in a first embodiment, the directional valve 208 has 
an overlapped spool 302A. In a second embodiment, as shown in FIG. 3B, the 
directional valve 208 includes a critically-lapped spool 302B. And in a 
third embodiment, the directional valve 208 includes an underlapped spool 
302C, as shown in FIG. 3C. Any one of the three spools 302A,302B,302C may 
be used. Their advantages/disadvantages are well known in the art and the 
choice of spool 302A,302B,302C is dependent upon the type of application. 
Returning to FIG. 2, in the first flow position 210, hydraulic fluid under 
pressure (generally denoted as P and typically below 10,000 psi) from a 
pump (not shown) passes through the directional valve 208 to the back end 
of the hydraulic cylinder 204. The hydraulic fluid from the pump acts to 
extend the piston 202 by exerting a force on the back end of the piston 
202 in the direction of the arrow labeled E. Hydraulic fluid from the 
front end of the hydraulic cylinder 204 passes through the directional 
valve 208 and back to a tank of hydraulic fluid, T. 
In the second or neutral flow position 212, the piston 202 is held at its 
current position. Since the hydraulic fluid and the pump are doing a 
minimal amount of work, the pump is destroked, thereby reducing flow to 
minimize losses (note that the pump may remain stroked to power other 
hydraulic circuits.) 
When the directional valve 208 is in the third flow position 214, hydraulic 
fluid passes through the directional valve 208 to the front end of the 
hydraulic cylinder 204 and exerts a force on the front end of the piston 
202 in the direction of the arrow labeled R. Hydraulic fluid from the back 
end of the hydraulic cylinder 204 also passes through the directional 
valve 208 to the tank, T. 
A means 215 senses the actual position of the piston 202 and produces a 
signal, p.sub.a (t), indicative of the relative measured extension or 
retraction of the piston 202. In one embodiment, the means 215 includes a 
radio frequency (RF) linear position sensor 218, as disclosed in U.S. Pat. 
No. 4,737,705, issued Apr. 12, 1988 to Bitar, et al. In another 
embodiment, the means 215 includes a potentiometer based sensor (not 
shown). And in a third embodiment, the means 215 includes a resolver (not 
shown). Use of both the resolver and the potentiometer based sensors are 
well known in the art and are therefore not further discussed. 
A means 216 senses the actual velocity of the piston 202 and produces a 
signal, v.sub.a (t), indicative of the measured velocity of the piston 
202. In one embodiment, the means 216 includes a velocity sensor 220. The 
velocity sensor 220 includes a DC generator which when rotated, generates 
a voltage indicative of the velocity of rotation (and therefore the linear 
velocity of the piston 202). In a second embodiment, the means 216 
receives the position signal, p.sub.a (t), and determines the velocity of 
the piston 202 by numerically filtering and differentiating the position 
signal, p.sub.a (t). 
A controller 222 processes the acquired velocity and position signals, 
v.sub.a,p.sub.a (t), produces a compensated velocity signal, v.sub.c (t), 
and a flow command signal, f.sub.c (t). The flow command signal, f.sub.c 
(t), is indicative of the desired flow of hydraulic fluid to the hydraulic 
cylinder 204 and is preferably proportional to the compensated velocity 
signal, v.sub.c (t). 
A means 224 receives the flow command signal, f.sub.c (t), and produces a 
pilot pressure signal, p.sub.p (t). In one embodiment, the means 224 
includes an electrohydraulic pilot system 226. The pilot system 226 
includes a proportional pilot pressure solenoid valve. The flow command 
signal, f.sub.c (t), actuates the solenoid valve, which in turn, delivers 
pilot pressure signals, p.sub.p (t), to the directional valve 208. The 
pilot pressure signals, p.sub.p (t), are in the form of hydraulic fluid 
under low pressure (typically, below 1000 psi). The hydraulic fluid acts 
on the spool 302 to place the directional valve 208 into one of the flow 
positions 210,212,214. The pilot system 226 is well known in the art and 
is therefore not further discussed. 
In the preferred embodiment, the controller 222 includes a microprocessor. 
The microprocessor receives the position signal, p.sub.a (t), from the 
position sensor 218. In one embodiment, the microprocessor also receives 
the velocity signal, v.sub.a (t), from the velocity sensor 220. In a 
second embodiment, the microprocessor computes the velocity of the piston 
202 by numerically filtering and differentiating the position signal, 
p.sub.a (t). 
The controller 222 receives a request signal, signals from one or more 
sensors 218,220 and produces the flow command signal, f.sub.c (t). In a 
first embodiment, the controller 222 receives the actual velocity signal, 
v.sub.a (t), and produces the flow command signal, f.sub.c (t), in 
accordance with a first control scheme 400, as shown in FIGS. 4, 5A, and 
5B. In the first embodiment, the controller 222 receives a flow request 
signal, f.sub.r (t), which is indicative of the desired velocity of the 
piston 202, and is typically proportional to the displacement of an 
operator actuated control handle. Operator actuated control handles are 
well known in the art and are therefore not further discussed. The 
velocity signal, v.sub.a (t), represents the actual velocity of the piston 
202. 
A means 402 receives the flow request signal, f.sub.r (t) and produces a 
desired velocity signal, v.sub.d (t). In the preferred embodiment, the 
means 402 implements a first transfer function, h.sub.fp (t) 404. The 
first transfer function, h.sub.fp (t) 404 scales the flow request signal, 
f.sub.r (t) to pro the desired velocity signal, v.sub.d (t). 
The La Place transform of the first transfer function, h.sub.fp (t) 404 is 
denoted as H.sub.fp (s) and is of the form: 
EQU H.sub.fp (s)=K.sub.1. 
Therefore, 
V.sub.d (s)=K.sub.1 .times.F.sub.r (s), where K.sub.1 is a constant, 
V.sub.d (s) is the La Place transform of the desired velocity signal and 
F.sub.r (s) is the La Place transform of the flow request signal. 
A means 406 subtracts the actual velocity signal, v.sub.a (t), from the 
desired velocity signal, v.sub.d (t), and produces a velocity error 
signal, v.sub.e (t) In the preferred embodiment, the means 406 includes a 
first summing junction 408 and implements a second transfer function, 
h.sub.fv (t) 410. The second transfer function, h.sub.fv (t) 410 provides 
velocity feedback compensation and, in the preferred embodiment, scales 
and integrates the output of the first summing junction 408 to produce the 
velocity error signal, v.sub.e (t). 
The La Place transform of the second transform function, h.sub.fv (t) is 
denoted as H.sub.fv (s) and is of the form: 
EQU H.sub.fv (s)=K.sub.2 /s, where K.sub.2 is a constant. 
Therefore, 
##EQU1## 
where V.sub.e (s) and V.sub.a (s) represent La Place transforms of the 
velocity error signal and the actual velocity signal, respectively. 
A means 412 receives the desired velocity signal, v.sub.d (t), and the 
velocity error signal, v.sub.e (t), and produces the flow command signal, 
f.sub.c (t). In the preferred embodiment, the flow command signal, f.sub.c 
(t), is a pulse width modulated (PWM) drive current applied to the 
solenoid of the pilot system 226. The flow command signal, f.sub.c (t), 
actuates the solenoid. The means 412 includes a second summing junction 
414 and a nonlinearity inverter 416. The second summing junction 414 adds 
the desired velocity signal, v.sub.d (t) and the velocity error signal, 
v.sub.e (t) to produce the compensated velocity signal, v.sub.c (t). The 
nonlinearity inverter 416 receives the compensated velocity signal, 
v.sub.c (t), and produces the flow command signal, f.sub.c (t). 
The nonlinearity inverter 416 compensates for the nonlinearities of the 
directional valve 208 and in the preferred embodiment includes a map in 
the controller 222. The steady-state characteristics, particularly, the 
deadband and flow gain characteristics, are measured and are stored in the 
map. The nonlinearity inverter 416 receives the compensated velocity 
signal and uses the map to determine the appropriate flow command signal, 
f.sub.c (t). 
The velocity sensing means 216 includes a third transfer function, H.sub.bv 
(s) 418. The third transfer function H.sub.bv (s) 418, mitigates sensor 
noise by filtering the output of the velocity sensor 220 to produce the 
actual velocity signal, v.sub.a (t). In the preferred embodiment, the 
third transfer function 418 is a second order filter with a corner 
frequency around 10 Hz. 
The La Place transform of the third transfer function is denoted as 
H.sub.bv (s) and is of the form: 
EQU K.sub.3 {s.sup.2 +(K.sub.4 .times.s)+K.sub.3 }, 
where K.sub.3 and K.sub.4 are constants. 
Referring to FIGS. 5A and 5B, the controller 222 delivers a controlled flow 
of hydraulic fluid to the hydraulic cylinder 204 using the following 
procedure: First, as shown in block 502, the flow request signal, f.sub.r 
(t) is retrieved. In the preferred embodiment, the flow request signal is 
stored in a memory location within the controller 222. The microprocessor 
reads the memory location to determine the desired flow. Then, the actual 
velocity of the piston 202 is determined by reading the velocity sensor 
220 (block 504). In block 506, the desired velocity signal is computed by 
scaling the flow request signal. 
In block 508 (FIG. 5B), the actual velocity signal is subtracted from the 
desired velocity signal. The result is scaled to produce a velocity error 
signal. The velocity error signal is then added to the desired velocity 
signal (block 510). The velocity error signal is used to calculate the 
flow of hydraulic fluid needed to correct the velocity error (block 512). 
The calculated flow of hydraulic fluid is then delivered to the hydraulic 
cylinder 204 via the pilot system 226, as described above. This process is 
then repeated. 
In a second embodiment, the controller 222 receives the actual velocity 
signal, v.sub.a (t), and the actual position signal, p.sub.a (t), and 
produces the flow command signal, f.sub.c (t), in accordance with a second 
control scheme 600, as shown in FIGS. 6, 7A, and 7B. In the second 
embodiment, the controller 222 is responsive to a position request signal, 
p.sub.r (t), which is indicative of a desired position of the piston 202. 
A means 602 receives the position request signal, p.sub.r (t) and the 
actual position signal, p.sub.a (t), produces a first desired velocity 
signal, v.sub.fd (t). The first desired velocity signal producing means 
602 includes a third summing junction 604 and implements a fourth transfer 
function H.sub.fp2 (t). The third summing junction 604 subtracts the 
actual position signal, p.sub.a (t) from the position request signal, 
p.sub.r (t), and produces a position error signal, p.sub.e (t). The fourth 
transfer function, h.sub.fp2 (t) 606 scales the position error signal, 
p.sub.e (t) to produce the first desired velocity signal, v.sub.fd (t). In 
an alternate embodiment, the fourth transfer function 606 also filters the 
position error signal, p.sub.e (t). 
The La Place transform of the fourth transfer function, h.sub.fp2 (t) 606 
is denoted as H.sub.fp2 (s) and is of the form: 
EQU H.sub.fp2 (s)=K.sub.5. 
Therefore, 
V.sub.fd (s)=K.sub.5 .times.P.sub.e (s), where K.sub.5 is a constant, 
V.sub.fd (s) is the La Place transform of the first desired velocity 
signal and P.sub.e (s) is the La Place transform of the position error 
signal. 
A means 608 provides feed forward compensation. The feed forward 
compensation providing means 608 includes a fourth summing junction 610 
and implements a fifth transfer function H.sub.ff (s) 612. The fifth 
transfer function, H.sub.ff (s) 612, scales and differentiates the 
position request signal, p.sub.r (t). The fourth transfer function 606, 
H.sub.fp2 (s) may have some inherent phase lag, particularly at low 
frequencies. The fifth transfer function 612 provides phase lead to 
improve system response and to compensate for the phase lag of the fourth 
transfer function 612. 
The fourth summing junction 610 adds the output of the fifth transfer 
function 612 to the first desired velocity signal, v.sub.fd (t), to 
produce a second desired velocity signal, v.sub.sd (t). 
The La Place transform of the fifth transfer function, h.sub.ff (t) 612 is 
denoted as H.sub.ff (s) and is of the form: 
EQU H.sub.ff (s)=s.times.K.sub.6, 
where K.sub.6 is a constant. 
Therefore, 
##EQU2## 
where V.sub.sd (s) represents the La Place transfer function of the second 
desired velocity signal. 
The position sensing means 215 includes a sixth transfer function 614, 
H.sub.bp (s). The sixth transfer function 614, H.sub.bp (s), mitigates 
sensor noise by filtering the output o the position sensor 218 to produce 
the actual position signal, p.sub.a (t). In the preferred embodiment, the 
sixth transfer function 614 is a second order polynomial with a corner 
frequency around 10 Hz (similar to the third transfer function 424). 
The second desired velocity signal, v.sub.sd (t) is processed, similarly to 
the desired velocity signal, v.sub.d (t) in the first control scheme 400, 
to produce the flow command signal, f.sub.c (t). 
Referring to FIGS. 7A and 7B, the controller 222 precisely controls flow of 
hydraulic fluid to the hydraulic cylinder 204 according to the following 
procedure: First, as shown in block 702, the position request signal, 
p.sub.r (t) is retrieved. In the preferred embodiment, the position 
request signal is stored in a memory location within the controller 222. 
The microprocessor reads the memory location to determine the desired 
position. Then, the actual position and velocity of the piston 202 is 
determined by reading the position and velocity sensors 218,220 (block 704 
. In block 706, the feed forward compensation is calculated by scaling and 
taking the derivative of the position request signal. As shown in block 
708, the first desired velocity signal is computed by scaling the position 
request signal. In block 710 the second desired velocity signal is 
generated by adding the feed forward compensation and the first desired 
velocity signal. 
In block 712 (FIG. 7B), the actual velocity signal is subtracted from the 
second desired velocity signal. The result is scaled to produce a velocity 
error signal. The velocity error signal is then added to the second 
desired velocity signal (block 714). The velocity error signal is used to 
calculate the flow of hydraulic fluid needed to correct the position and 
velocity errors (block 716). The calculated flow of hydraulic fluid is 
then delivered to the hydraulic cylinder 204 via the pump and pilot system 
226, as described above. This process is then repeated. 
INDUSTRIAL APPLICABILITY 
With reference to the drawings, and in operation, the present invention is 
adapted to control the linear extension of a hydraulic cylinder. In the 
excavator discussed above, each appendage is controlled by at least one 
hydraulic cylinder. Each hydraulic cylinder 204 has an associated 
hydraulic fluid delivering means 206, an actual position sensing means 
215, and an actual velocity sensing means 216. The controller 222 receives 
appropriate signals (v.sub.a (t)) from each velocity sensing means 216 and 
delivers appropriate signals (f.sub.c (t)) to each hydraulic fluid 
delivering means 206. The first control scheme 400 is duplicated and 
adapted for each linkage. 
A flow request signal (f.sub.r (t)) for each hydraulic cylinder is received 
by the controller 222. The flow request signals may be delivered by a 
manual, semiautomatic, or automatic control system. The source of these 
signals, however, is not relevant to the present invention. 
With reference to one hydraulic cylinder, desired velocity signals and 
velocity error signals (v.sub.fd (t),v.sub.sd (t),v.sub.e (t)) and a 
compensated velocity signal will be produced. 
The nonlinearity inverter 416 receives the compensated velocity signal and 
delivers a flow command signal, f.sub.c (t), to the pilot system 226. The 
pilot system 226 delivers pilot pressure signals to the directional valve 
208 and the directional valve 208 provides flow of hydraulic fluid to the 
hydraulic cylinder 204 to minimize the velocity error, v.sub.e (t).