Low Cost Plotter Electronics Design

by Neal J. Martini, David M. Eflement and Peter L. Ma f HE ELECTRONICS for the 74 70A Graphics Plotter had to be designed as inexpensively and ruggedly as possible to remain consistent with the philosophy of a low-cost, high-performance, high-reliability product. The basic guidelines for the design were to use a single printed circuit board for the entire electronic system, minimize overall parts count, eliminate the need for a cooling fan, and use custom and semicustom electronics with high performance/cost ratio wherever possible.

Fig. 1 is a block diagram of the electronic design of the 7470A. All of the circuitry is contained on a single, two-layer printed circuit board 178 mm wide and 298 mm long. The plotter intelligence is provided by a 1-MHz 6802 microprocessor, an 8K x8 ROM. and a IK x8 RAM. Some of the main functions controlled by the program code stored in the ROM are;

. Servo vector generation and servo system control Interpretation and execution of the 1IP-GL* plotter programming language (scaling, character generation, windowing, line type, et cetera]

I/O control for"the HP-IB,** RS-232-C/CCITT V.24, and HP-iL*** interfaces Pen-lift control.

The outside world communicates with the plotter via an HP-IB control chip and buffers. This integrated circuit handles ihe bus protocol and allows the microprocessor to be free from normal data transfer overhead.

"Hewlell-Packard Graphics Language

■-Hewieu-Paiiiara Interface Bus. HPs implemantaiian ol IEEE Stanaaia 466 (1973) ""Hewiett'PacKaici Interface Loop (see article ori page 16)

Servo

Two servos (Fig. 2) are used in the 7470A. one to move the paper and the other to move the pen carriage. The electronics for this consists of the microprocessor generating and sending digital move commands to the VLSI NMOS servo chips via the CMOS gate arrays. The two gate arrays contain all the circuitry needed to support the VLSI circuits and driver sections of the electronics (see box on page 25], The servo chips output pulse-width-modulated (PVVM) and direction signals back to the gate arrays. The gate arrays take these signals and generate the appropriate signals to control the switching motor drivers. In addition, the gate array circuits modify the pulse widths to adjust the servo gain to compensate for power supply variations and stabilize the slow axis movement. As the mechanical system moves, optical encoders (see article on page 26) mounted on the back of each dc motor send back digital pulses to the servo chips to close the servo loops.

Regulating the motor supply voltage would have been a duplication of effort because the servo already modulates this voltage, usually to an average value less than full supply voltage. It is less expensive to adjust the servo gain to compensate for power supply variations. For the gate array logic to know how much to change the servo chip pulse width, the microprocessor must know the level of unregulated voltage supplied to the motors. A voltage sensing circuit, consisting of a 1-bit analog-to-digital converter, provides this data. The output of the converter also serves the dual purpose of controlling the front-panel error light.

The servo system (Fig. 2) was modeled as a third-order system, with two-slate feedback. The electrical time con-

Motor

+5V (1A) Linear -5V Linear - +12V Linear + Z2V (1 A) (Unregulated Motor Drive)

Motor

Motor

Motor

Fig, 1, Block diagram of electronic system for the HP 7470A Graphics Plotter Most of the system logic is implemented by two custom gate-array ICs

Command

Gate Array and Gain Adjust

Motor Driver dc Motor, Load

Servo Controller IC

Encoder

Fig. 2. Block diagram of servo sysfem for each plotting axis in the 7470A. Pulse-width-modulation (PWM) drivers are used because their switching action offers much greater efficiency than linear drivers This reduces power consumption and heat generation stant of the dc motor is such that it could not be neglected. However, adequate performance is achieved with only position and velocity feedback.

A servo controllerlC supplied by HP's Corvallis Division1 is used to close the loop, it provides the interface to the microprocessor, decodes the encoder signals, sums position errors, estimates velocity and sums it, and transforms the servo error to a pulse-width-modulated output. The velocity constant and PWM gain, both 1C mask programmable. were changed to adapt the original servo controller chip to this servo system.

The PWM output of the servo chip is processed in the gate array to provide an adjustable forward path gain kK. There

Charge Path

Charge Path

To -SV Regulator

Regulator

To -12V, -SV Regulators

Fig. 3, Charge pump circuits are used to supply negative voltages for servo and HO circuits - (a) Hall-wave pump for HP-IB and HP-IL versions of 7470A (b) Full-wave pump for RS-232-CiCCITT V.24 version

To -12V, -SV Regulators

Fig. 3, Charge pump circuits are used to supply negative voltages for servo and HO circuits - (a) Hall-wave pump for HP-IB and HP-IL versions of 7470A (b) Full-wave pump for RS-232-CiCCITT V.24 version are two reasons why it is necessary to adjust the gain. First, because the motor driver supply voltage is unregulated, the forward gain ka through the motor driver varies with its supply voltage. This variation is too wide to maintain adequate performance without compensation. Second, below some threshold velocity, the servo controller chip provides no velocity feedback. This leaves the servo underdamped.and enhances limit cycling. Reducing the forward gain when the servo is idle reduces this effect. Through the gate array, the processor can control the forward gain and compensate for both problems.

Pen Lift

The pen lift in the 7470A is actuated by a solenoid. To minimize the size of the solenoid it is necessary to drive it first with one current to pull the plunger in, and then with a smaller current to hold it in. This is accomplished by a PWM voltage drive. The PWM duration is set by the gate array logic. One duty cycle is used for pulling in and another for holding. These duty cycles are under microprocessor control and are modulated to eliminate the effects of unregulated 24Vdc supply variations.

Power Supply

Four voltages are generated by the power supply module. Low-current supplies provide the +12V and -5V required for the NMOS servo chips. These supplies are also used in the RS-232-C/CCITT V.24 version of the 7470A. In addition, there is a linear 5V supply. The linear supply design is attractive because of its simplicity, and can be used because the total 5V load current is low. nominally 600 mA (the CMOS gate arrays and the NMOS servo chips require very little power). The fourth voltage is the unregulated +24Vdc supplied to the main drive motors. Since this voltage has to supply about 1 ampere rms, it is less expensive if it does not have to be regulated. This is possible because of the voltage-sensing and servo gain adjust schemes described above.

To supply the negative voltages required for the I/O and the servo chips, charge-pump circuits [Fig. 3) were chosen to run off the secondary of the transformer. A half-wave pump is used for the HP-IB and HP-IL interface versions of the 7470A. The RS-232-OCCITT V.24 version requires a full-wave pump. The part sizes for operation at the low-frequency limit of 47 Hz were acceptable, so there was no need to consider operating at a higher frequency. The extra parts needed for the charge pumps cost less than adding an extra transformer winding.

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