Help with analog circuit design for servo valve current drive

First off, let me state that this is not homework. At least it's not homework in the sense that I am not being graded for this. It is homework in the sense that this is a problem that's been bugging me off and on for 30 years (mostly off as it was something I didn’t quite understand 30 years ago, but it was only a minor issue and we were able to work around it, however, as time went on and I would go to a customer’s site to deal with quasi related problems, I would sometimes return back to trying to understand how this circuit worked, or was supposed to work).

Secondly, I am a degreed mechanical engineer who has worked in the controls/motion field since 1990. I do understand basic circuit theory and can work with and troubleshoot general analog circuit card, I lack understanding of nuances in certain areas, which is what I believe is causing me to not understand this design.

The design is an analog circuit to drive an electro-hydraulic servo valve which does not, DOES NOT, have a feedback wire in it. Software is used to close the loop. The circuit card has the following states of operation:

1. computer running, no damping, no playback
2. computer running, no damping, no playback
3. computer running with playback
4. computer not running,

The main two of concern are states 1 and 4.

There are switches that connect certain electrical pathways based on the above states. Those switches will either switch in a signal (servo command, position feedback, velocity feedback, force stabilization) or switch in a diode. The output of all the switches get summed together to feed an operational amplifier circuit which drives the current amplifier feeding the servo valve coils.

Some areas of confusion for me are the following:

1. What is the purpose of the diodes?

2. How does operation change if the diodes are oriented in the opposite direction? I ask because the board design this circuit was based on is essentially identical with the exception of the diodes being oriented the opposite way.

State 1

This circuit has the following components:

Operational amplifier U14A (SE5532) with these signals connected to the

• non-inverting input:
• ground through R55 (1K ohm)
• inverting input:
• ground through R90 (2K ohm)
• servo command (Servo Cmd) signal goes through R8 ( 52.4k ohm)
• a stabilizing input (band pass of a load cell signal) through R91 (10.5k ohm)
• bias offset signal (Potentiometer R51 and fixed resistor R52)
• Diodes (CR8, CR7 and CR10), 1N4148, to ground
• Feedback from servo valve coil via R53 (15k ohm)
• Output:
• R56 (100 ohm)

Current amplifier U48 (LH0002) with these signals connected to the:

• Input
• R56 (100 ohm) from U14A
• Output
• Servo valve coils (in parallel)

The way this state is supposed to operate is the servo command provides the primary signal for moving the servo valve. However, in my humble opinion, the 3 diodes will keep the inverting input signal essentially grounded thus not allowing the system to operate. That being said, the system DOES operate, so what am I not understanding?

State 4

This circuit has the following components:

Operational amplifier U14A (SE5532) with these signals connected to the

• non-inverting input:
• ground through R55 (1K ohm)
• inverting input:
• position feedback signal which goes through R89 (52.3k)
• velocity feedback signal which goes through R87 (4.99k)
• bias offset signal (Potentiometer R51 and fixed resistor R52)
• Diodes (CR11, CR12, CR9 and CR10), 1N4148, to ground
• Feedback from servo valve coil via R53 (15k ohm)
• Output:
• R56 (100 ohm)

Current amplifier U48 (LH0002) with these signals connected to the:

• Input
• R56 (100 ohm) from U14A
• Output
• Servo valve coils (in parallel)

The way this state is supposed to operate is when the computer is off, the servo command is switched out and one of the diodes (CR9) is switched in, the resistor, R90, is switched out and diode CR11 is switched in, the stabilizing circuit is switched out and diode CR12 is switched in. Diodes CR7 and CR8 are switched out and the position and velocity feedback signals are switched in. This provides an analog damped position feedback look control the valve which will in turn hold position of the actuator the valve is mounted to.

Again, in my humble opinion, the diodes will keep the inverting input signal essentially grounded thus not allowing the system to operate. That being said, the system DOES operate, so what am I not understanding?

I apologize if I’ve not described the operation clearly. It’s clear in my head, so I may have been too brief in my elaboration. Also, I noticed the images I've included are not very clear. I will see if I can figure out what format to use to get a clearer image.