1. INTRODUCTION

The problem, to reduce the spacecraft (SC) system mass-dimension parameters, is especially acute in the space field. One current solution is to replace electro-mechanical drives in different SC units forpiezo-drives which, in its turn, would decrease the mass-dimension parameters n-fold times [1,2]. Peripheral cord tensioner unit (PCTU) in the reflecting surface shape regulator system for large-sized transformable spacecraft antenna reflectors provide the preliminary tension of the spacecraft reflector surface itself. Peripheral cord tension isF_нат – 300N, unit mass- 250g, total reflector spoke mass- 40kg and pushrod displacement with increment of Х_нат – 12mkm.

2. PROBLEM STATEMENT

The problem targets involve the following: conducting microlinear piezo-drive experiments based on piezo-pack APM-2-7 in inertia load modes under different inertia load values; identifying the characteristic oscillatory system frequencies; determining oscillatory system (OS) resonant frequencies and vibration displacement amplitudes within resonant frequencies.

3. TEST STAND CONSTRUCTION-DESIGN DESCRIPTION OF PERIPHERAL CORD TENSIONER

Oscillatory system of test stand was calculated on the basis of 3D and one-dimension mathematical models[3–5]. This test stand is designed to investigate the microlinear piezo-drive (MLPD) operation modes (fig. 1). Microlinear piezo-drive (MLPD) operation modesdepend on the following OS parameters: type and power of piezo-pack;weight (loads) mass; preliminary stack tension forces; tension on piezo-pack; piezo-pack current; frequency effect[6–9].Operation mode parameters are recorded as electrical signals in experiment testing of MLPD: tension on piezo-pack; piezo-pack current; signal strength from piezo-pack sensing unit; vibration acceleration weight (loads). Another important parameter to be considered is preliminary tension force.

The following data was obtained during the MLPD experiment:

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  weight (loads) frequency -responses;

  
  


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  weight vibration displacement frequency responses;

  
  


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  frequency responses of forces on weight.

  
  

Based on the results of these frequency-responses the operating OS frequency responses and vibration displacement amplitudes were determined.

Varying the OS parameters and frequency exposure on the test stand, numerous system operation modes were investigated.Based on the test results the optimal frequency exposure on given load was determined, at which the maximum forces on loads, maximum vibration displacement load and maximum power on load were observed.

Figure 1. Test stand, operating MLPD at inertia load: 1 – inertia load mass (weight); 2 – pushrod; 3 – support structures; 4 – adjustment screw; 5 – frame; 6 – piezo-pack APM- 2-7; 7 –force sensor; 8 – rigidity; 9 – accelerometer AR 1019.

Figure 1 illustrates the test stand. Piezo-pack (6) is installed into the frame (5) on the adjustment screw (4), through which the force sensor (7) rests upon the pushrod (2), then the pushrod (2) through rigidity (8) acts on the inertia load (1) (weight). Accelerating inertia load (1) is measured by accelerometer (9). Through adjustment screw (4) preliminary piezo-pack tension force is established. Force sensor piezo-pack calibration, accelerometer calibration and preliminary tension setting procedure should be described in relevant places later.

Detailed diagram of test stand is illustrated in fig. 2, where minor details are included- energy brushes and MLPD radiator, as well as power source (alternating current 12 and direct current 13) and data acquisition and storage system 11.

Figure 2. Layout diagram of experimental test stand for prototype MLPD: 1 – inertia mass load (weight); 2 –guiding weight; 3 – energy brushes; 4 – frame; 5 –piezo-pack; 6 – force sensor;7 – radiator; 8 – pushrod; 9 – preliminary tension rigidity; 10 –accelerator; 11 – data acquisition and storage system; 12- DC power source.

4. EXPERIMENT RESULTS

Varying the AC frequencies input to piezo-pack, the piezo-pack generates the disturbance force, which, in its turn, through the radiator-pushrod, acts on the inertia load imposing vibration at given frequency. Oscillograms are presented on figure 3 (a and b). Measuring the inertia load acceleration the vibration displacement acceleration is determined. Based on the experiment results frequency-response characteristics are plotted.Piezo-pack current was maintained at 0.5 A, whereas the power supply decreased proportionally to piezo-pack capacitance. Parameters for 1^st operation mode (fig. 3а): f = 10.7khz,I = 0.5 A; preliminary tension forceFo = 240 H; force sensor signal F_н = 2.82 Н, Ẍ= 57 m/c²; weight forceGпр = 0.4kg.; Х = 0.012mkm.Parameters for 2^nd operation mode (fig. 3b): f = 1400H, I = 0.5 A;preliminary tension force Fo = 240 H, F_н = 2.82 Н, Ẍ = 57.4 m/с²;weight force G_пр= 3kg., Х = 0.742 mkm.

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   b)

   
   

Figure 3. Signal distribution from oscillatory system transducer to electron oscillograph screen: а) 1st operation mode; b) 2nd operation mode

1- acceleration, 2 – piezo-pack current; 3- piezo-pack force, 4 – piezo-pack tension

MLPD frequency-response characteristics without preliminary tension and mass load of 0.5kg. is illustrated in fig. 4. These characteristics have four distinct resonances. This effect could be based on the fact that separate piezo-pack elements in SC resonate differently and the system does not haveunified operation mode. The most energetic SC mode is at the frequency of 750 Hz (at currentI = 0.5 A and load mass Gпр = 0.5 kg.). It is within the range of this frequency that the vibration displacement and forces on the load are more significant (fig. 4).I n the specific frequency points the vibration displacement has the following values: 1 – 29 mkm, 2 – 9.32 mkm, 3 – 0.735 mkm, 4 – 0.002 mkm, 5 – 0.0013 mkm, 6 – 0.0013 mkm.

Figure 4. Frequency characteristics of MLPD vibration acceleration, operating on inertia load without preliminary tension

Increasing inertia load furthers decreasing resonant frequency and increasing vibration displacement (fig.5), at currentI = 0.5 A and preliminary tension F₀ = 240 N.

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   b)

   
   

Figure 5. Frequency-response characteristics of operating MLPD at different inertia loads and preliminary tension; a) mass loadG_пр = 0.4kkg; b) mass weightG_пр=0.5kg.

If inertia mass is 3 kg., the resonant frequency is 300 Hz, load force= 550 N (fig.6- curve 2) and vibration displacement = 12mkm (fig. 7, curve 1). Experimental resonant frequency characteristics and vibration displacement amplitudes in dependence to inertia mass weight values are illustrated in fig. 7 (a and b).

Figure 6. Experimental frequency characteristics of vibration displacement load and forces on load of operating MLPD: I = 0.5 A and G_пр= 3 kg. Preliminary tension Fo = 240 N.

Frequency effect interval– 100 Hz

Figure 7.Experimental dependence: а) operating MLPD resonant to mass weight dependency;

b) weight vibration displacement on operating MLPD resonant to mass weight dependency. Mass weight – up to 3 kg.

The basic MLPD operating frequency interval will be 50-100Hz for a SC with corresponding of peripheral cord tensioner unit (PCTU) load parameters, as this is the most energetic operating interval of this frequency system. In this case, the excitation force on the load itself would be more than 550 Hz, vibration displacement interval – more than 12 mkm.

5. CONCLUSION

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   Experiment showed that the test stand is designed as a spacecraft (SC)with only inertia load, however, the operating SC modes are quite similar to the operating modes with combined loads.SC resonant frequencies decrease with inertia mass (typical for inertia load), while vibration displacement amplitudes increase with inertia load which indicates the fact that elastic component exists in the SC, i.e. preliminary tension elasticity, elasticity of stand supporting legs, piezo-pack elasticity which, in its turn, is characteristic of combined load.

   
   


2. 
   

   The basic MLPD operating frequency interval will be 50-100Hz for a SC with corresponding of peripheral cord tensioner unit (PCTU) load parameters, vibration displacement interval – more than 12 mkm.

   
   


3. 
   

   Experimental data is in good agreement with the numerical experiment results on 3D and one-dimension mathematical models. In the case when SC has inertia load and is a resonant system, according with calculation results it is possible to match the MLPD design to desired operating frequency resonant intervals.

   
   


4. 
   

   The analysis of experimental data and numerical experiments showed that designed MLPD specifications could meet the requirements applicable to the peripheral cord tensioner unit in the reflecting surface shape regulator system for large-sized transformable spacecraft antenna reflectors.