Service Data Vickers Servo Valves Single Ended Linear DC Servo Amplifier EM-A-0 Revised 09/0/ I-0-S
General This manual is written primarily to establish a logical troubleshooting procedure for the solid state EM (electronic modular) amplifier. Complete systems are beyond the scope of this manual and will not be covered. Adequate information is presented for an Electrical Technician to repair the EM-A-0 amplifier. EM-A-0 (0) Linear Servo Amplifier A. Description The EM-A-0 is a special purpose DC servo amplifier designed specifically for a Vickers SE or SF flapper type servo valve. The EM-A-0 consists of a high gain summing amplifier, a low gain DC amplifier and a power output stage. The amplifier module also contains a 0 volt regulated power supply which may be used for development of input signals through an external 000 ohm potentiometer. The complete amplifier and power supply are contained on a plug-in module whose approximate dimensions are -/ x inches. Refer to table for electrical and mechanical specifications. B. Specifications Input Impedance Input Signal Level Gain: Continuously Adjustable: Input Input Output Output Current 0 Ω load Output current limits Dither current adjustable Output drift at max. gain During warm-up (0 min.) vs. temperature vs. time (after 0 min.) vs. supply voltage Frequency Response DC - 00 Hz max. gain full load Regulated Output Supply into 00 Ohm load Pin to common Pin to common Temperature Range Operating Storage Power Supply Req mnt Voltage Range Mechanical Specs Module Module Size Module Weight Controls Min. Typ. Max. Units.9 9. 00 0.0 0 Amps/volt Amps/volt Single ended with respect to common 0 0 +9 9 9 0 + 0 0 00 +0 0 +9 9.0 0. +00 00 00 00 0 0 + + + +0 0 Special printed circuit card.0 x. x.0 ox. Screwdriver adjusted Dither Gain KΩ KΩ V-peak p-p / F / hrs. DB C C Table. Electrical and Mechanical Specifications for the EM-A-0. C. Installation The EM-A-0 servo amplifier is designed for mounting on a power supply plate such as the EMP-A-. Input and output connections to the amplifier circuitry are provided by printed circuit pin connections on the module. These pin connections, when installed into a plug-in receptacle, must be connected as shown in Table. TB wiring interconnections, located on the EMP-A- power supply plate, are shown for convenience. Portions of the E-0 servo amplifier are of the incapsulated construction and must be replaced as complete assemblies. Amplifier A and A shown on the schematic diagram Figure are examples of this type of construction. Replacement of A and A require factory adjustments to be performed to the resistance values designated by an asterisk (). Therefore, should replacement of either amplifier be required, it is recommended that the installation be accomplished by Vickers. Replacement EM-A-0 amplifiers are available. EMP-A- TB (J)A (J)B (J)C Plug-in receptacle pin conn tions a b c d e f h j k l m n p r s Plug-in module pin conn tions 9 0 Signal Dither ( Vac) N.C 9 input Input # Input # Summing Junction Regulated +DC output Regulated DC output N.C. N.C. Negative output to coil Positive output to coil +9 input Slotted for polarizing key Common Table. The EM-A-0 Plug-in Receptacle and Terminal Board Interconnecting Wiring D. Circuit Description Amplifier Section - The EM-A-0 is a DC amplifier, consisting of a high gain summing pre-amplifier feeding a unity gain buffer amplifier which drives a power output stage. The output stage is statically adjusted to produce 00 Milliamperes () of current through the 0Ω servo valve coil. (Vickers type SE/SF servo valve) and varies from this 00 value with variations in input signal. An explanation of the circuitry follows: Refer to the pictorial diagram, Figure, the simplified schematic diagram Figure, and the complete schematic diagram schematic diagram Figure.
Command and feedback signals are connected through R and R to the input of amplifier A. Resistors R, R, and amplifier A s input resistance form a summing network which permits a difference potential to be developed across amplifier A s input resistance. The amplitude of this potential is determined by the polarity and amplitude of the input signals, the input coupling resistance R and R, and the input resistance of amplifier A. Under normal operating conditions, the junction of R and R is maintained at virtual ground within a few millivolts by A. Diodes D and D9 limit amplitude extremes at the input of A to approximately. volt minimum, if the amplifier output saturates. Amplifier A inverts the output signal with respect to the input signal, permitting gain adjustment to be obtained by a negative feedback arrangement through resistors R and R. A portion of the inverted output is allowed to develop across the input resistance of amplifier A. This negative feedback subtracts from the input signal subsequently reducing the gain of the amplifier. The output of A is used to drive amplifier A which in turn provides the current necessary to drive power transistor Q and Q. (Shown on the complete schematic diagram Figure ). Amplifiers A and A are of the same type. But amplifier A s gain differs from that of amplifier A, due to wiring arrangement. Amplifier A s gain is essentially the ratio of feedback to input resistance or: A Gain - Input (Approx. gain of ) A Gain - Input (Approx. gain of 0) = = R + R + R Ra R + Ra R R Ra R + Ra Amplifier A is connected to provide a non-inverting, unity gain amplifier characteristic. Unity gain amplifiers (gain of ) provide isolation between circuits (buffer action) and will permit a source with low current capacity (A) to drive a heavy load (Q and Q). Transistor Q and Q develop the necessary output current for the servo coil. Resistors R0 and R0a bias amplifier A and establish the 00 null current through Q, D, the servo coil, and resistor R (the current sensing resistor). A voltage is developed across R directly proportional to the current through it. The voltage is reduced through voltage divider action by R and R, and is fed to pin two () of the unity gain amplifier A reducing its gain. This gain reduction (negative feedback) improves the linearity of the driver stage A and the output transistor circuit Q and Q. Ra and Rb are adjusted to limit the maximum current. When the voltage across Ra and Rb exceeds the threshold voltage of D and D, the diodes conduct limiting a further current rise in the output circuit. Dither - The dither signal (0 to 00 Hertz) is connected to pin of the plug-in module. A variable resistive divider network R and R provide adjustment of the value of the dither. The dither is applied to pin two () of amplifier A through resistor R. Dither signal is used to keep the servo flapper in constant motion, thus preventing the flapper from magnetizing in a locked condition against the orifice. Constant motion of the flapper will also reduce the effect of silting (the particle build-up around the orifice). R Power Supplies - Four regulated power supplies are provided on the plug-in module. Refer to the schematic diagram figure. The amplifier section utilizes both a positive ten (+0) and a negative ten (0) volt supply for operation. A positive ten (+0) and a negative ten (0) volt supply is also available for external use. The externally connected supplies may be used for amplifier control circuitry if desired, thus providing the control voltage and amplifiers necessary for a complete system in one plug-in module. All the regulated supplies operate in a similar manner, therefore, explanation of only one will be presented. Upon application of negative nineteen (9) volts DC to pin of the plug-in module, Zener Diode Z conducts through R establishing a regulated source voltage for the base of Q. A portion of this regulated voltage is applied to Q through the voltage divider network of R9, D, and R. Diode D and resistor R shunt the base resistance of Q and reduce the base drive as the temperature rises. This reduction in drive prevents thermal runaway of transistor Q. Emitter resistor R swamps the emitter base junction resistance and prevents a large increase in emitter current, particularly at low temperatures. Q and R act as a variable voltage dropping resistor for Zener Diode Z, and maintain a constant current through Z with varying input voltages. The combined action of Q and Z provide a regulated -0 volt source at pin of the plug-in module. E. Troubleshooting Procedure Determine if the EM-A-0 module is functional. Refer to the schematic diagram figure and the pictorial diagram Figure. Note The EMP-A- power supply or its equivalent must be used to perform the following test. Minor wiring changes may be required if an equivalent supply is used.. Remove electrical power from the system.. Remove the input signal connections,,,, and. Tape the wire ends and symbolize to prevent error.. Connect a linear taper, 000 ohm test potentiometer as shown in figure.. Remove the EM-A-0 plug-in module. Use the ohmmeter on the low ohm scale to check the resistance of the load as follows: Connect the ohmmeter between J - m & n. A reading of approximately 0 ohms is considered normal. If the reading is normal, reinsert the plug-in module and proceed with the test.
Note The characteristics of this amplifier are such that once conduction starts, a very small change in input signal level will cause a very large change in output current. Therefore, the 000 ohm command test potentiometer (shown in figure ) will seem to have no effect on the measured output voltage level until the center of the control is reached, then the voltage level will change rapidly from 0 to. volts. To obtain 0. volts (00 ) reading, the control must be varied very slowly when the center of the control is reached.. Connect a volt-ohmmeter between TB- and TB- (common ground reference). Apply power and measure for negative. volts (00 ). The voltage should vary from approximately zero (0) volts at one end of the test potentiometer adjustment range to approximately. volts at the other. If the amplifier performs as indicated, it is operating normally.. Remove AC power from the system.. Connect symbolized wiring removed in step E.. Summing Junction Sig. Input Sig. Input Z +D.C. Com D.C. Z R. K Supply Out R. K R. K Z Supply Out R9.K D Z R.K R 0K +%,W R K + D.C. Supply In R0 R R.K Ω 0Ω D R Ω Q Q R 0Ω R K +% W Q Z Z Q D D9 R K R K A R 0 K R C.00 µf a b R %/0W R0 R0a K GAIN R 00K R a ALL RESISTORS + %, / W UNLESS OTHERWISE SPECIFIED FACTORY ADJUST C 0PF R.K A D C 0. µf R.K D D Rb Q R 0 Ω Ra R.0 Ω W Q D R Ω W NEG OUT POS OUT D C Supply In Figure. Complete Schematic Diagram for EM-A-0
Figure. Pictorial Diagram of the EM-A-0 +0 R K 0K R R K A R 0K R0a R0 K R 00K 0/ INV. R K R.K R K A R.K Load R Ω Factory Adjust Figure. Simplified Schematic Diagram of the EM-A-0
EMP-A- TB- Volt-Ohm Meter (VOM) Load Neg. Out Load Pos. Out Servo Coil 0Ω Common ground Input signal Test Potentiometer Remove signal wiring at terminals &. Symbolize the wires removed to prevent error. 000 Ohm Linear Potentiometer +DC DC Note If the EMP-A- power supply is not used, connect the power source as shown to test the EM-A- amplifier. Figure. Test Potentiometer Wiring Diagram