Ola Fremming's homepage

Servo test results

Presented below are the test-results of the servo's tested so far, good and bad, old and new. Description of test method and how the results are presented can be found here.

Manufacturer specifications

My test results (all at 5.0V)

Servo designation

Weight [g]

 Size (LxHxW) [mm]

Speed [s/60°]

 Torque [kgcm]

Gearbox

Start-stop Speed [s/60°] 

 Speed [s/60°]  Speed [s/60°] (@2kgcm) Accuracy [°] (@2kgcm) Complete Results Comments
Futaba FP-S128 53 40.5x40.5x20 0.24 3.5 Plastic 0.29 0.26 0.45* 9.5 FP-S128  *Tested at 1.5kgcm, failed test at 2kgcm
 Futaba S9102 45 41x25x23 0.16 2.8 Plastic  0.19 0.17 0.39 6.2 S9102  Not new,  overshoots when stopping 
Futaba S9602 31 36x30.7x15 0.09 2.7 Metal 0.14 0.11 0.26* 15.5* S9602 *Tested at 1.0kgcm    **Tested at 1.5kgcm
Futaba S9650 26 35.5x28.5x15 0.11 4.5 Metal 0.17 0.13 0.30 8.3 S9650  
Futaba S9451 56 40x36.5x20 0.10 8.7 Metal 0.16 0.12 0.17 2.6 S9451  
Futaba S9551 45 41x25x20 0.14 4.8 Metal 0.16 0.13 0.19 1.6 S9551 VERY precise
                                   
 Hitec HS-645MG  60 40.5x38x20    0.2  9.6  Metal  0.29 0.22 0.30 5.8 HS-645MG    
                         
Hextronik D-MG16 19 29x29x11.2 0.08 2.9 Metal 0.16 0.12 0.47 8.6 D-MG16
Hextronik HX-5010 39 40x38x20 0.16 6.5 Plastic 0.26  0.21 0.34 8.5  HX-5010
                        
Hyperion DS16FCD 25.5 32x31.5x16 0.18 4.5 Carbonite 0.25 0.18 0.31 7.6 DS16FCD
Hyperion DS20xSMD 52.5 40x38.5x19.5 0.13 7.0 Metal 0.18 0.12 0.18 2.4 DS20xSMD Very good!
Sanwa SRM-141AL 33 31x31x15 0.25 3.2 Metal 0.28 0.24 0.45* 7.0 SRM-141AL *Tested at 1.5kgcm, failed test at 2kgcm   
Sanwa ERG-XT 65 39x37x20 0.12 7.3 Metal 0.17 0.14 0.26 6.3 ERG-XT   
                       
Graupner/JR C512 45 40x38x19 0.25 2.7 Plastic 0.25 0.21 0.53 16.3 C512   
JR NES 331 18 30x28.5x13 0.23 3.25 Plastic 0.31 0.27 0.52* 9.6 NES-331 *Tested at 1.5kgcm, failed test at 2kgcm  
Graupner/JR DS368 22 30x28.5x13 0.24 3.8 Metal 0.33 0.27 0.58* 4.8 DS368 *Tested at 1.5kgcm, failed test at 2kgcm
JR NES-7100 43 44x26x22 0.14 4.8 Metal 0.19 0.15 0.17 4.8 NES-7100
JR DS-3421SA 26 33x26x15 0.18 4.6 Plastic/metal 0.21 0.17 0.27 4.0 DS-3421SA  
JR DS-9411SA 39 36x26x19 0.15 5.9 Plastic/metal 0.21 0.17 0.25 3.1 DS-9411SA  
JR DS-9421 38 36x26x19 0.17 5.0 Plastic/metal 0.21 0.16 0.24 2.78 DS-9421
JR DS8900G 59 41x38x20 0.05 3.5 Plastic 0.08 0.057 0.14 3.8 DS8900G   
JR DS-8401 49 39x35x19 0.19 11.0 Plastic 0.22 0.18 0.22 2.4 DS-8401  
JR DS-8455 60 39x35x19 0.10 6.5 Metal 0.14 0.096 0.15 2.2 DS-8455  
JR DS-8355 49 34.5x39x19 0.09 6.2 Plastic 0.12 0.09 0.12 2.4 DS-8355

NB : Servo torque is presented with the unit kg-cm, to convert to oz-in multiply by 13.86 !

During development of the system and testing of the servo's I had lying around I learnt a lot of interesting things, here are some :

By far most servo's are supplying power to the motor in pulses, these pulses match the update frequency of servos. The amount of power supplied during each pulse is controlled by the pulse-width as can be seen in the two graphs below. The first one is on a JR DS-8401with a 2[kgcm] load, the second is with more load on the same servo. As can be seen the max current in each pulse is very similar, the pulse-width is different. By counting the pulses per a certain time-frame the update frequency of the servo can be calculated, from the graph below : 31 Pulses in 102 [ms]  (31 / 0.102) indicates an update rate of 303[Hz]. The more pulses you count the more accurate this result becomes.

 

The current consumption during start of movements (as during speed testing) shows that the servo are doing it's very best to start immediately, there is no need for precision involved, just start as fast as possible .  When the servo is to stop the picture is quite different, the the task is not only to stop, but to stop at the correct place. Inside the servo there is a regulating system (in the literature for control-systems, a so called servo-system:-) )this performs the math (and control) needed to make the servo stop in the wanted position. For each current-pulse it's calculating the pulse-width needed to perform the requested positioning.  The graphs below shows how a JR DS-8401 start it's movement in 30-40 [ms] with long pulses, and stops in 40-50[ms] with pulses that are of different length. 

 

When making the test-rig I have made an effort to supply the servos with as stable voltage as I could.  To verify this the graph below shows the current consumed and the voltage deviation from 5.0[V] during start of movement.  The voltage drop is just above 20[mV] (that's 0.02V) during a 1.7[A] current pulse, which is far better than to be expected in a real life situation in out models.  Our batteries and cable resistance makes it far worse leading to slower and weaker servos than what my test results indicate (unless if you run a higher voltage initially, of course).